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Th e azide radical yields nitrogen. Th e other inter-m e d i a t
e , C T ( N H ~ ) ~ ~ + ,s very labile and decomposest o give
Cr(1I) an d a m m ~ n i a . ~
The f ate of Cr(I1) is not know n because the photolyzedpale
green solution has not yet been identified. How -ever, we assume
that Cr(I1) is oxidized by air to binu-clear complexes. Such
reactions are typical for theoxidation of Cr(1I) by airin a cidic ~
o l u t i o n . ~
Strong support for the intermediate formation ofCr(J1) is given
by another experiment.If the redox
photolysis of Cr(NH3)5N32+n a slightly acidic mediumat 320 mp is
carried out in the presence of Co(NH&-HzO +, which is not
photosensitive under these condi-tions, extensive formatio n of Co
(II) does occur. Cr(I1)is kn ow n t o reduce C O ( N H ~ ) ~ H ~ O
~ + .o
The occurrence of a photoredox decompositionofCr(NH3)5N 32+ pon
i r radia t ion in the C T band maybe connected to the observation
that Cr(NH3)5C12+1and Cr (NH&Br*+l 2 show a large increase of
halideaquation upon irradiation in the CT band. Bothobservations
are consistent with a cage mechanismwhich was first proposed for
the photochemical redoxreactions of Co(II1) complexes.'3.'4 The
absence ofazide aquation for Cr(NH3):N3+ upon irradiation in
the CT region could be explained by the exceptionalshor t
lifetimeof the azide radical.1 5 , 1 6 After homolyticsplitting of
the Cr3+-N, bond, the azide radical mayreact fast enough to yield
nitrogen before a chargerecombinat ion Cr2+. .N3t t Cr3+N3-can tak
e place.
9) M . Ardon and R. A. Plane, J . A m e r. Chem. Soc. ,81, 3197
1959).10) A. Zwickel and H. Taube, ibid., 81, 1288 1959); J. P.
Candlin,
11) H . F. Wasgestian and H. L. Schlafer, 2 . Phys . Chem.
Frank-
1 2 ) P. Riccieri and H . L. Schlafer, Inorg. Chem.,9 727
1970).1 3 ) A. W. Adamson and A. H. Sporer, J . Amer. Ch em. So c .
, 80,
14) A. Vogler and A. W. Adamson, J . Phys . Chem.,74,67
1970).15) S. A. Penkett and A. W. Adamson, J . Amer. Chem.
SOC.,87,
16) A . Treinin and E. Hayo n,J. Ch em.Phps. , 50, 538
1969).
J. Halpern, and D . L. Trimm, ibid.,86, 1019 1964).
fur t am Main) , 62, 127 1968).
3865 1958).
2514 1965).
Arnd VoglerFachbere ich Cliemie , Univers i th Regensburg
8400 Reg en sb u rg , Ge rman yReceiced Ju ly 6 1971
Tris[3-
trifluoromethylhydroxymethylene)-d-camphorato]europium(III). A
Chiral ShiftReagent for Direct DeterminationofEnantiomeric
Compositions'
Sir
We wish to report an nmr method for direct deter-mination of
enantiomeric compositions (optical purities)
which we have applied successfully to several typesof compounds
including alcohols, ketones, esters,epoxides, and amines. This me
tho d involves use of anew chiral nmr shift
reagent,tris[3-(trifluoromethyl-hydroxymethylene)-d-camphorato]europiun~
iII) 1).Similar methods based on chemical-shift nonequivalenceof
enantiomers (in chiral solvents2 or in the presenceof a chiral
shift reagent,tris[3-
tert-butylhydroxymethy-lene)-d-caniphorato]europium(III)2)3) have
been re-
( I ) Supported by the Research Committee of the Graduate
Schoolof the University of Wisconsin,
2) W . H. Pirkle and S . D. Beare, J . A m e r . Ch em. Soc. 91,
51501969); W. H. Pirkle, R. L. Muntz, an dI . C. Paul,
ibid.,93,2817 1971).
(3) G. M . Whitesides and D. W. Lewis, ib id . ,92, 6979
1970).
b ) I
) AH
1 I I I I I I I I
120 110 1 9 8 0 7 0 6 5 0 4 0 3 0 2 0PPM
Figure I Spectra of 0.54 M 2-phenyl-2-butanol In CCla in a)
thepresence of 0.13 M tris dipivalomethanato)europium III) and
(b)0.42 M 1, and c) spectrum of 0.3 M
1,2-dimethyl-exo-2-norborna-no1 in the presence of 0.42 M 1.
porte d. However, these appear to beof limited ap-plicability.
Ma gnitud es of nonequivalence in chiralsolvents are small 20.04
ppm )2 which limits the use-fulness of this technique for
determining enantiomericcomp ositions. Large pseudocontact-shift
differencesfor enantiomeric amines are observed with2 . 3 How-ever,
with neutral compounds magnitudes of nonequiv-alence are generally
too small to be useful.On theother hand, with 1 we have observed
pseudocontactshift differences for enantiomeric alcohols of
>0.5ppm. Moreover, there is very little line broadeningand in
most cases we have achieved complete resolutionof signals for enan
tiotopic4 protons with a 60-MH zinstrument.
Compound1 was prepared by reactionof
3-trifluoro-methylhydroxymethylene-d-camphor(3) with europium-(111)
chloride in th e presence of base.5 Th e chelateis an amorphous
solid that softens at100 and is verysoluble in nonpolar solvents.
The nmr spectrum of1ranges from 3 to -1 ppm from TMS. Compound3 was
obtained by co nden satio n6 of d-cam phor withethyl
trifluoroacetate. Anal. Calcd for C12HljF302:C, 58.06; H, 6.09. Fo
un d: C, 58.17; H,6.0 9.
Parts a and b of Figurel 7 show spectra of carbontetrachloride
solutionsof dl-2-phenyl-2-butanol 4) in
4) M . Raban and I
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0 PPM
11
5 4 3 0 2 0I~ ~~
P P M6 0
Figure 2. Spectra of CC14 solut ions of 0.9 M 5 and 0.6 M 6 n
thepresence of 0.5 M 1 Expanded spectra at left are for the
down-field methyl resonances. Unassigned resonances are due to
1
the presence of tris dipivalomethanato)europium III)and in the
presence of 1. This comparison showsthat similar pseudocontact
shifts(As) are observedwith the two reagentsa8As illustrated by
spectrumb, in the presence of1 pseudocontact-shift differencesfor
enantiomers are observed. The enantiotopicamethyl singlets are
separated 0.29 ppm and th e P-methyltriplets are separated 0.22 ppm
which correspondsto -2J and gives rise to a quintuplet.A more
dramat icexample of nonequivalence is illustrated by the
lowerspectrum in Figure 1. This spectru m of partly active1
2-dimethyl-exo-2-norbornanolg inthe presence of 1shows large shift
differences for corresponding methylgrou ps of the enantiomers. It
is interesting to notethat nonequivalence is largest (>0.5 ppm)
for the leastshifted methyl group (presumably the 1-methyl).
Effects of 1 on the nmr spectra of some other typesof compounds
are summarized in TableI. This tableshows pseudocontact-shift
differences(AAs) for the
Table I. Pseudocontact -Shift Diffe rences forEnantiomers (AA8)
in the Presence of l
Compound Proton
2-Octanol a-CH3/l-CHa
1,2-Dimethyl-e/zdo-2-norbornanol lZCH31-Phen yle tha nol
CY-H
I-Phenylethyl acetate -C02CCH3
1-Methyl-2-norbornanone 1-CH33,3-Dimethyl-2-aminobutane
a-CH3cis-/3-Met hylstyrene oxide P-CHa
AA6,PPm
0 . 110.370 . 3 30 . 3 0
0.18
0 .170.280 .27
a Concentration of 1, 0.4 M 200 mg/0.6 ml of CC1,). Molarratio
of l/substrate, >0.6.
(8) For informa tion regarding pseudocontact shifts and nmr
shiftreagents see a) J . K. M . Sanders and D. H . Williams, J .
Amer. Chem.Soc., 93, 641 1971); (b) R. E. Rondeau and R. E.
Sievers, ibid., 93,1522 1971); and c) J . Briggs, G. H. Frost, F.
A. Hart, G. P. Moss,and M . L. Staniforth, Chem. Commun., 749
1970), and references inthese papers.
9) H. L. Goering, C . Brown, S. Chang, and J. V. Clevenger, J .
Org.Chem., 34, 624 1969).
indicated enantiotopic proto ns. Nonequivalence wasno t observed
with ethers. Magnitudes of pseudo-contact shifts (As) and of AAs
depend on the ratioof 1 to substrate. Con dition s have been
optimizedonly for 4. In this caseA s and AAs (for both
methylgroups) increase with the1/4 ratio until the latterreaches
-0.7 after which there is no chang e. Thissuggestssb tha t a t
ratios >0.7 (optimum conditions)essentially allof the substrate
is coordinated . In thisconnection it is significant that
nonequivalence wasnot observed for protons that are enantiotopic by
in-ternal comparison, e .g . , 2-propanol a nd dimethyl
sulf-oxide.
Nonequivalence of enantiomers is also observed withthe
praseodymium analog of1 and shift differencesare at least as large
as with1; however, resolution isgenerally poorer. In this case
induced shifts are inthe upfield d i r e ~ t i o n . ~ ~ * ~ ~
Th e use of1 for direct determination of
enantiomericcompositions is illustrated by Figure2 which
showsspectra of optically active methyl 2-methyl-2-phenyl-butanoate
(5) and 3-methyl-3-phenyl-2-pentanone 6 )in the presence of1. Both
c ompou nds were preparedfrom th e same sample of partially
resolved 2-methyl-2-phenylbutanoic acid (excessS isomer)'O and
havethe same optical purities. Fo r5, nonequivalence isobserved for
the 0-methyl R , 5.68 ppm; S, 5.56ppm) and 2-methyl protons R ,
2.98 ppm; S, 2.93ppm ). Similarly, for 6 nonequivalence is
observedfor the acyl-methyl R , 5.62 ppm; S , 5.49 ppm) and3-methyl
protons S, 3.72 ppm; R , 3.62 ppm ). Ex-pan de d sweep widths of
the downfield methyl resonancesare shown to the left of the
corresponding spectrum.'Peak areas of the expanded signals
correspond to o pticalpurities of 27.7% for5 and 27 .3x fo r 6 as
comparedto 25.8 for 5 and 25.4 for6 determined from rota -tions.l0
It is note wort hy that in the spectrum ofthe se nse2 of
nonequivalence is reversed for th e acyl-
methyl and 3-methyl singlets. This indicates th
atnon-equivalence results from intrinsicly different
magneticenvironments for coo rdina ted enantiomers. Differencesin
stability constants for complexes of enantiomersmay also con tribut
e to nonequivalence.
9a) NOTE ADDED N PROOF. We have also investigated the
europiumand praseodymium chelates derived fro m 3-heptaf
luoropropylhydroxy-methylene-d-camphor. These chiral chelates have
nmr shift propertiessimilar to the chelates derived from 3.
Nonequivalences for enantio-mers are of about the same magnitude
and resolution is similar forcorresponding chelates.
10) D. J. Cram and J. Allinger, J . Amer. Chem. S O C . 76,
45161954); D. J. Cram, A. Langemann, J. Allinger, and I
