T~~drnn Lenen, Vol. 35, No. 21, pp. 3513-3516,1994 Eisevier Science Ltd

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~oly~enum Oxadiene Catalysts fbr the Chemt3selective Hydrosilylation of a&Unsaturated Ketones and Aldehydes

Max-Phnck-Institut fttr Kohlenfors&ung, Kaiser-Wilhelm-Platz 1, D-45470 Mlllheim an da Rub, Gennaay

Ahstrad: Csrbonyl molybdeaum and hmgsten oxadiene cotnplexcs exhibit high cstalytic activity in hydmilylation of t#Iumma Wanes and aldehydr?s, even those gcwmctdtiy fixed in s-(mns- amngement, using PbSiwJ and Ph@$. In the case 64-pbenylbut-3~-Z-onc as s&@ate, variable chemwelecdvity is oLlaerva3 dqmding on the specific catalyst employed

Hydrosilylation of unsaturati carbony compounds provides access not only to saturated and unsaturated

silyl ethers. Hydrolysis of these primary products 2 and 3 may also lead to satum&d ca&onyi compotmds U and

sllylic alcohols 5, as depicted in Scheme 1. Platinum metals, specB&y rhodium and platinum complexes are

usually employed as catalystsl for ~ tllese Gactions.

Hexacatbonyl molybdenum has been reportedz to catalyze silane additions to unsaturated ketones almady

some years ago. Unfortunately, relatively large amounts of catalyst (4-9 mol-96) aud a large excess of silane

were needed to a&eve these results. In contrast, byte oxadiene camplexes, e.g. compounds 6a3, 7a4,

~8,exhibitan~~~activi~aswellasa~~ychanged cbemc&&vity in by~lation

reactions, even with only a slight excess of silane. These oxadiene compiexes am crystall& orange to bmwn

compou&easily o~~bydirectconq~~~oftheoxadienesand~~~kfops~paio&sof~.

Some important reactivity results are ~~inTable1.Inatypical~~~tfieu~~

carbonyl compound (3 mmol) is dissolved in a solution of the catalyst in benzene or toluene (5 ml) under argon.

The s&me is added via syringe and the clear yellow to brown mixture kept at reaction tempemture with &ring.

Then, after dilution with ether (5 ml), 5 96 aqueous hydrogen fluoride (5 ml) is added and the two phase

mixture SW for 3 h. Neutralisation, extraction with ether and conce&ration of the combined extracts &ords

3513

3514

the crude reaction product. It can further be purified by chromatography, but already tums out to be essentially

free of silicon compounds.~

Table 1. Hydrosilylation of a$-Unsaturated Ketones Catalyzed by Dicarbonylbis(oxadiene) Molybdenum and

Tungsten Comp1exes.a

/ d . .$ O\

M

-5-L ,co

co

i+ 0’ \ I co 0 -CO

9 sa(M=Mo) 7u(M=Mo) 8

eb(M-W Rx(M=W)

Entry Substrate Silaneb Catalyst (mot-%), Conversion PrcductG [Product Ratio] Conditions f%]

P

Et3SiH 6b (1), 48 h, 70°C 2 [S&34]

Ph2SIHg 6b (1), 16 h, 70°C 50 [65:35]

, 0 PhSIH3 Sa (1). 18 h, 2O“C 95 0 [55:45]

s PhSiH3 6b (l), 16 h, 7O“C 77d

IOa lob [62:38]

5 P hSiH3 7a (l), 3.5 h, 20°C

6

6X 0

11 PhSiH3 7b (1). 7 h, 20°C

7 Ph2SiH2 Sa (2). 10 h. 50°C [25:75]

8 0

7

Ph2SiH2 6b (1). 16 h, 70°C [21:79]

9 \ PhSiH3 Sa (l), 1 h.70°C [28:72] 10

Ph P hSiH3 6b (I), 12 h, 70°C ‘j ‘up + Jo” (23:77]

Ph Ph 11 100

14 P hSiH3 7a (1), 6 h, 50°C [60:40]

12 PhSiH3 8 (l), 3 h, 50°C 97 15 16 [45:55]

a additional Experimental Conditions: see text:

b the following amounts (mol-%) were used: Et3SiH (1 lo), Ph2SiHp (70) and PhSiHS (50);

’ products and product ratios as determined by NMR and GLC after hydrolytic workup:

d 1,4- and 1 p-reduction product obtained In a 3 : 2 ratio;

* 50 % combined yield of 12 and 13; several minor products are formed:

f 13 is formed as a mixture of diastereomers (3 : l), major isomer Is shown above.

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Several conclusions can be deduced from the results in Table 1. Firstly, with sterically hindered, less

reactive silanes like triethyl silane, only a stoichiomettic maction could be achieved (entry 1). Reactions with

diphenyl silane are again slower than those with plnmyl silane (entries 2, 4). Molybdenum complexes, e.g.

dicarbonylbis[(R)-(+)-pulegone]molybdemun 6n, am much more active catalysts than the corresponding,

isostructural tungsten.complexes (entries 3,4). Therefore, most of the results presented arise from the use of

molybdenum catalysts. Comparison of entries 1.2 and 3 shows that there is no sign&ant influence of the s&me

on product distribution, l&addition takes place in all cases, an observation quite unusual for catalytic

hydmsilylation.7 With the pinocarvone complexes 7, reactions proceed even faster, the tungsten complex again

being less active and selective (entries 5, 6). So far, the oxadiene substrates and the oxadiene ligands of the

catalyst have been chosen to be identical. Entries 7 to 12 with a diffemnt substrate show this selection not to be

essential, 4phenylbut-3-en-2-one (14) may serve as example. Most interesting in these cases, the product

mixtures obtained with diffmt oxadiene molybdenum carbonyl catalysts (entries 9, 11, 12) widely differ with

respect to product ratio (H/16), ranging from only 21% to about 60% of allylic alcohol 16 at almost complete

conversions, thus contrasting previous Rndingsz with Mo(CO)6, where the saturated ketone 15 has been

reported as exclusive product. Even ~6benxenetricarbonyl molybdenum shows a relatively low catalytic

activity in the above reaction, again leading to a diffemnt product ratio, i.e. a 1 : 1 mixture of 15 and 16.

Mechanistically, these results can be taken as evidence for at least one of the oxadiene ligatxls staying

coordinated to the metal centre during the catalytic cycle. As complexes 6 and 7 are enantiomerically pure, this

might allow for their use in asymmetric hydrosilylation. Experiments in order to explore this possibility am

underway. Loss of both oxadiene ligands, on the other hand, should lead to the formation of identical

catalytically active species even from different oxadiene complexes and themfore give rise to identical product

ratios, which is not observed. It yet remains unclear if the ligand stays in a ~‘&ordination mode which might

be essential in order to produce sufficient rigidity to allow asymmetric induction in this catalysis.

The terpene ketone R-(-)-cawone (17) with an oxadiene fragment fixed in s-trans geometry is another

interesting substrate in several respects. Its reaction in the presence of phenyl silane leads to the formation of a

mixture of diastemomers of 1,2- and 1,4addition products (Scheme 2). Especially the formation of the latter is

remarkable, as it has not been observed so far with group VI metal catalysts. A simultaneous interaction of the

metal with both carbonyl and alkene fragment, which is unlikely in s-zrans oxadienes, has pmviouslyZs been

assumed as being necessary for the hydmsilylation to pmceed in order to explain why s-rruns oxadienes could

not be 1.4hydrosilylated or -hydrogenated with those catalysts. Secondly, the carvone reaction shown hem is

specific for the oxadiene fisgrnenk no hydtosilylation takes place at the isopropenyl group. The overall

reactivity thus strongly parallels that observed with noble metal catalysts, e.g. of rhodium or palladium P

b- l.lmoC%6a.

, 0 0.5 eq. PhS& benzene 0

b

OH / 2.5%aq.HF +

A 12 h, 50°C A (3 : 5) A

17 18 (3 : 2) la (3 : 1)

convara&n: low, Yield: 85%

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With unsaturated aklehycles 20 or 22 as substrates, the reactions exclusively produce the comqonding

allylic alcohols 21 and 23 in high yieb (Scheme 3). This selectivity might be arising Finn the formation of an

in- ql- or q2-bound aldehyde complexes leading to an exclusive activation of the carbonyl fragment

of the oxadiene. Such ql- and q2-aldehyde complexes are known as stable compounds for a variety of metals1o

including tungsten, whereas only few iron complexes with q4-bound unsaturated aldehydes exist.

1.1 mol-%60, 0.5 eq. PhSIHg benzene

2.5%aq.HF b

20 (R - Me) 12 h, 70°C

21 (R = Me) Conversion: 95%, Yield: 92%

22 (R = Ph)

Scheme 3

29(R=Ph) Conversion: 99%, Yield: 90%

In conclusion, molybdenum oxadiene complexes are not only active catalysts for the hydrosilylation of

unsaturated carbonyl compounds, but allow a tuning of chemoselectivity by selection of the catalysts ligands.

This may lead to interesting preparative applications as well as to an improved understanding of the influences

arising from the ligand sphere in an active hydrosilylation catalyst.

Acknowledgement. ‘Ibis work has been generously supported by the Max-Planck-Gesellschaft and the Max-

Planck-Institut f[ir Kohlenforschung, Millheim an der Ruhr. Additional tinan@ support from the Fonds der

Chemischen Industrie is gratefully acknowledged.

References and Notes.

1.

2. 3. 4. 5.

6.

7,

8.

9.

10.

a) Maminiec, B. Comprehensive Handbook on Hydrosilylation; Peqamon Press: Oxford. 1992, pp. 32- 61; b) Marciniec, B., G&i&i, J. J. Organomet. Chem. 1993,446,15-2 1. Keinan, E., Perez, D. J. Org. Chem. 1987,57,2576-2580. Schmidt, Th., Krtlger. C., Betz, P. J. Organornet. Chem. 1991,402, 97-104. Schmidt, Th., Bienewald. F. GZ2’Fach.z. Lab. 1993.37.76 l-762. The complexes of pinocarvone and 5-methylhex-3-m-2-one am prepared similar to those of pulegone (as &scribed in Ref. 3) in comparable yields. When mono- or diphenyl silane are used, more than one of the hydrogen atums at silicon is active in the hydrosilylation reaction, permitting the use of substoichlometric amounts (50 and 70 mol-%, respectively) of these silanes. e.g.: Ojima, I.: The Hydrosilylation Reaction. In The Chemistry of Organic Silicon Compounds, Patai, S., Rappoport, Z. Eds.; John Wiley & Sons: Chichester, 1989; Part 2, pp. 1479-1526. a) Keinan, E. Pure Appl. Chem. 19%9,61,1737-1746, b) Sodeoka, M., Shibasaki. M. J. Org. Chem. 1985.50,1148-l 149. a) Ojima, I., Kogure, T. Organometallics 1982, 1, 1390-1399; b) Keinau, E.. Greenspoon, N. J. Am. Chem. Sot. 1986,108,7314-7325. e.g.: Honeychuck, R.V., Bonnesen, P.V., Famhi, J.. Hersh, W.H. J. Org. C’hem. 1987, 52, 5296-5298; Gladysz, J.A., Huang, Y.-H. J. Chem. Educat. 1988, 65, 298-303; Bullock, R-M., Rappoli, B.J. J. Am. Chem. Sot. 1991,113,1659-1669.

(Received in Germany 17 February 1994; accepted 25 March 1994)