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J . Org. Chem. 1980, 45 , 1035-1041
1035
of ketone 11. Why e nolization is greater for the cyclohexoxy
derivative compared to the other hydrides is not clear,
especially in its comparison to LiMgH,OPh. One expla-
nation is that the amount of axial alcohol produced in the
reaction of every ketone with L iM gH 20 Ph is very nearly
the same as tha t observed for MgH,. Th us
if
MgH, is the
reducing agent for LiMg HzO Ph and MgH, gives 100%
yield in each case,
i t
is not surprising t ha t those hydride
reagents that disproportionate
to
MgH, give no enolization
as is observed in the case of MgH,.
It
was desired to determine if the degree of stereose-
lectivity q - ,
as
higher in the initial stages of the reaction than
after equilibrium had a chance to take place. In this
connection,
4-tert-butylcyclohexanone
as allowed
to
react
with a 10090 excess of lith ium
(2,2,6,6-tetrabenzylcyclo-
hex0xy)dihydridomagnesiate. The results are given in
Table VI. As can be seen from the data , the lower the
temperature, the greater the observed enolization to re-
ductio n ratio. A t -25 C, for example, the major product
(64%) af ter quenching is the s tar ting ketone , 4 - te r t -b ~ -
tylcyclohexanone. It should also be noted tha t the initial
reaction is very fast and t hat af ter 30 s l i t t le more tha n
5% change was observed in the yield of produc t. Also
between
30
and
18000
s,
less than 5% change was observed
in the ratio of alcohols. Th e therm odyn am ic produ ct is
the equatorial alcohol, and if
4-tert-butylcyclohexanone
is allowed to react under the equilibrium conditions in-
herent for
Meerwein-Ponndorf-Verley
or Birch reductions,
the equ atorial alcohol is produced in 98-99% yield. For
lithium alkoxymagnesium hydride reductions, never less
than 86% of the axial alcohol is observed; therefore,
if
equilibration is taking place,
it
is taking place very slowly.
Tabl e VI1 (expt 64-68) lists the results of the reactions
of ketones I, 11, 111, and IV with lithium dialkoxy-
magnesium hydrides prepared according to eq
5 .
The m ost
selective reagents were the bis tetramethylcyc1ohexoxy)
and bis tetrabenzy1cyclohexoxy) ydrides (expt 67 and 68)
which reduced ketone I to provide 89 and 85% axial al-
cohol, respectively. However, a large am oun t of enolization
accompanied th e reactions
(70
and 63 %, respectively). On
the oth er hand, exp t 65 shows that the bis di- ter t-butyl
derivative enolized only 20% of the ketone w hile reducing
the ke tone to
81%
of the axial alcohol. Th e sodium
reagent (expt 66) not only produced a 5 54 5 axial to
equa torial alcohol ratio bu t also enolized 90% of the ke-
tone. When L iH and LiOR were allowed to react under
similar conditions (expt 64), a 74:26 ratio of axial to
equatorial alcohol was observed, but 72 of the ketone
was enolized.
Wh en ketones 11,111, and IV were allowed to react w ith
these reagents, lesser amounts
of
enolization were observed
with very stereoselective results. All the reage nts stud ied
produced 99 and
100%
axial alcohol when allowed to react
with ketones I1 and 111,respectively. Th e reactions with
campho r (ketone IV) produced greater tha n
90
exo al-
cohol with little enolization except for exp t 64 and 66 which
produced 70 and 64%, espectively, of th e sta rtin g ketone.
These reagents represent a me thod of using l i thium a nd
sodium hydrid e for reduction which has n ot been previ-
ously reported.
Acknowledgment. We wish to thank the National
Science Foundation (Grant No. MPS 7504127), Union
Camp, and Alcoa for financial support of this work.
Registry No LiMgHz(O CH3), 72749-25-8; LiMg Hz(O-i-P r),
72749-26-9; LiMg Hz(O-t-B u), 72749-27-0; LiM gHz(O CHz-t-B u),
72749-28-1; LiMgH2(OCHPhz),72749-12-3; LiMgHZ(O-c-CGHll),
72749-13-4; LiM gHz (OP h), 72749-14-5; lithium [(2-methylcyclo-
hexy1)oxylmagnesium hydride, 72749-15-6; lithium (2,6-diiso-
propy1phenoxy)magnesium hydride, 72749-16-7; lithium (2,6-di-
tert-buty1phenoxy)magnesium hydride, 72749-17-8; l i thium
[(2,2,6,6-tetramethylcyclohexyl)oxy]magnesium
ydride , 72749-18-9;
lithium [(2,2,6,6-tetrabenzylcyclohexyl)oxy]magnesium ydride,
72749-19-0; lithium methoxide, 865-34-9; lithium isopropoxide,
2388-10-5; lithium tert-butoxid e, 1907-33-1; lithium 2,2-dimethyl-
propoxide, 3710-27-8; ithium diph enylm ethox ide, 2036-66-0; ithium
cyclohexyl oxide, 4111-51-7; lithium phenoxide, 555-24-8; lithium
2-methylcyclohexyl oxide, 72727-48-1; lithium 2,6-diisopropylphen-
oxide, 72727-49-2; lithium 2,6-di-tert-butylphenoxide, 5894-67-2;
lithium
2,2,6,6-tetramethylcyclohexyl
xide, 72727-50-5; lithium
2,2,6,6-tetrabenzylcyclohexyl
xide, 72727-51 6; magnesium hydri de,
7693-27-8; cis-4-tert-butylcyclohexanol, 37-05-3; trans-4 -tert-bu -
tylcyclohexanol, 21862-63-5; cis-3,3,5-trimethylcyclohexanol,33-
48-2; trans-3,3,5-trimethylcyclohexanol, 67-54-4; 2,2,6,6-tetra-
benzylcyclohexanol, 3849-12-5;
cis-2-methylcyclohexanol,
443-70-1;
trans-2-methylcyclohexanol,443-52-9; endo-1,7,7-trimethylbicyclo-
[2.2.l]heptan-2-01, 507-70-0; eno-1,7,7-trimethylbicyclo[2.2.l]hep-
tan-2-01, 124-76-5; [(2,2,6,6-tetramethylcyclohexyl)oxy]magnesium
hydride, 72727-52-7; lithium bis[ 2,2,6,6-tetramethylcyclohexyl)-
oxylmagnesium hydride, 72749-24-7; 2,2,6,6-tetramethylcyclo-
hexanone, 1195-93-3; 2,2,6,6-tetrabenzylcyclohexanone, 382-13-0;
2,2,6,6-tetramethylcyclohexanol,
948-41-0.
Hydrometalation. 5. Hydroalumination of Alkenes and Alkynes with
Complex Metal Hydrides of Aluminum in the Presence of Cp,TiCl,
Eugene C. Ashby* and Steph en A. Noding
School of Chemistry,
Georgia
Institute
of
Technology,
A t lan ta , Georgia 30332
Received July 2, 1979
Ter min al alkenes and inter nal alkynes are reduced rapidly a nd in high yield by reaction with LiAlH4, NaAlH,,
LiA1Me3H, NaAlMe3H, LiAlH 2(NR z)z, aA lH2(N Rz)2,
r
Vitr ide [NaA1Hz(0CHzCHz0CH3)z]n the presence
of
a catalytic amo unt
of
CpzTiClz n TH F at room temper ature. When these reactions were quenched with DzO
or
I*,
quantitative yields of the corresponding deuterium or iodine compounds were obtained in most cases. Thi s
method provides a convenient and high-yield route t o alkyl- and vinylaluminum comp ounds as intermediates
in organic synthesis.
Considerable interes t in recent years has been directed
toward the development of carbometalation and hy-
drometalation reactions involving alkenes and alkynes.
Th e reasons for this inte rest are clear: first, alkenes and
0022-3263/80/1945-1035 01.00/0
alkynes are very fundamental and economic building
blocks for more complex organic compounds, an d second,
carbometalation and hydrometalation provide routes to
form carbon-metal bonds which can then be functionalized
980 A me r ic a n C he mic a l Soc ie ty
8/19/2019 JOC 1980,45,1035
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1036 J . Org. Clzem.,
(eq
1
and
2)
to form
Vol.
45 , N o . 6 , 1980
numerous classes of compounds.
-
( H i
Carbo metala tion reactions have evolved significantly in
just the last few years due to the efforts of numerous
workers; however, hydrometalation reactions have received
muc h less atte ntio n. Th e work of Brown’ in the devel-
opment of hydroboration as a synthetic tool in organic
chemistry stands alone in terms of the degree of develop-
me nt of the m ethod a nd in terms of its utility. However,
the produc t of hydroboration (R3B)cont ains C-B bonds
which are not easily functionalized com pared t o C-Mg or
C-A1 bonds. In additio n, diborane is quite expensive, thu s
providing sufficient impetus to develop other hydrome-
talatio n reactions, particularly those tha t can form more
active C-M bonds an d whose metal hydride reagent is not
so
expensive.
Recently, Schwartz and co-workers have developed a
hydrometalation reaction involving the addition of
Cp2Zr(H)C1 o alkenes an d alkynesS2 Th e reaction pro-
ceeds well for terminal alkenes and uses stoichiometric
am oun ts of catalyst. In addition , Sat o and co-workers3
have effected th e reduction of alkenes and alkynes with
LiA1H4 in th e presence of transitio n-me tal halides. Al-
thou gh one might assume tha t hydrometalation products
are produced as intermediates, deuterolysis of the reaction
mixture shows that only TiC1, and TiC1, catalysts produce
a deuterium-incorporated product.
Some time ago we reported the direct synthesis of
NaAlH, (eq 3) an d a co nvenie nt synth esis for LiA1H4 (eq
4).4 It is clear th a t these complex metal hydrides, espe-
140 “C
N a
+
A1
4-
2Hz aAlH,
(3)
EbO
NaAlH, LiCl LiA1H4 NaC l (4)
cially NaAlH,, should be a most inexpensive source of
soluble metal-hydrogen compound . In addition, if all four
hydrogens can be mad e to rea ct w ith olefin, NaA1H4 should
inde ed represe nt th e least expensive source of soluble or
insoluble metal hydride available for hydrometalation
reactions (eq
5 ) .
It has been known for some time that
NaAlH4 f 4RHC=CH2
-
aA1(CH2CHzR)4 ( 5 )
NaAlR, compounds react m uch like R3A1 compounds and
are easily functionalized. However, it should be noted that
the ease of functionalization for the alkyl groups is best
for the first group an d progressively more difficult for the
remaining ones.
Bis(dialky1amino)alanes are produced in nearly quan -
titativ e yield and in a high state
of
purity by th e reaction
(1)
H. C. Brown, “Hydroboration”, W.
A.
Benjamin,
New
York, 1962.
(2 ) D.
W. Ha r t a nd
J.
Schwartz,
J .
A m .
C h e m . SOC. 6,
8115 (1974).
(3)
F.
Sato,
S.
Sa to , a nd
M .
Sa to ,
J . O r g a n o m e t . C h e m . , 131,
C26
(1977);
ibid.,
122, C25 (1976);
F.
Sato. S. Sato,
H.
Kodema, and M. Sa to ,
ibid.
142, 71 (1977).
(4)
E.
C. Ashby,
G.
Brendel , and
H.
E.
Re dma n,
Inorg.
Chem. ,
2 , 4 9 9
(1963).
(5)
E.
C. Ashby and
S.
Noding.
T e t r a h e d r o n Le t t . ,
4579 (1977).
Ashby and Noding
of Al, Hz , and Rz NH compounds a nd a re also readily
available, inexpensive, and reactive metal hydrides. We
reported earlier th at bis(dialky1amino)alanes add to olefins
and alkynes in the presence of catalytic amounts of
Cp,TiCl, in benzene solution in high yield (eq
6).
We have
CpzTiCIz
HAl(NR2)Z
+
RHC=CHz CHZCH2Al(NR2)2
(6)
just reported the details of HA1(NRzl2 ydroalumination
of alkenes6 and now we wish to repo rt our latest results
in hydroalumination of alkenes and alkynes with NaAlH4,
LiAlH,, and some of their derivatives.
Experimental Section
Apparatus. All reactions were performed under nitrogen or
argon at the b ench by using Schlenk- tube techniques or in a
glovebox equip ped with a recirculating system to remove oxygen
and solvent vapors.’ Calibrated syringes equipped with stain-
less-steel needles were used to transf er reage nts. All glassware
an d syringes were heated in an oven an d cooled under a flow of
nitrogen or argon. All inorganic and organic compounds, including
internal standa rds for GLC, were prepared by weighing the reagent
in a tared volumetric flask and diluting with the appropriate
solvent.
Proton NM R spectra were obtained by using a Varian Model
A-60 60-MHz spectrometer or a
JEOL
Model PFT-100 100-MHz
Fourier transform spectrom eter. All chemical shift values are
expressed in part s per million
6
values) relative
to
Me,Si a s the
internal standard . All mass spectra were obtained by using a
Hitachi RMU -7 mass spectrometer. GLC analyses were obtained
by using an F&M Model 720 gas chromatograph.
I R
spectra were
obtained by using a Perkin- Elmer Model 621 IR spectrometer.
High-pressure reactions were carried o ut by using an au toclave
rated to 15000 psi obtained from th e Superp ressure Division of
the American Ins tru ment Co.
Analytical Methods. Gas analyses were carried ou t by hy-
drolyzing samples w ith 0.1 M HC1 on a s tand ard vacuum l ine
equipped with a Toepler pump.‘ Aluminum was determined by
adding excess standard EDT A solution to hydrolyzed samples
and then back-titrating with standa rd zinc acetate solution at pH
4 with dithizone as an indicator. Amines were analyzed by in-
jecting hydrolyzed samples with an internal standard on the gas
chromatograph. Carbon and hydrogen analyses were carried out
by Atlantic Microlab, Inc.
All produ cts arising from the q uenching of reactions
of
hydrides
and alkynes with HzO,
DzO, Iz,
or
COz
were identified by GLC
and compared to authentic samples obtained commercially or
synthesized by known methods. All NM R spec tra were obtained
in CDC13 or b enzene- d,.
Lithiu m and sodium trimethylaluminohydrides were prepared
by the equimolar addition of a benzene, diethyl ether, or TH F
solution of trimethylaluminum (obtained from Ethyl Corp. and
distilled under v acuum in a drybox) to a lithium or sodium hydride
slurry in the app ropriate solvent. Th e addition was carried out
in a one-necked roun d-bo ttome d Cask equippe d with a magnetic
stirring bar a nd a pressure-equalizing additio n funnel while being
cooled with an ice-water bath . Th e addition funnel was fitted
with a rubber ser um cap which was attached to an argon-filled
manifold connected to a mineral oil filled bubbler by a syringe
needle. After th e addition and stirring (usually 10 min) the
reaction mix ture became a clear, pale brown solution which was
analyzed for aluminum by EDT A titration an d for lithium and
sodium by flame-ionization ph otometr y.
Sodium
bis(2-methoxyethoxy)aluminohydride
Vitride
T)
was
obtained as a 70% toluene solution from Matheson Coleman and
Bell.
Lithium and sodium bis(diethy1amino)- and bis(diisopropy1-
amin0)aluminohydride w ere prepa red by adding, with stirring at
0
“C, a stoichiometric amou nt of diethylamine or diisopropylamine
to a THF solution of lithium or sodium aluminum hydride.
(6)
E.
C. Ashby and
S.
Noding,
J . Org. Chem. .
44,
4364 (1979).
(7)
F.
W. Walker and
E.
C. Ashby,
J . C h e m . Educ . .
45, 654 (1968).
8/19/2019 JOC 1980,45,1035
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H y d r o m e t a l a t i o n
Additionally these compounds were prepared by adding a stoi-
chiometric a mo unt of bis(diethy1amino)- or bis(diisopropy1-
amino)alane to activated lithium or sodium hydride. Th e clear,
pale yellow-brown T H F solutions were analyzed for l i thiu m,
aluminum, and hydrogen by the standard m ethods described. If
benzene solutions were desired, the T H F was removed under
vacuum an d replaced by freshly distilled benzene. Th is procedure
was repeated three times. Th e amount of T H F which remained
a c co rd ing to G LC w a s 55 % .
Act ivated I i H was prepared by the hydrogenat ion of te r t -
butylli thium using Me,Si as the internal standard.
Materials. Solvents. Fisher reagent-grade benzene and
hexane were stirred over concentrated H zS04, washed with
NaZCO3 nd then disti lled water, dried over anhydrous MgS 04,
and th en disti l led from NaA1H4 under nitrogen. Th e catalyst
CpzT iClz was obtained from Alfa Inorganics.
Alkenes. 1-Oc tene (b p 122-123 C), 1-methyl-1-cyclohexene
(bp 110-111 C), styrene (bp 145-146 C), cis-2-hexene (bp 6 7 4 8
C), trans-2-hexene (bp 107-103 C), 2-ethyl-1-hexene (bp 11S 120
C) , cyclohexene (b p 82-83 C), 3,3-dim ethyl-l-buten e (neo-
hexen e) (b p 40-41
C),
and methylenecyclohexane (bp 102-103
C) were obtained from Chemical Samples Co. or Aldrich
Chem ical Co. and disti l led and stored over 4A molecular sieves.
Alkynes. 2-Hexyne (b p 83-84 C), 4-octyne (bp 132-133 C),
1-phenyl-1-propyne (b p 185-186 C), 1-octyne (bp 124-125 C),
and phenyle thyne (b p 170 C a t 19 mm ) were obta ined from
Chemical Sam ples Co. or Aldrich C hemical Co. and distilled a nd
stored over 4A molecular sieves.
1-(Trimethylsily1)-1-octyne as prepare d as discussed in ref
6. Anal. Calcd for Cl1Hs#i: C, 72.44; H , 12.16. Found: C, 72.23;
H , 12.13.
Preparation of Com plex Aluminohydrides. LiA1H4 (Alfa
Inorganics) solutions in T H F or diethyl ether and NaAlH4 (Alfa
Inorganics) solutions in T H F were prepa red by refluxing LiAlH,
an d NaAlH, in the app ropr iate solvent for at least 24 h followed
by fil tration through a frit ted-glass fil ter funnel in the drybox.
The resulting clear solutions were standardized for aluminum
conten t by EDT A and for hydrogen by stan dard vaccum-l ine
techniques.* Activated LiH was prepar ed by the hydrogenation
of tert-butylli thium or n-butylli thiu m a t 4000 psi of hydrogen
for 12 h a t rm m tem perature in hexane. Th e resulting L iH slurry
was removed via syringe under an argon atmosp here or in a
drybox.
Sodium hydride
as
a 50% oil dispersion was obtained from Alfa
Inorganics. Th e oil was removed by repeated washing and de-
cantation using freshly disti l led hexane.
General Reactions of Alkenes and Alkynes w ith Complex
Metal H ydrides. A 10
18
mm test tube with a Teflon-coated
magn etic stirring bar w as flamed an d cooled under a flow of argon
or nitrogen.
A
saturated solution of CpzTiClz n TH F (0.125 M )
was prepared ( th e solutions had to be made fresh each day). One
or two milliliters of th e Cp,TiCl,-THF solution was introd uced
into the vessel and the n th e alkene or alkyne added. Immediately
afte r additi on, a violet color developed but diminished in intensity
with t ime. The n the reac t ion mixture was st ir red a t room tem-
peratur e or at higher temperatures, depending upon the reactants,
for up to 40 h in some cases. In general, the reactio ns involving
internal alkynes were complete in 20 min. Th e reactions were
quenched by various ineans (see General Quenching Techniques
section) an d worked up by the regular m ethod (additio n of water,
extraction with dieth yl ether or hexane, and drying over MgS0,).
Most products were separated by GLC using a 6 ft, 10 Apiezon
L 60-80 column with a helium flow rate of 45 mL /mi n: 1-octene
(110 C, oven tem pera ture) , 1-methyl-1-cyclohexene
(50
C),
2-ethyl-1-hexene
(50
C), and cyclohexene (50 C).
A
20-ft, 10%
T C E P column with a he l ium f low ra te of 45 mL/min was used
for I- hexen e, cgs-2-hexene, rans-2-hexene, neohexene (45 C, flow
ra te 25 mL/ni in) , 2-hexyne (70 C), 1-octyne
(70
C), 1-( t r i -
methylsily1)-1-octyne (100 C), an d diphenyl-1-propyne (125 C).
A 10-ft,
5
Carbowax 20M column was used for diphenylethyne
(200 C, flow rate 60 mL/m in). Th e yield was calculated by using
a su it a bl e hydroc arbon in te rna l s t a nda rd for ea ch c ase ( I Z - C ~ ~ H ~ ,
n-C14H30,or T Z - C I ~ H ~ ~ ) ,nd the products were identified by
J . Org. Chem., Val. 45, N o. 6, 1980 1037
comparing t he retention times of authentic sam ples with those
of the produc ts under similar conditions and /or by coinjection
of products and authentic samples obtained commercially or
synthesized by known methods.
General Quenching Techniques. Quenching with HzO.
After the desired reaction tim e for the catalytic hydrom etalation
reaction described above, th e reaction was quen ched with wa ter
or a sa tu ra ted so lu t ion of ammonium chlor ide to produce the
proton ated species. Th e am oun ts of recovered startin g mate rial
and products were determined by the m ethods described above.
The same procedure used for
quenching with HzO was followed. Th e amou nts of recovered
starting material and products were determined by the GLC
methods and conditions described above. Each produc t was
collected from the gas chromatograph and submitted for mass
spectral analysis. Th e corrected percent of deuterium incorpo-
ration for the product was calculated by comparing the mass
spectrum of the proto nated species with the m ass spectrum of
the de uterated species and by sub tracting the con tributions of
naturally occurring isotopic components from each molecular ion
peak. Thi s procedure was followed for all unsaturated substrates
observed und er these reaction conditions.
The regioselectivity of reactions was monitored by the use
of
NM R for 1-phenyl-1-propyne and
1-(trimethylsily1)-1-octyne
fter
the produc ts were isolated by preparative GLC. For l-pheny l-
1-propyne, 95% of the m ajor product, cis-1-phenyl-1-propene,
was deuterated, with 90% of the deu terium located on C -1 as
indicated by a quar tet of triplets at 5.78 ppm J = 7 Hz, 1 H )
and a double t a t 1 .89 ppm ( J
=
7 Hz, 3 H). The o ther i somer
(deuterium located on C -2) showed a singlet at 1.89 ppm and a
multiplet a t 6.36-6.54 ppm. No trans-1-p henyl-1-p ropene was
observed. However the two major products, 1-phenylprope ne
(15%)
and 3-phenyl-1-propene
( 1 5 )
contained 55 and
85
deuterium, respectively. Th e deuterium in the 3-phenyl-1-propene
was located on C- 3 as indicated by a multiplet at 4.4-5.8 pp m
( 3 H ) .
T he products from t he reduction of I- trimethylsily1)-1-octyne
w ere p re p ar ed i n d e ~ e n d e n t l y . ~is-1-(Trimethylsily1)-1-octene
was prepared by the hyd rogenation of 1-(trimethylsily1)-1-octyne
with 5 Pd /C in 95% ethanol and the reaction monitored until
the desired am oun t of hydrogen was absorbed. Th e cis isomer
was collected an d purified via GLC under the aforem entioned
conditions. Th e tran s isomer was also detected by GLC . Th e
cis isomer, when coinjected under GLC conditions with the
product of the hy droalumination reaction, showed a trace char-
acteristic of only the cis isomer. T he tran s isomer obtained from
the hyd rogenation reaction had a reten tion time identical with
th at of the minor product from the hydroalumination reaction.
Th e following data were obtained for cis-1- trimethylsily1)-1-
octene: IR (neat,
film
2960
(s),
2940
(s),
2860 (m ), 1600 (m). 1470
(m ), 1260 [for octene: IR (neat, fi lm) 2960 (s) , 2860 (m ), 1600
(m ), 1470 (m), 1260 (s), 850 (br,
s)];
NMR (CCl,, Me&) 6 0.14
s,
9 H), 0.74-2.66 (m, 11H), 2.15 (9,
2 H ,
J
= 8
Hz), 5.49 (d, 1
H , J
=
13.0 Hz) , 6.33 (d t, 1 H ,
J
= 14 and 7 Hz); mass spectrum,
m / e
(relative intensity) 184 (M+,2), 170 (13), 169 (70), 141 (4) ,
125 (4), 114 (23), 109 (13), 99 (2 6), 85 (15), 73 ( loo ) , 67 (9), 59
(91),44 (21), 41 (14). An al. Calcd for CllHXS i: C, 71.65; H , 13.12.
Foun d: C, 71.54; H , 13.14.
Th e NMR spectrum of the trans isomer matched t he spectrum
found in the 1iterature:'O NM R (CC14, Me&) 6 0.16
(s,
9 H ) ,
0.6-1.6 (m, 11H ) , 2.1 ( m , 2 H) , 5.6 (d , 1 H ,
J =
18 Hz), 6.0 (d t ,
1
H, J
= 18
and 6 Hz) .
Quenching with Is.
A
known concentration of iodine in
benzene was prepared, an d a stoichiometric amou nt was added
to the catalytic hydroalumination reaction after th e desired time.
Th is mixture was the n allowed to stir at room tem perature for
1h. Afterward, water was added followed by a satur ated sodium
thiosulfate solution. T he organic layer was the n separated an d
dried over sodium sulfate and analyzed by GLC or NMR. Thi s
proced ure was followed for 2-hexyne. Th e iodoalken es were
identified by NM R in the following way. When th e catalytic
hydro alum ination reaction of 2-hexyne was quenched with
DzO,
Quenching with D20.
(8) D. F. Shriver , The Manipula t ion
of Air
Sensit ive Compo unds ,
McGraw-Hill,
New
York,
1969.
(9)
R.
A . Walker and
R.
A . Hickner ,
J .
Am .
Chem.
SOC.
0, 5298
(10) F.
A.
Carey and J. R. Toler ,
J . Org.
Chem., 41, 1966 (1976).
(1958).
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1038 J .
Org. Chem.,
Vol. 45, No. 6 , 1980
the produc ts were the cis alkenes which were confirmed by co-
injection of authentic samples in the gas chromatograph. From
Zweifel's work, it is known that hydroaluminated com pounds
quen ched with iodine main tain their regiochemistry. Ther efore,
the iodoalkene obtained from th e quenching with I of th e reaction
involving 2-hexyne was 2- and 3-iodo-cis-2-hexene. N M R was
used t o distinguish between th e two isomers:
Ashby
a n d N o d i ng
S c h e m e I.
Proposed M echanism of Hydrom eta la t ion o f
Olefins by LiAIH, in the Presence of Cp,TiC12
L I A I H ~+ C p 2 T 1 C 1 2 C P ~ T I ( H ) C
+
L I A I H ~ C I ( 8 )
\
/ I
/c=c
H
BU \Me
methyl s ing le t a t 2 . 3 6 ppm
I \
c=c / H
BU
\ M e
m ethyl doublet at 1 . 6 3 p p m ,
J =
7 Hz
Anal. Calcd for C6Hl1I: C, 34.30; H , 5.28. Found: C, 34.51;
H. 5.25.
R e s u l t s
and
Discus s ion
Sat o and co-workers3 reported th at ti ta nium te tra-
chloride and zirconium tetrachloride catalyze the addition
of LiA1H4 to olefinic double bonds to produce the corre-
sponding lithium organoaluminate. They reported, for
example, that 1-hexene produced 99% n-hexane when
allowed to react with LiA1H4 n T H F at room temperature
for 30 min in t he presence of
2
mol
7'0
of T iC14 followed
by hydrolysis (eq
7).
However, the reactions were not
TiCI,
H 2 0
C4H9CH=CHz LiAlH,
-
6H14
( 7 )
quenched with
DzO
which in effect monitors the produc-
tion of the hydrometalated intermed iate. When these
reactions were quench ed with bromine, only 70 of the
corresponding bromide was obtained in most cases.
Therefore, the prod uct obtained from the quenching of the
reacti on with Hz O could be misleading in term s of the
formation of the intermed iate hydrom etalated product.
We have recently reported th at th e reaction of LiAlH4 with
olefins in the presence of NiClz results in th e formation
of the reduction product in quantitative yield in almost
every case; however, addition of
DzO
to the reaction
mixtu re resulted resulted in very low deute rium incorpo-
ration in the product.12
Since previous work h as no t definitely shown the pres-
ence of a hyd rometalation intermediate in the reduction
of olefins with complex metal hydrides, we investigated
the hydrometalation reaction of unsatu rated molecules in
more detail, particularly looking for a different hydride
and/or catalyst that would give a higher percentage of
hydrometalated intermediate as evidenced by deuterium
incorporation in the product. Lithium and sodium alu-
minum hydride, li thium and sodium trimethylalumino-
hydride, li thium and sodium
bis(dialky1amino)alumino-
hydride, a nd Vitride [sodium bis(2-methoxyethoxy)-
aluminohydride] were allowed t o react with a series of
alkenes and alkynes in a 1:l ratio in th e presence of 5 mol
of CpzTiC lz in TH F. Th e results are presented in
Tables
I
and
11.
Titanocene dichloride was chosen as the
catalyst because it worked
so
well in the previously studied
hydrometalation reaction with
bis(dialkylamino)alanes,6
an d since th en Sato3 has investigated other catalysts.
T he results
of
Tables
I
and I1 show that all of the hy-
drid es behaved similarly. Wh en allowed to react with
termina l alkenes, the reactions were over in 10 min a t room
11) G.
Zweifel
and C. C.
Whitney,
J . A m . Chem.SOC.,
9, 2753 (1967);
G . Zweifel and
R.
B. Steel, ibid.. 89, 2754 (1967): G. Zweifel. J.
T.
Snow.
a n d
C.
C. Whitney, ibid.,
90,
7139 (1968).
( 12 )
E.
C.
Ashby and
J. J.
Lin, Tetrahedron Lett., 51, 4481 (1977) .
L
R C H ~ C H ~ A I H ~ L IC P ~ T I I O )
'H
R C H ~ C H ~ A I H ~ L I D 2 O --.- RCH2CH2D 11)
temperature. After quenching with
DzO,
each product was
analyzed via mass spectrom etry and the m ass spectrum
of the product compared to the mass spectrum of the
deuterated species obtained from quenching the corre-
sponding Grignard reagent with
DzO.
The percent deu-
terium incorporation in the product was considered an
indication of the intermediate formation of hydrometalated
species. All ter min al alkenes, i.e., 1-octen e, 1-hexe ne,
styrene, methylenecyclohexane, and 2-ethyl-1-hexene
(except neohexene), provided high yields of deuterium
incorporation pro duct (95-100% ). A mechanism for this
reaction is proposed in Schem e
I.
Th e low yield of deu -
terium incorporation product
( 5 5 )
in the case of neo-
hexene is undoubtedly due to the steric influence of a
tert-buty l group attach ed to the C=C double bond
(I) .
c1
\
/ c p
\CP
CH31
,'
JH3 ,
i \ b / H
C H 3
I
Another observation in this study which parallels the
bis(dialky1amino)alane alumination reaction is that the
overall yield of the alkane decreased in the following
manner: octane (98% ) hexane (99% ) > ethylbenzene
(85 )
> methylcyclohexane
(70%)=
2-ethylhexane
(70%)
>
neohexane (60%).
The internal alkenes (cis-2-octene, cis-2-hexene, and
trans-2-hexene) were no t
as
readily hydrometalated
as
he
terminal alkenes. In all cases only low yields ( 1 4 % ) of
the corresponding alkanes were observed. Th e reactions
were carried out as described for the terminal alkenes
except the reaction time was increased
to
36 h.
In
addition,
the reaction tem per atu re was increased to 60 OC, bu t to
no avail. Sato 3 found th at in terna l alkenes could be hy-
droaluminated if the reactions were carried out for 120 h
a t 60 C in th e presence of TiC14 or ZrC1,. We were hoping
that CpzTiClzwould allow th e reaction to ta ke place under
milder conditions, but unfortunately this did not happen.
For term inal alkynes (1-octyne and phenylethyne ), re-
sults similar to those observed for the hydroalumination
reactions with
[
(i-Pr)zN]zAIHwere realized, namely, de-
protonation
of
the acetylenic proton followed by addition
to the unsaturate d linkage. Only small
amounts
of alkenes
(3-1070)
nd alkanes (4-13
7 0)
were observed. T he starting
alkyne showed a large amount (85 )
of
deuterium in-
corporation a t the terminal carbon atom when th e reaction
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1040 J . Org.
Chem. , Vol .
45, No ,
1980
Ashby and Noding
Scheme I
D / H
P h
'Me
b
p -c
A '
D 2 @
B
lcjc
\ / D
,c=c
P h 'Me
ti
B'
Table 11. Reac t ions of Selected Alkenes and Alkynes
wi th Com plex
(Dialky1amino)aluminohydrides
n t h e
Presence of 5 mol Cp.TiCl ,=
D
com plex a lkene o r y ie ld , incorp ,
a lum inum hydr ide a lkyne p roduc t
L i A I H , ( N E t , ) , 1 - o c t e n e o c t a n e 9 6 9 8
1 - o c t y n e 1 - o c t e n e 10
oc tane 11
4 - o c t y n e 4 - o c t e n e 9 5 9 7
c i s -2 -oc tene oc tane 5
LiAlH,(
N-i -Pr? ') : 1 -oc tene oc tane 92 96
1 - o c t y n e 1 - o c t e n e 1 2
o c t a n e 11
4 - o c t y n e 4 - o c t en e 9 7 9 7
c i s -2 -oc tene oc tane 2
N a A I H , ( N E t , ) , 1 - o c t e n e o c t a n e 15
1 - o c t y n e 1 - o c t e n e
15
o c t a n e 1 3
4 - o c t y n e 4 - o c t en e 9 3
c i s . 2 -oc tyne
octane t race
NaAlH, (N- i -P r , ) , 1 -oc tene oc tane 95 96
1 - o c t y n e 1 - o c t e n e
10
o c t a n e 1 3
4 - o c ty n e 4 - o c te n e 9 1 9 3
c i s -2 -oc tene oc tane 2
he reac t ions were ca rr i ed ou t in THF a t room tem -
pera tu re f o r 2 h in a
1
1 m ola r ra t io
of
com plex a lum ino-
hydr ide to a lkene or
a lkyne .
pheny l ring. Also, th e bulky cyclopentadie nyl groups on
the tita nium can ad apt a staggered arrangement with the
phenyl group if the tit ani um is closer to the number one
carbo n, whereas this kind of arrang eme nt would be more
difficult and would involve the methyl group if the ar-
rangem ent were on the opposite side. Therefore, the least
hindered position would be th e one where the titanium
would be located closer to the number one rather tha n the
number two carbon. Once A and B form , hydrogen m i-
gration can take place, resulting in the formation of an
1VI
CHZ
M
H
H \ I
i
i - C - C = C H z
P
h H t '
intermediate with C being the most stable resonance
contributor. Interm ediate C can then undergo the
transmetalation step (Scheme
I)
with [(i-Pr),NI2A1H fol-
lowing the @-hydride dd ition
to
yield
D
on quenching with
D 2 0
or
abstract hydrogen from the solvent to form
E.
Once
E,
a terminal alkene, is formed,
it
may also undergo
catalytic hydrometalation to yield, after similar steps, the
protonated or de uterated 1-phenylpropane. I n all cases,
it is probably m ost likely t ha t the protonated products
result from a homolytic cleavage
of
the intermediate ti-
tanium species before the transmetalation step takes place.
Therefore, the hydroalumination of internal alkynes shows
great promise for the formatio n of vinylaluminates which
are
known to
undergo further reaction with
C 0 2 ,
yanogen,
and halogens to form a,@-unsaturated arboxylic acids,
nitriles, and vinyl halides.ll
As noted earlier, the terminal oc tyne reactions were no t
clean. Therefore,
1- trimethylsily1)-1-octyne
as prepared
by the reaction of 1-octynyllithium with chlorotri-
methylsilane (eq
13).
Whe n this alkyne was allowed to
C6HI3C=CH n-Bu Li 6HI3C=CLi
e3SiC1
-CIHlO
LiCl
C6Hl3C=CSiMe3 (13)
react under the conditions state d in Tables
I
and 11, the
reaction was slow even at 60 C. Th e best results obtained
showed a 35% yield of cis-1-(trimethybily1)-1-octene
65%
deuter ium incorporat ion) and a
15%
yield
of
1-( t r i -
methylsily1)hexane
(20
deuterium incorporation). Pre-
sumably because of the bulkiness of the trimethylsilyl
group, some homolysis of the interm ediat e alkenyl tita -
nium compound takes place, resulting in hydrogen ab-
stractio n from solvent, causing poor de uterium incorpo-
ration of the product when
D 2 0
s added (eq
14 .
In conclusion, hydroalumination reactions using complex
aluminohydrides work extremely well for terminal alkenes
and internal alkynes but do not work well for internal
alkenes and terminal alkynes. An interesting observation
which is a result of this work is that t,he regiochemistry
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J .
Org.
Chem. 1980,45, 1041-1044
1041
/ H
S H
C
p2T
-
C
H
/ c = ,
- C=C
-
/’ ‘SiMe3
\
H13C6
S i +le3 Hx
14)
involved for both the bis(dialkylamino)alane4 and the
alu min ate reactions is approximately the same. Th is in-
dicates th at t he regiospecific step m ust be th e sam e for
both reactions. We suggest th at the complexation of the
intermediate titanium hydrido compound
or
the formation
of the alkyltitanium compound is the rate-determining
step, after which the subsequent steps of Scheme
I
follow.
Acknowledgment. We are indebted to the Nat ional
Science Foundation (Grant No.
MPS 7504127) ,
Union
Ca mp Corp., and the Alcoa Foundation for financial sup-
\
/ H
/ c = c
H
H1326 ‘SiMe3
port of this work.
Regis t ry
No.
I-Octen e, 111-66-0; octane, 111-65-9; 1-hexene,
592-41-6; hexane, 110-54-3; styren e, 100-4 2-5; thylb enzen e, 100-41-4;
methylenecyclohexane, 1192-37-6;methylcyclohexane, 108-87-2;2-
ethyl-1-hexene, 1632-16-2;2-ethylhexane, 589-81-1;neohexene, 558-
37-2; 2,2-dimethylbutane, 75-83-2; cis-2-hexene, 7688-21-3; ran s-2 -
hexene, 4050-45-7;cyclohexene, 110-83-8;cyclohexane, 110-82-7;1-
methyl-I-cyclohexene, 591-49-1; I-octyne , 629-05-0; phenylethyne,
536-74-3; 4-octyne, 1942-45-6; cis-4-octene, 7642-15-1; 2-hexyne,
764-35-2; 1-phenyl-1-propy ne,673-32-5; 1-phenylpropane , 103-65-1;
1-deuterio-cis-1-phenyl-1-propene,
2087-52-6; 2-deuterio-cis-l-
phenyl-1-propene, 72090-05-2;3-phenyl-l-propene, 300-57-2; -( tri -
methylsily1)-1-octyne, 5719-55-8;
1-(trimethylsilyl)octane,
429-76-3;
cis-1- trimethylsily1)-1-octene,
7365-48-7; I-iodoo ctane, 629-27-6;
2-iodo-cis-2-hexene, 72087-50-4; 3-iodo-cis-Z-hexene, 72087-51-5;
Cp2T iClz, 1271-19-8; LiAIH,, 16853-85-3; NaAlH,, 13770-96-2;
LiAlMe3H,62816-22-2; NaA1Me3H, 66484-08-0; NaA I(OC H2C H20 C-
H3)2Hz, 22722-98-1; cis-2-octe ne, 7642-04 -8; LiA 1HZ (NE tz),, 2749-
21-4; LiA1Hz(N-i-Prz)2, 2749-22-5; NaA1H2(NEt&, 62259-84-1;
NaA1 HZ(N-i-Pr2),, 2749-23-6.
Hydrometalation.
6.
Evaluation of Lithium Hydride as a Reducing Agent
and Hydrom etalation Agent
Eugene C. Ashby* and Stephen A. Noding
School
of
Chemistry, Georgia Institute
of
Technology, Atlan ta, Georgia
30332
Received June
11
1979
Th e reactions of activa ted lithium hyd ride with carbonyl compounds (aldehydes, ketones, esters, and enones),
alkenes, and alkynes in the presence of transition -metal halides were investigated. Significant reaction involving
the above substrates w as accomplished only when a n equimolar amo unt of VCI, was used in con junction with
th e lithium hydrid e. Aldehydes were reduced to their corresponding alcohols in high yield (95-97%), and ester s
were also reduced to their corre spond ing alcohols in high yield (93-95%) with a small amou nt (5-7%) of th e
corresponding aldehyde (of the carboxylic acid portion) formed. Reductions of cyclohexanones were highly
stereoselective. In this connection, 4-tert-butylcyclohexanoneas reduced to the axial alcohol in 96% yield and
with 82% stereoselectivity. T he only enone
to
be reduced w as cinnamaldehyd e, which gave the 1,2-reduction
product
in
90% yield. Termin al olefins were reduced to alkanes whereas interna l olefins were completely unreactive.
In thi s connection, 1-octene was reduced to oc tane in 95% yield. In this example,
30%
deuterium incorporation
of the product was observed when hydrolysis was effected with DzO. Such a result indicates formation of th e
intermed iate octyllithium in 30% yield. Since alkynes and inte rnal olefins are not reduced a t all with LiH and
VCl, and te rminal olefins are, reduction of enynes and dienes with LiH a nd VCl, could serve as a selective reduction
metho d for the red uction of a terminal double bond in t he presence of a triple bond a nd also for the selective
reduction of a terminal doub le bond in the presence of an internal double bond.
Lith ium, sodiu m, an d potassium hydrides have been
widely used as bases in sy ntheti c organic chemistry, but
have no t been used
as
reducing agents. Some time ago we
developed a simple metho d for the prep aratio n of a very
active form of lithium hydride.’ Th is method involves the
room-temperature hydrogenolysis of tert-butyllithium (eq
1).
Th e resulting white solid is easily filtered from th e
room
t e m p
t -C4HSLi
+ H2
i -C4HI0 LiH
(1)
reaction mix ture b ut is most conveniently used as a slurry
due to its high degree of pyroph oricity. Th is form of
lithium hydride app ears to be much more reactive tha n
the lithium hyd ride th at is commercially available and
which is prepared at about
450
“C. A preliminary ex-
periment? showed that benzophenone is reduced to
benzhydrol in
6%
yield when stirred w ith activated lithium
1)E. C. Ashby a nd
R.
D. Schwartz ,
Inorg.
Chem., 10, 355 (1971).
(2)
E. C. Ashby and
R.
Boone, unpublished results.
0022-3263/80/1945-1041 01.00/0
hydride for
2 h,
whereas no trace of benzhydrol was found
when commercial lithium hydride was used under th e same
conditions.
Recently Caubere and co-workers showed that the
reagent NaH-R ONa-M X, is capable of reducing
halide^,^^^
ketones, alkenes, and
alkyne^.^^^
Although these reactions
were not catalytic, they d id provide a
75-95
yield of the
corresponding alkane s, alcohols, alkanes, an d alkenes , re-
spectively.
In our continuing search for new stereoselective reducing
agents and new hydride systems to effect hydrometalation
of alkenes and alkynes, we investigated the reactions of
activated LiH with various carbonyl compounds, alkenes,
and alkynes in the absence and presence of catalysts. The
(3) G.
Guillaumet, L. Mordentia , and
P.
Caubere ,
J .
Organomet.
(4)
G.
Guillaumet, L. Morde nt ia, a nd
P.
Caubere ,
J .
Organomet.
( 5 )
J.
J.
Brume t ,
L.
Lordenti , B. Loubinaux, and P. Caubere ,
Tetra-
(6)
J. J.
Brume t a nd
P.
Caubere ,
Tetrahedron Lett., 3947 (1977).
Chem., 92,
43 (1975).
Chem. , 102, 353 (1977).
hedron Lett.,
1069
(1977).
980
Am erican Chem ica l S oc ie ty