9440
J. Am. Chem. Soc. 1996, 118, 9440-9441
Table 1. B(C6F5)3-Catalyzed Hydrosilation of Aromatic
Aldehydes, Ketones, and Esters
Tris(pentafluorophenyl)boron-Catalyzed
Hydrosilation of Aromatic Aldehydes, Ketones, and
Esters
Daniel J. Parks and Warren E. Piers*,1
Department of Chemistry, UniVersity of Calgary
2500 UniVersity DriVe N.W.
Calgary, Alberta, T2N 1N4, Canada
ReceiVed May 7, 1996
Hydrosilation of carbon-oxygen double bonds is a mild
method for selective reduction of carbonyl functions. Although
the reaction is exothermic, a catalyst is generally required to
achieve adequate rates; consequently, many catalysts have been
developed for this important reaction.2 Nucleophilic/electro-
philic hydrosilation catalysis is characterized by a synergic
mechanism in which a nucleophile polarizes the Si-H bond of
the silane reagent, while a Lewis acid electrophile activates the
carbonyl by binding to oxygen. Among the nucleophiles that
have been employed are amines3 and halide ions,4 and Lewis
5
acids used include ZnCl2 and BF3‚Et2O.6
Tris(pentafluorophenyl)borane7 is a convenient, commercially
available Lewis acid of comparable strength to BF3 but without
the problems associated with reactive B-F bonds. Although
its primary commercial application is as a cocatalyst in
metallocene mediated olefin polymerization,8 its potential as a
catalyst for organic transformations is beginning to be appreci-
ated.9 Herein we report its use as a catalyst for mild addition
of Ph3SiH to aromatic aldehyde, ketone, and ester carbonyl
functions and the evidence we find for an unusual nucleophilic/
electrophilic mechanism by which the reduction takes place.
Aromatic aldehydes, ketones, and esters were hydrosilylated
at room temperature in the presence of 1-4 mol % B(C6F5)3
and 1 equiv of Ph3SiH; data for a selection of substrates are
shown in Table 1.10 For the aromatic substrates employed in
this study, B(C6F5)3 compares favorably with the best hydrosi-
lation catalysts in terms of conversion rates and selectivity.2
Limitation of the silane reagent to 1 equiv was essential for
clean reactions since further reduction of the silyl ether or silyl
acetal products was observed when excess silane was present.
So limited, isolated yields of the silyl ethers were excellent,
reflecting the >98% selectivity for product as determined by
gas-liquid chromatography. Ester reductions were extremely
rapid and were >90% selective for the acetal products; these
could subsequently be converted to aldehydes in synthetically
useful isolated yields.
a Mole percent of catalyst based on [Ph3SiH]. b For n ) 1, concentra-
tion ) 0.385 M. c Turnover frequency. d First-order rate constant.
e Isolated yield. f Reaction done at 0 °C. g Reactions too fast to follow
by GC. h Yield of benzaldehyde.
acetophenone > benzaldehyde. Since it is generally assumed
that coordination of the carbonyl oxygen to the electrophile is
the means by which Lewis acids activate carbonyl substrates,
this observation is surprising given that the strength of substrate
binding to B(C6F5)3 is in the opposite order. 1H NMR
experiments show that, in solution, binding of these substrates
to B(C6F5)3 is reversible and exchange between bound and free
substrate is rapid;11 the equilibria, however, strongly favor the
adducts 1. Equilibrium constants12 for eq 1 are 2.1(1) × 104,
For the unsubstituted substrates (X ) H, entries 1a, 2a, and
1.1(1) × 103, and 1.9(1) × 102 for R ) H, Me, and OEt,
respectively. Furthermore, competitive binding experiments
revealed that the preferred order of binding is benzaldehyde >
acetophenone . ethyl benzoate,13 consistent with experimentally
determined binding energies of these compounds with BF3.14
Since these equilibria favor the adduct, isolation and full
spectroscopic and structural characterization15 of the acid-base
3a, Table 1), the order of reactivity was ethyl benzoate .
(1) Corresponding author: FAX: 403-289-9488. e-mail: wpiers@
chem.ucalgary.ca.
(2) (a) Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: New York, 1989. (b) Marciniec, B.
ComprehensiVe Handbook on Hydrosilation; Pergamon: New York, 1992.
(3) Boyer, J.; Breliere, C.; Corriu, R. J. P.; Kpoton, A.; Poirier, M.; Royo,
G. J. Organomet. Chem. 1986, 311, C39.
(4) Corriu, R. J. P.; Perz, R., Reye, C. Tetrahedron 1983, 39, 999.
(5) Calas, R. Pure Appl. Chem. 1966, 13, 61.
(6) (a) Fry, J. L.; Orfanopoulo, M.; Adlington, M. G.; Dittman, W. R.;
Silverman, S. B. J. Org. Chem. 1978, 43, 374. (b) Doyle, M. P.; West, C.
T.; Donnelly, S. J.; McOsker, C. C. J. Organomet. Chem. 1976, 117, 129
and other papers in this series.
(11) Rates of exchange (kex ) π/2∆ν) between PhC(O)Xbound and PhC-
(O)Xfree as determined by VT 1H NMR spectroscopy: X ) H, 390 s-1
290 K; X ) CH3, 119 s-1, 265 K; X ) OEt, 811 s-1, 203 K.
,
(12) Measured by 1H NMR according to: Drago, R. S. Physical Methods
for Chemists, 2nd ed.; Saunders: New York, 1992; p 257.
(7) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1966, 5, 218.
(8) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015. (b) Ewen, J. A.; Edler, M. J. Canadian Patent App 2,027,145,
1991; Chem. Abstr. 1991, 115, 136998g.
(13) Equilibrium constants for the competitive binding experiments: 1-H
+ PhC(O)CH3 T PhC(O)H + 1-Me, 5.45(5) × 10-2; 1-H + PhC(O)OEt
T PhC(O)H + 1-OEt, 9.5(7) × 10-3; 1-Me + PhC(O)OEt T PhC(O)CH3
+ 1-OEt, 1.8(2) × 10-1
.
(9) (a) Ishihara, K.; Hananki, N.; Yamamoto, H. Synlett. 1993, 577. (b)
Ishihara, K.; Funahasi, M.; Hanaki, N.; Miyata, M.; Yamamoto, H. Synlett
1994, 963. (c) Ishihara, K.; Hananki, N.; Yamamoto, H. Synlett 1995, 721.
(d) Ishihara, K.; Hanaki, N.; Funahasi, M.; Miyata, M.; Yamamoto, H. Bull.
Chem. Soc. Jpn. 1995, 68, 1721.
(14) Maria, P.-C.; Gal, J.-F. J. Phys. Chem. 1985, 89, 1296.
(15) Structurally characterized adducts of boron Lewis acids with
carbonyl compounds are rare: (a) Corey, E. J.; Loh, T.-P.; Sarshar, S.;
Azimioara, M. Tetrahedron Lett. 1992, 33, 6945. (b) Reetz, M. T.;
Hu¨llmann, M.; Massa, W.; Berger, S.; Radmacher, P.; Heymanns, P. J.
Am. Chem. Soc., 1986, 108, 2405.
(10) See supporting information for further details.
S0002-7863(96)01536-3 CCC: $12.00 © 1996 American Chemical Society