Tetrahedron: Asymmtq Vol. 3. No. 6. Pp. 693-6941992 09574166/92 S5.OCb.W

Printed in Great Britain PergammlPre-3sw

OpticaUy The

Enantiosclective Iiydrosilylatiw Of Ketones.

Anna Iuliano and Piero Salvadori*

C.N.R. Centro Sludi iWmomoIec0le Stem ed otticameate Al&, Dipaftimento di Chimica e Chimica

Industxiale, via Riiento 35.56126 PEA, lTALY.

(Received 10 April 1992)

Abstract. In the asymmetric hydrosilylatioa of ketones employing lithium (lS,ZS)-l~-diphenyl-lo-

ethanediolate as chiral ligand of trimethoxysilane, alkylarylcarbinols were obtained with yield up to 95%

and e.e. ranging from 12 to 82%.

The reduction of prochiral ketones, by means chirally modified hydrides, constitutes an useful system for the

preparation of optically active alcohols’. Recently this reaction was carried out employing “chiral” silicon

hydrides, which am obtained by treating uimethoxysilane with an optically active lithium dialcoholatez: in

this way, a pentacoordinate hydrosilane was obtained, which acts at the same tim as Lewis acid, activating

the carbonyl group of the substrate toward the nucleophilic addition, and as nucleophile. traatbrring the

hydride ion on the activate carbonyl group3. In this tmmmnication we report the mults obtained in the

reduction of alkyarylketones, employing as chiral ligand of trimethoxysilane the lithium dialcoholate of

(lS,2S)-1,2diphenyl-1,2-ethanediol, 1, which is easily obtainable’ and has not been yet used in this reaction.

The results obtained’ (yield, e.e., absolute contIguration of the prevailing enantio~r) are summa&cd in the

Table. The chemical yields depend on the bulkyness of the carbonyl subs&ems: as a matter of fact when

the aryl fragment is the I-naphthyl group, which is crowded owing to the presence of the peri hydrogen near

to the site of reaction, lower yields of the corresponding alkylarylcarbinol am obtained. The lower yields are

attributable3 to the difficult coordination of the carbonyl oxygen to the silicon atom: if this coo&nation is

practically prevented, as in the case of highly crowded l-naphthyl-t-butylketone, the starting material is

quantitatively recovered. Very good yields am obtained by reacting phenyl and 2-naphthylalkylketones,

except in the case of phenyl-t-butyllcetone; this anomalous behaviour can be explained considering a different

reaction mechanism for this substrate. As far as the enantioselectivity is concerned, low chemical yields

correspond to low e.e. (12-2246): and it seems reasonable that a weak coordmadon of the carbonyl oxygen to

the silicon is also responsible of the low asymmetric induction from the chiral ligand 1. In the other cases

the e.e. of the product change depending on the nature of the alkyl substituent in the order t-Bu< Me< i-Pr,

the aryl tiagment being the same: this behaviour has been already observed in other kinds of enantioselective

reductions of alkylaryl ketone?. In all the considered cases, the carbinol having the S absolute configuration

was pmailingly obtained. Therefore, the use of the chill pentacoordinate hydrosilane obtained from 1 can

693

694 D. Pl~l et al.

constitute a practical and efficient route to obtain allcylarylcarbinols in very good yields and e.e. ranging

from 12 to 82%. In addition must be considered that the chiral ligand, although it is used in stoichiometric

amount, is recovered easily and almost quantitatively. Finally, the accessibility to optically active 1,2diols,

by means of the cis-dihydroxilation of pnxhiral olekrs by the Sharpless meth&, allows to study the

influence of their structure on the course of the reaction.

TABLE. Enantioselective hydtosilylation of akylarylketones by trimethoxysilane and 1.

0

K 1 , HSi(OMe)J

Ar R OT, 20h l Arx:

Ar

Phenyl

Phenyl

Phenvl

R

Methyl

LPropyl

t-Butvl

Yield %‘ e.e. %2 ax.’

93 28 S

95 42 S

59 12 S

2-Naphthyl

2-Naphthyl

2-Naphthyl

1 -Naphthyl

1-Nauhthyl

Methyl 93 65 S

i-Pmpyl >95 82 S

t-Butyl >95 20 S

Methyl 59 22 S

i-Propyl 50 20 S

1 -Naphthyl I t-Butyl I I I I

1. Percen@e detemdued by ‘H NMR enalysk 2. By HPLC analysis usbq ionic Plrkle DNPBG column (see ref. 7); 3. Absolute umfi8uration essl@ed from elution order (see ref. 7).

REFERENCES

1. M. Nogradi, “Stesoselective Synthesis”, VCH, Weinhenn. 1986. pp. 53-148; H. Haul Top. Stmochm, 1986,14.231.

2. S. Kolua, H. Hayashidu. Y. Tominaga and A. Hosomi, Temhedron Lert., 1988,29,89.

3. H. Sakurai in “Selectivirie.q in Lewis Acid promoted Reactions” D. Schinzer (ed) pp. 203-226.

4. E. N. Jacobsen, I. Mark& W. S. Mungall. G. Schmder and K. B. Shaqkss. J. Am. Chem. Sot., 1988,110, 1968.

5.In a typical lxoceiltue the dili&m salt 1 was pnparad from opdcally active (l!QS)-1~dipheny&?-ethan&ol(1.2 mmol) and n-butyllithium (2.4 mmol) in dry THF (10 ml) at (PC. To this soludon, a solution of tie ketone (0.5 mmol) and MmcUxmy.silane

(0.6ml3lol)indryTHF(lOml)wasslowlyadderlat~aadthareectionmixtunwa9stimdatthcsamctempenulaefar20h. Diethyl ether (20 ml) and 10% HCl(10 ml) wert added end, after usual work-up. the alkylarykarbinol was sepal@ by means column c hromatography (SK&, CHCl,) of the CNde product. from the chimJ Qand, which was recovered In 90% yield and

unchanged apical Purity.

6. K. K&to. T. Yoshida, S. Yama8u&i, S. Miyeno and I& Hashimoto. J. Org. C&m.. 1985,50,3013.

7. W. H. Firkle and J. M. Fiin, J. Org. Ckm., 1981,46,2935.