Selective reduction of carbonyl compounds by polymethylhydrosiloxane in the presence of metal hydride catalysts

Full text of DOI:10.1021/jo982314z

2582
J . Org. Chem. 1999, 64, 2582-2589
Selective Red u ction of Ca r bon yl
Com p ou n d s by P olym eth ylh yd r osiloxa n e in
th e P r esen ce of Meta l Hyd r id e Ca ta lysts
Hubert Mimoun‡
Firmenich S.A, CH 1283-La Plaine-Geneva, Switzerland
Received November 24, 1998
In tr od u ction
Polymethylhydrosiloxane (PMHS) is a safe1 and inex-
pensive polymer coproduct of the silicone industry rep-
resenting ca. 5% of the world production.2 Its use as a
reducing silylating agent of carbonyl compounds (not
esters) in the presence of organotin compounds has been
first described by Nitzsche and Wick in 19573 and later
developed by other authors with tin,4 alkali or ammonium
n
fluorides,5 and titanocene catalysts activated by BuLi6
ally considered to be key intermediates in hydrosilylation
reactions,10 and PMHS is known to reduce organotin
oxides to tin hydrides.11 We reasoned that a metal
hydride hydrosilylation catalyst, particularly zinc hy-
dride, should be preformed from the reaction of metal
salts with an alkali metal hydride,12 considering the fact
that NaH-ZnCl2-RONa mixtures have been used for the
or RMgBr.7 Recently, nonactivated Ti(OR)4 complexes
have also been shown to catalyze the reduction of esters
by PMHS, albeit with lower turnover numbers (TON )
1-4).8 Here we present an air stable, general, convenient,
and inexpensive industrially important method for the
highly selective hydrosilylation-reduction of aldehydes,
ketones, esters, lactones, triglycerides, and epoxides to
the corresponding alcohols in the presence of zinc hydride
catalysts conveniently generated from the reaction of
soluble zinc carboxylates with a hydride reducing agent
(eqs 1 and 2).9 The present catalytic systems generally
do not disproportionate PMHS and therefore do not cause
gummy deposits and adventitious solidification of the
reaction medium, thus allowing multiton operations
under safe conditions.
13
reduction and hydrosilylation of carbonyl compounds
and that zinc compounds catalyze the reductive silylation
of ketones by a LiH-Me3SiCl mixture.14 Considering also
the fact that zinc borohydride is an excellent reagent for
the selective reduction of carbonyl compounds including
esters,15 we have undertaken the study of the catalytic
properties of various zinc compounds as such, or preac-
tivated by reaction with various reducing agents such as
NaBH4, BH3, CH3MgCl, AlEt3, LiH, LiAlH4, or sodium
dihydrobis(2-methoxyethoxy)aluminate (SDMA)16 in the
reduction by PMHS, using methyl benzoate as a sub-
strate model for esters. The reaction was carried out by
reacting first 0.2 mmol of Zn compound with 0.2 mmol
of additive unless otherwise stated (see Table 1) in
isopropyl ether (IPE), then adding 10 mmol of methyl
benzoate and 24 mmol of PMHS. The resulting mixture
was heated at 70 °C for 4 h, hydrolyzed by an excess of
aqueous NaOH 30%, and then analyzed by GC.
Resu lts
Meta l Hyd r id e Ca ta lysts for th e Red u ction of
Meth yl Ben zoa te by P MHS. Metal hydrides are gener-
‡ E-mail: hubert.mimoun@firmenich.com. Fax: (22) 780 7291.
Phone: (22) 780 7497.
(1) In contrast to other alkali metal hydride reductants, PMHS is
stable to air and water, easy to pump, and soluble in most organic
solvents.
Zinc compounds such a ZnCl2,17 Zn(RCO2)2, ZnEt2, Zn-
(BH4)2 or ZnH2 (insoluble) are not active for the reduction
(2) PMHS is industrially prepared by water hydrolysis of MeHSiCl2,
a coproduct of the Rochow “Direct Process” between Si and methyl
chloride, giving Me2SiCl2 as the major building block for the silicone
industry. The main industrial use of PMHS involves hydrofugation of
clothes and building materials, and silicone curing reagents by hy-
drosilylation of polyvinylsilanes.
(3) Nitzche, S.; Wick, M. Angew. Chem. 1957, 69, 96.
(4) (a) Grady, G. L.; Kuivila, H. G. J . Org. Chem. 1969, 34, 2014.
(b) Lipowitz, J .; Bowman, S. A. Aldrichimica Acta 1973, 6, 1.
(5) (a) Corriu, R. J . P.; Perz, R.; Re´ye´, C. Tetrahedron 1983, 39, 999.
(b) Kobayashi, Y.; Takahisa, E.; Nakano, M.; Watatani, K. Tetrahedron
1997, 53, 1627. (c) Drew, M. D.; Lawrence, N. J .; Fontaine, D.; Sekhri,
L.; Bowles, S. A.; Watson, W. Synlett 1997, 989. (d) Drew, M. D.;
Lawrence, N. J .; Watson, W.; Bowles, S. A. Tetrahedron Lett. 1997,
38, 5857.
(9) Mimoun, H. Patent WO 96/12694 (1995) to Firmenich S.A. The
technology involved in this patent has been licensed exclusively to
Morton International and is presently commercialized under the trade
name of Venpure ERS Technology.
(10) (a) Ojima, I. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1989; Chapter 25,
Part 2. (b) Hiyama, T.; Kusumoto, T. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991;
Vol. 8, Chapter 3.12.
(11) Hayashi, K.; Iyoda, J .; Shiihara, I. J . Organomet. Chem. 1967,
10, 81.
(6) (a) Berk, S. C.; Kreutzer, K. A.; Buchwald, S. L. J . Am. Chem.
Soc. 1991, 113, 5093. (b) Berk, S. C.; Buchwald, S. L. J . Org. Chem.
1992, 57, 3751; (c) 1993, 58, 3221. Enantioselective reduction of ketones
in the presence of chiral titanocene catalyst activated by n-BuLi: (d)
Carter, M. B.; Schiott, B.; Guttie´rrez, A.; Buchwald, S. L. J . Am. Chem.
Soc. 1994, 116, 11667. Reduction of lactones to lactols: (e) Verdaguer,
X.; Berk, S. C.; Buchwald, S. L. J . Am. Chem. Soc. 1995, 117, 12641.
(f) Verdaguer, X.; Hansen, M. C.; Berk, S. C.; Buchwald, S. L. J . Org.
Chem. 1997, 62, 8522.
(12) ZnCl2 has been previously shown to catalyze the hydrosilylation
of γ- and δ-lactones by triethylsilane at high temperatures (>120 °C).
Frainnet, E.; Calas, R.; Berthault, A. C. R. Acad. Sci. 1964, 258, 613.
(13) (a) Caubere, P. Top. Curr. Chem. 1978, 73, 50 and references
therein. (b) Brunet, J . J .; Besozzi, D.; Caubere, P. Synthesis 1982, 721.
(14) Okhuma, T.; Hashiguchi, S.; Noyori, R. J . Org. Chem. 1994,
59, 217.
(15) Ranu, B. C. Synlett. 1993, 885 and references therein.
(16) SDMA (NaAlH2(OCH2CH2OMe)2 is available as a 70% active
solution from Morton International (Alkadride Solution) or Cambrex
Chemical Corp (Vitride).
(7) Barr, K. J .; Berk, S. C.; Buchwald, S. L. J . Org. Chem. 1994, 59,
4323.
(8) (a) Breedon, S. W.; Lawrence, N. J . Synlett 1994, 833. (b) Reding,
M. T.; Buchwald, S. L. J . Org. Chem. 1995, 60, 7884.
(17) Chandrasekhar, S.; Ravindra Reddy, Y.; Ramarao, C. Synth.
Commun. 1997, 27, 2251.
10.1021/jo982314z CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/18/1999

Notes
J . Org. Chem., Vol. 64, No. 7, 1999 2583
Ta ble 1. Red u ction of Meth yl Ben zoa te by P MHS
P r om oted by Zin c Ca ta lystsc
Ta ble 2. Hyd r osilyla tion of Meth yl Ben zoa te by Va r iou s
Sila n es in th e P r esen ce of Zn (2-EH)2 + Na BH4 Ca ta lystc
a
b
Rhodorsil 628 from Rhone Poulenc. GC yields after hydroly-
sis by NaOH 15%. c Reaction conditions: methyl benzoate ) 10
mmol, Zn(2-EH)2 ) 0.2 mmol, NaBH4 ) 0.2 mmol, silane ) 22
mmol (equiv of SiH), solvent ) isopropyl ether (2 mL); 70 °C
(reflux), 4 h.
into monomeric tetrahedral zinc complexes ZnR2(diamine)
in which the diamine acts as a bidendate ligand.20 These
complexes thus appear to catalyze efficiently the reduc-
tion of methyl benzoate without the need for hydride
activation.
We also tested isolated zinc hydride complexes as
catalysts for the PMHS reduction of methyl benzoate.
Zinc borohydride Zn(BH4)2 1 was found inactive despite
its reducing properties toward esters (entry 16), as well
21
22
a
b
as [HZnOC2H4NMe2]2 2 (entry 17) and [PhZnH.Py]3
Equivalents of additive based on the zinc concentration. GC
yields after hydrolysis by 15% NaOH. c Reaction conditions:
methyl benzoate ) 10 mmol, zinc compound ) 0.2 mmol, equiv of
additive (see table), PMHS ) 22 mmol, solvent ) diisopropyl ether
(2 mL); 70 °C (reflux), 4 h.
(run 20). Other zinc hydride complexes such as [HZnN-
(Me)C2H4NMe2]2 3 (entry 18),23 and (PhZnH)2‚TMEDA22
(entry 19) were found active for the hydrosilylation but
less effective than the hydride species generated from the
reaction of Zn(2-EH)2 + NaBH4.
of methyl benzoate by PMHS (entries 1-3). Addition of
an equimolecular amount of NaBH4 to Zn(2-ethylhex-
anoate)218 (hereinafter Zn(2-EH)2) results in the evolution
of 0.5 mol of H2/mol of Zn which stops after 0.5 molar
equiv of NaBH4 has been added and the formation of a
very active soluble catalyst which allows almost quan-
titative transformation of methyl benzoate first to ben-
zyloxypolysiloxane and then to benzyl alcohol after
hydrolysis (entry 4). The reaction can be operated in
aprotic solvents such as hydrocarbons and ethers, or
without solvent, in the presence of a catalytic quantity
of zinc + reductant as low as 1 mol % based on the
substrate. Protic solvents such as alcohols and carboxylic
acids are not suitable, since they are silylated by PMHS
in the presence of zinc hydride catalysts (vide infra).
Other soluble zinc carboxylates such as benzoate, isobu-
tyrate, 2-methylbutyrate, and undecylenate, are active
as well, while insoluble zinc acetate is almost inactive.
Activation of Zn(2-EH)2 by CH3MgCl also results in the
formation of a very active catalyst (entry 7), but AlEt3,
BH3, and DIBAL-H (entries 8-10) are less efficient
activators. ZnCl2 can be activated by 4 equiv of LiH (entry
11), but in the presence of LiAlH4 it gave inferior results
(entry 12).
Thus, soluble zinc hydride species, best generated from
the reduction of soluble zinc carboxylates by NaBH4, as
well as dialkylzinc coordinated by 1,2-diamines are very
effective catalysts for the reduction by PMHS of esters
to the corresponding alcohols.
Although PMHS is the most economical, safest, and
most effective silylating agent, other silanes such as
(EtO)3SiH and (MeO)3SiH hydrosilylate methyl benzoate
in the presence of Zn(2-EH)2 + NaBH4 catalyst as well
(Table 2, entries 3-4), while Et3SiH and Ph3SiH were
found completely ineffective (entries 6-7), in contrast to
Ph2SiH2 (entry 5). A silane copolymer where Si-H and
Si-Me groups alternate randomly (Rhodorsil 628, HSi-
Me:SiMe2 ) 5, entry 2) was also operative in the
hydrosilylation of methyl benzoate, provided the fact that
SiH/ester ratio is > 2. Thus, in contrast to the recently
reported NBu4F- or Triton B-catalyzed hydrosilylation of
carbonyl compounds,5c there is no clear evidence here of
a considerable rate acceleration with PMHS in compari-
son with other silylating agents, favoring a “zipper”
mechanism where the catalyst jumps from one Si-H to
the next adjacent one along the polymeric silicon back-
bone.
While ZnEt2 alone does not catalyze the present
reaction (entry 2), it can be activated by ethylenediamine
(entry 14) or TMEDA (entry 15).19 Diamines are known
to transform the linear C-Zn-C backbone of dialkylzinc
(20) (a) O’Brien, P.; Hursthouse, M. B.; Motevalli, M.; Walsh, J . R.;
J ones, A. C. J . Organomet. Chem. 1993, 449, 1. (b) Motevalli, M.;
O’Brien, P.; Robinson, A. J .; Walsh, J . R.; Wyatt, P. B.; J ones, A. C. J .
Organomet. Chem. 1993, 461, 5.
(21) (a) Bell, N. A.; Coates, G. E. J . Chem. Soc. A 1968, 823. (b)
Goeden, G. V.; Caulton, K. G. J . Am. Chem. Soc. 1981, 103, 7354.
(22) De Koning, A. J .; Boersna, J .; Van Der Kerk, G. J . M. J .
Organomet. Chem. 1980, 195, 1.
(23) Bell, N. A.; Moseley, P. T.; Shearer, H. M. M.; Spencer, C. B. J .
Chem. Soc., Chem. Commun. 1980, 359.
(18) Metal soaps M(2-EH)n used in this paper are commercially
available 2-ethylhexanoates used as drying agents in the paint industry
or easily prepared from the metal oxide or carbonate and 2-ethylhex-
anoic acid (see Experimental Section).
(19) TMEDA ) tetramethylethylenediamine.

2584 J . Org. Chem., Vol. 64, No. 7, 1999
Notes
Ta ble 3. Red u ction of Meth yl Ben zoa te by P MHS.
Ca ta lytic Activity of Va r iou s Solu ble Meta l Sp ecies
Ta ble 5. Red u ction of Keton esa
a
Activa ted by Na BH4
entry
metal compound
convn (%)
selectivity (%)
1
Zn(2-EH)2
Mn(stearate)2
Fe(2-EH)2
100
100
73
8
100
100
100
100
83
2
3b
4
Cd(propionate)2
5
Ti(OiPr)4
Sn(2-EH)2
Zr(OiPr)4
6
6
0
0
7
a
Reaction conditions: methyl benzoate ) 10 mmol, metal
compound ) 0.2 mmol, NaBH4 ) 0.2 mmol, PMHS) 22 mmol,
b
solvent ) diisopropyl ether (2 mL); 70 °C (reflux), 4 h. Solidifi-
cation occurred during the reaction.
Ta ble 4. Red u ction of Ald eh yd esc
a
Reaction conditions: same as in Table 4.
geraniol-nerol mixture (entry 2), methylcinnamic alde-
hyde 6a to the corresponding alcohol 6b, and campholenic
aldehyde 7a to campholenic alcohol 7b. Sterically hin-
dered aldehydes such as (+)-formylpinane 11a are much
more readily reduced to the corresponding alcohol 11b
with trimethoxysilane (95% yield) than with PMHS (20%
yield). This suggests that the silane and the active zinc
species are associated in the catalytic intermediate.
a
b
E:Z ) 50:50 (synthetic citral). Isolated yields. c Reaction
Red u ction of Keton es. The use of Zn(2-EH)2
+
conditions: aldehyde ) 100 mmol, Zn(2-EH)2 ) 2 mmol, NaBH4
)
NaBH4 catalyst also applies for the selective reduction
of ketones by PMHS, as shown in Table 5. Both saturated
and R,â-unsaturated ketones are very efficiently reduced
to the corresponding alcohols. 2-Cyclohexen-1-one 13a ,
2-pentylcyclopent-2-ene-1-one 14a , pseudoionone 15a ,
geranylacetone 16a , â-ionone 17a , and isophorone 18a
are almost quantitatively reduced to the corresponding
secondary alcohol. The reduction is very sensitive to steric
effects, as shown by the absence of reactivity of the
encumbered ketone 20a with PMHS (Table 5, entry 10),
whereas trimethoxysilane gave 72% yield of the corre-
sponding alcohol 20b. 4-tert-Butylcyclohexanone 20a is
reduced to an 87:13 trans:cis mixture of 4-tert-butylcy-
clohexanol isomers, in a manner similar to that of NBu4F
and Triton B catalysis but in contrast to the L-Selectride
reagent.25
While zinc, manganese, and iron carboxylates activated
by NaBH4 are best able to catalyze the reduction of
methyl benzoate to benzyl alcohol (Table 3), various metal
hydrides allowed the reduction of ketones by PMHS, but
with different selectivities, as shown in Table 6. Taking
2-cyclohexen-1-one as a model, we could distinguish
between metals capable of 1,2-reduction which gave
2-cyclohexen-1-ol as product and metals capable of 1,4-
reduction which gave cyclohexanone first and then cy-
clohexanol in some cases.
2 mmol, PMHS ) 110 mmol, solvent ) diisopropyl ether (20 mL),
70 °C (reflux), 4-6 h.
Various combinations of other soluble metal carboxy-
lates with NaBH4 (2 mol % each) were tested for the
reduction of methyl benzoate by PMHS, as shown in
Table 3. Although zinc catalysts were found to be the
most effective, manganese(II) stearate and iron(II) 2-eth-
ylhexanoate reduced by NaBH4 also catalyzed the hy-
drosilylation reaction (Table 3, entries 2-3). Manganese
carbonyl complexes, e.g., (PPh3)(CO)4MnC(O)CH3, have
recently been shown to catalyze the hydrosilylation of
aldehydes and ketones by monomeric silanes.24 Other
metal precursors such as Cd, Ti, Sn, Co, and Zr carboxy-
lates or alkoxides activated by NaBH4 were not nearly
as effective for this transformation (entries 4-7). This
contrasts with the known catalytic activity (not for esters)
of [Bu2Sn(OAc)]2O operating in alcoholic solvents4 and
that of titanocenes activated by BuLi or RMgX reagents.6
Red u ction of Ald eh yd es. Table 4 shows that a large
variety of aldehydes are almost quantitatively reduced
by PMHS (1 molar equiv) in the presence of Zn(2-EH)2
+ NaBH4 catalyst. Conjugated or nonconjugated double
bonds present in the substrate are not affected by the
reduction. Thus, (E)-2-hexenal 4a is transformed to (E)-
2-hexenol 4b (Table 4, entry 1), citral 5a to a 50:50
(24) Dibiase Cavanaugh, M.; Gregg, B. T.; Cutler, A. R. Organome-
tallics 1996, 15, 2764.
(25) Brown, H. C.; Krishnamurthy, S. J . Am. Chem. Soc. 1972, 94,
7151.

Notes
Ta ble 6. Red u ction of Cycloh exen -1-on e-2 by P MHS.
J . Org. Chem., Vol. 64, No. 7, 1999 2585
Ta ble 7. Red u ction of Ester s a n d La cton esa
In flu en ce of Va r iou s Meta l Hyd r id e Ca ta lysts: 1,2 vs 3,4
Red u ction a
a
Reaction conditions: 20 mmol of 2-cyclohexen-1-one, 22 mmol
of PMHS, 0.4 mmol of metal compound (except for Ru and Pd
where 0.04 mmol were used), 0.4 mmol of NaBH4, solvent )
diisopropyl ether (2 mL), 70 °C, 4 h.
a
Reaction conditions: substrate ) 20 mmol, PMHS ) 44 mmol,
Zn(2-EH)2 ) 0.4 mmol, NaBH4 ) 0.4 mmol, solvent ) diisopropyl
ether (2 mL), 70 °C, 4 h.
(i) Activated metals which give 2-cyclohexen-1-ol by
1,2-reduction are Zn(II), Co(II), Mn(II), Sn(II), Fe(II), and
Cd(II) carboxylates (entries 1-6). Zr(OiPr)4 was found
to be only slightly active and Cp2TiCl2 was inactive.
(ii) Activated metals which give conjugate reduction
to cyclohexanone with almost no consecutive reduction
to cyclohexanol are Cu(2-EH)2 and Pd(OAc)2(PPh3)2 (en-
tries 10-11). The peculiar behavior of copper hydride
catalysts in the hydrosilylation of enones by PMHS is
reminiscent of the catalytic property of copper(I) hydride
such as [CuH(PPh3)]6 in the selective 1,4-reduction of
enones to their corresponding saturated ketones by
molecular hydrogen.26 This conjugate reduction has been
shown to occur via a Cu(I) enolate resulting from the
conjugate addition of Cu(I) hydride to the enone. Here,
enol polysilyl ethers are presumably formed from the
reduction of enones by PMHS in the presence of Cu(2-
EH)2 + NaBH4 catalysts.
(iii) NaBH4-activated RuCl2(PPh3)3 and Ni(2-EH)2 which
give a mixture of 1,2- and 1,4-reduction products: 2-cy-
clohexen-1-ol, cyclohexanone, and cyclohexanol.
(iv) NaBH4-activated molybdenum(VI) naphthenate,
chromium(III) 2-ethylhexanoate, and Ce(2-EH)3 were
found to be completely inactive for the reduction of
2-cyclohexen-1-one.
Thus, the use of soluble metal hydrides as catalysts
for the hydrosilylation by PMHS reveals, by the variety
of their behavior, unexplored ways for fine-tuning the
reduction of carbonyl compounds and requires further
studies for other specific substrates.
corresponding alcohols in the presence of Zn(2-EH)2 +
NaBH4 catalyst, as shown in Table 7. This reduction,
which requires 2 molar equiv of PMHS per mol ester,
occurs without affecting CdC double bonds in their
position and stereochemistry. Thus, methyl (Z)-3-hex-
enoate 22a is selectively transformed into (Z) -3-hexen-
1-ol 22b, methyl undecylenate 23a into undecenyl alcohol
23b, methyl oleate 24a into oleyl alcohol 24b, methyl
cinnamate 25a into cinnamyl alcohol 25b, phenethyl
caproate 26a into phenethyl alcohol and n-hexanol, and
geranyl acetate 27a into geraniol. As previously shown
for the encumbered ketone 20a (Table 5, entry 10), the
reduction of esters is very sensitive to steric effects. For
example, methyl â-cyclogeraniate 30a is not hydrosily-
lated under these conditions (Table 7, entry 8).
While no aldehyde was detected in the partial reduc-
tion of esters by 1 molar equiv of PMHS, lactols can be
clearly identified from the half-reduction of lactones and
isolated in yields of ca. 70% (entry 10). Addition of
another equivalent of PMHS led to complete reduction
to the corresponding diol and solidification of the reaction
medium, owing to the cross-coupling of two polysiloxane
chains by the resulting diol. This contrasts with Buch-
wald’s Cp2Ti(p-ClC6H4O)2/TBAF system which requires
a large excess of PMHS and stops at the lactol stage.6e,f
One of the most peculiar properties of the present Zn-
(2-EH)2 + NaBH4 catalyst is its ability to catalyze the
hydrosilylation-reduction of fatty oils to the correspond-
ing natural fatty alcohols in high yields (eq 3).
Red u ction of Ester s a n d La cton es. Esters and
lactones are very efficiently reduced by PMHS to the
(26) (a) Strycker, J . M.; Mahoney, W. S.; Daeuble, J . F.; Brestensky,
D. M. In Catalysis of Organic Reactions; Pascoe, W. E., Ed.; Marcel
Dekker: New York, 1992; pp 29-44. (b) Mahoney, W. S.; Strycker, J .
M. J . Am. Chem. Soc. 1989, 111, 8818. (c) For heterogeneous version
of the 1,4-reduction of R,â-unsaturated ketones in the presence of Cu/
SiO2 catalyst, see: Ravasio, N.; Antenori, M.; Gargano, M.; Mastrorilli,
P. Tetrahedron Lett. 1996, 37, 3529.

2586 J . Org. Chem., Vol. 64, No. 7, 1999
Notes
C24:1
Ta ble 8. Red u ction of Tr iglycer id esa
yield,
entry
oils
mol %
C10
C10
C12
C14
C16
8.5
31.6
7.9
13.0
8,1
5.7
C18
C18:1
C18:2
3.0
C18:3
C20:1
C22:1
1
2
3
4
5
6
7
8
triolein
coconut
95
94
92
78
94.
81
75
95
94.0
13.3
30.9
12.1
49.1
24.6
16.9
6.5
12.2
48.8
6.8
19.3
17.1
-
cocoa butter
palm kernel
peanut
sunflower
linseed
33.1
1.3
6.2
4.6
3.1
53.3
1.4
26.3
62.8
14.0
55.0
J ojoba
59.4
26.8
3.9
a
Reaction conditions: oil ) 1 molar equiv, PMHS ) 7 molar equiv (based on the saponification value), Zn(2-EH)2 ) 4 mol % based on
the ester content, NaBH4 ) 4 mol %, solvent ) diisopropyl ether, 70 °C, 4 h.
Ta ble 9. Red u ction of Ep oxid esa ,b
The reduction of triglycerides is carried out in diiso-
propyl ether or toluene solvent and requires at least 2
molar equiv of PMHS per mole of ester (based on the
saponification value) and ca. 4 mol % of activated Zn
catalyst. The reaction can be operated safely, inexpen-
sively, and conveniently in a multipurpose reactor,
without the need for dangerous molten sodium in the
Bouveault and Blanc synthesis, and without the need for
transesterification to methyl fatty esters before high
pressure and temperature hydrogenation in the presence
of copper chromite catalysts which affects the Z double
bonds.27 As shown in Table 8, the natural alcohols
obtained by this reduction by PMHS accurately cor-
respond, in their constitution and stereochemistry, to the
fatty esters precursors. Thus, triolein is reduced in almost
quantitative yield to oleyl alcohol. Coconut, cocoa butter,
and palm kernel oils are transformed to a mixture of C10-
C18 alcohols (Table 8, entries 2-4). Peanut oil gave a
mixture of alcohols composed of 13% hexadecanol-1 (C16),
6% octadecanol-1 (C18), 49% of 9(Z)-octadecen-1-ol (oleyl
a
Reaction conditions: epoxide ) 100 mmol, Zn(2-EH)2 ) 4
mmol, NaBH4 ) 4 mmol, PMHS ) 110 mmol, solvent ) diisopropyl
ether (20 mL), 70 °C (reflux), 4-6 h. Isolated yields.
b
alcohol, C18:1), and 26% of 9(Z),12(Z)-octadecadien-1-ol
(linoleyl alcohol, C18:2). Linseed oil was reduced to a
mixture of alcohols containing 55% of 9(Z),12(Z),15(Z)-
octadecatrien-1-ol (linolenyl alcohol, C18:3), a high value
precursor for many important natural products. J ojoba
oil, an ester mixture of straight chain C18-C24 (Z)-9
monounsaturated acids and alcohols, was cleanly and
almost quantitatively converted into a mixture of oleyl
alcohol (C18:1, 6.5%), (Z)-9-icosen-1-ol (C20:1, 59.4%), (Z)-
9-docosen-1-ol (C22:1, 26.8%), and (Z)-9-tetracosen-1-ol
(C24:1, 3.9%).
Red u ction of Ep oxid es. Table 9 shows that only
terminal primary epoxides such as 1,2-epoxydecane 30a ,
phenylglycidyl ether 31a , and isopropylglycidyl ether 32a
are efficiently reduced by PMHS only to the correspond-
ing secondary alcohol, with no evidence of primary alcohol
formation. Other substituted epoxides such as R-pinene
epoxide, â-pinene epoxide, and trimethylcyclododecatri-
ene epoxide remain unaffected by the reduction condi-
tions (Table 9, entry 5).
we screened many combinations of metal carboxylates
+ NaBH4 catalysts, are not reduced to the corresponding
phosphines.
Deh yd r ogen a tive Silyla tion of Alcoh ols by P MHS
in th e P r esen ce of Zin c Hyd r id e Ca ta lysts. We have
previously mentioned that, in contrast to the catalysis
by tin compounds,3,4 the ZnH-catalyzed reduction of
carbonyl compounds cannot be operated in protic solvents
such as alcohols and carboxylic acids. A considerable
effervescence of hydrogen occurs when PMHS is added
to the carbonyl compound dissolved in alcohol in the
presence of Zn(2-EH)2 + NaBH4 catalyst. We have
observed that this hydrogen evolution results from the
dehydrogenative silylation of the alcohol by PMHS,
affording the corresponding alkoxylated polysilyl ethers
according to eq 4.
Oth er Su bstr a tes. Carboxylic acids such as 2-ethyl-
hexanoic acid are not or only slightly reduced by PMHS
in the presence of Zn(2-EH)2 + NaBH4 catalyst, since they
are first hydrosilylated by PMHS with evolution of
hydrogen. Anhydrides such as butyric anhydride and
maleic anhydride are also inert.
Nitriles such as benzonitrile, lactams such as N-
methylpyrrolidone or succinimide, and nitro compounds
such as nitrobenzene are unreactive under these condi-
tions. Also, phosphine oxides such as Ph3PO, for which
Silylation is a well-known procedure for the protection
of alcohols,28 but the present method is rapid, quantita-
tive, inexpensive, forms no salts, and does not generally
require any solvent or aqueous workup.29 The reaction
is easily carried out by first dissolving 1 mol % of Zn(2-
EH)2 and 1 mol % of NaBH4 in the alcohol. Then 1 equiv
(28) Green, T. W.; Wuts, P. G. M. Protective groups in Organic
Synthesis, 2nd ed.; J ohn Wiley & Sons: New York, 1991; pp 68-87.
(29) Mimoun, H. Patent WO 96/28497 (1996) to Firmenich S.A.
(27) Noveck, K.; Ridder, H. Ullmann’s Encyclopedia of Industrial
Chemistry, 5th ed.; 1987; Vol. A11, p 277.

Notes
J . Org. Chem., Vol. 64, No. 7, 1999 2587
Sch em e 1
of PMHS is slowly added at room temperature (exother-
mic reaction!) and the mixture is stirred until no more
hydrogen is evolved. The amount of evolved H2 exactly
corresponds to the expected stoichiometry of eq 4, and
we have used this volumetric method to evaluate the
available SiH bonds in PMHS. The resulting liquid,
semiviscous, polysiloxane resulting from the addition of
1 mol of PMHS to 1 mol of alcohol can be removed from
the zinc catalyst by thermal treatment at 150 °C for 1 h
and filtration of the gray precipitate through Celite. No
silylation of the alcohol is observed if the zinc carboxylate
is not activated by NaBH4. The reactivity of the alcohols
is in the order primary > secondary . tertiary.
The polysiloxanes obtained by silylation of alcohols by
PMHS are easily characterized by 1H and 13C NMR. The
1H NMR spectra of the polysiloxanes show the disap-
pearance of the PMHS Si-H signal at 4.72 ppm, similar
1
to the H NMR spectra of the alcohol precursor, with a
of double bonds. Other metal 2-ethylhexanoates such as
Mn(II) and Fe(II) activated by NaBH4 were also found
to be catalytically active for the dehydrogenative silyla-
tion of alcohols.
broadening of the signals, owing to the oligomeric nature
of the polysiloxane, and with a downfield shift of ca. 0.1
ppm of the Si-OCH2 protons. The 13C NMR spectra also
resemble those of the alcohol precursor, with a demulti-
plication of the PMHS Si-CH3 quartet from 1 ppm in
PMHS to several signals for the polysiloxane between -5
and +2.5 ppm.
Discu ssion
The present work illustrates various possibilities for
generating active catalysts for the hydrosilylation of
carbonyl compounds. Among the soluble metal carboxy-
lates activated by hydride reducing agents, zinc has been
found to be the most effective in a large scope of broad
applications.
Hydrosilylation catalyzed by alkali or ammonium
fluorides,5 hydroxides,5d and alkoxides30 is generally
thought to proceed through pentavalent anionic alkoxy-
hydridosilicates [A] which have been isolated and char-
acterized, and which act as efficient hydrogen transfer
reagents toward carbonyl compounds, including esters
(Scheme 1).31 Whether the carbonyl substrate is activated
by coordination to a sixth position at silicon in the
alcoxyhydridosilicates,31a or by the alkali cationic coun-
terpart, as recently shown in the Li(binol)-catalyzed
enantiomeric hydrosilylation of acetophenones by (MeO)3-
SiH,30b is still unclear, since chiral N-benzylquininium
fluoride having no coordination ability behaves also as
an asymmetric reduction catalyst.5d
The coordination ability for carbonyl compounds exists
for transition metals, and this will determine the par-
ticular selectivity observed in the hydrosilylation such
as ability for ester reduction, 1,2 vs conjugate reduction
of enones, reduction of epoxides, steric effects, etc.
Titanium alkoxides are good catalysts for transesterifi-
cation of esters,32 and Ti(OPh)4 forms 1:1 adducts with
acetone.33 ZnCl2 forms with aldehydes 1:1 complexes
which have been characterized by X-ray structure,34 and
this behavior constitutes the basis of the enantioselective
The 1H and 13C NMR spectra of the polysiloxane
obtained by dehydrogenative silylation of alcohols are
similar to those of the polysiloxane obtained by reduction
of the carbonyl precursor, as shown in the case of
citronellol where the resulting polysiloxane 40 is similar
to that obtained from the reduction of citronellal by
PMHS (eq 5 and entry 4, Table 10, Supporting Informa-
tion).
1
In the case of esters, the H and 13C NMR spectra of
the polysiloxane 41 obtained by the reduction of methyl
(Z)-3-hexenoate (eq 7) differ from that obtained by
dehydrogenative silylation of (Z)-3-hexen-1-ol (eq 6) only
by the additional presence of the signals of the Si-OCH3
group resulting from the methyl ester (entry 5).
(30) (a) Hosomi, A. In Reviews on Heteroatom Chemistry; Oae, S.,
Ed.; MYU: Tokyo, 1992; Vol. 7, p 214 and references therein. (b)
Shiffers, R.; Kagan, H. B. Synlett 1997, 1175.
(31) (a) Corriu, R. J . P.; Guerin, C.; Henner, B.; Wang, Q. Organo-
metallics 1991, 10, 2297, 3574. (b) Corriu, R. J . P.; Guerin, C.; Henner,
B.; Wang, Q. J . Organomet. Chem. 1989, 365, C7. (c) Corriu, R. J . P.,
Young, J . C. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989; Chapter 20, p
1241.
(32) (a) Otera, J . Chem. Rev. 1993, 93, 1449. (b) Siling, M. I.;
Laricheva, T. N. Russ. Chem. Rev. 1996, 65, 279. (c) Duthaler, R. O.;
Hafner, A. Chem. Rev. 1992, 92, 807
Several polysilyl ethers have been prepared from the
reaction of various alcohols with PMHS in the presence
of 2 mol % of Zn(2-EH)2 + NaBH4 catalyst and character-
ized by NMR, as shown in Table 10. As for the reduction
of aldehydes, ketones, and esters, the silylation of alco-
hols in the presence of ZnH catalysts is perfectly tolerant
(33) Masthoff, R.; Kohler, H.; Bohland.; Shmeil, F. Z. Chem. 1965,
122.

2588 J . Org. Chem., Vol. 64, No. 7, 1999
Notes
Sch em e 2
Sch em e 3
resulting zinc alkoxide [G] will share the oxygen atom
of the alkoxide group with silicium, forming four-
membered transition state [H] in which a pentacoordi-
nated-activated hydridosilicate is generated. Owing to the
higher bond strength of Si-O vs Zn-O, the alkoxide
group is transferred to silicon in [H] forming the polysi-
loxane [I] and the initial zinc hydride catalyst [D] is
regenerated, thus completing the catalytic cycle depicted
in Scheme 2.
Steric interaction in the transition state [F ] between
the carbonyl group and the bulky PMHS may explain the
low reactivity of the encumbered formylpinane 11a (Table
4), ketone 20a (Table 5), and ester 28a (Table 7), whereas
(MeO)3SiH was found active as silylating agent. This
difference cannot be interpreted if the hydride transfer
to carbonyl occurs from ZnH separated from PMHS.
Examination of the catalytic cycle (Scheme 2) permits
us to envisage two entries for the active species: the zinc
hydride entry in intermediate [E] and the zinc alkoxide
entry in intermediates [G] or [H]. Such metal hydride-
metal alkoxide interchange in the hydrosilylation process
has also been suggested to occur in titanium chemistry
by Buchwald.6f Thus, the catalytic activity of several
systems not involving hydride activation (Table 1) could
be interpreted by their ability to generate zinc alkox-
ides: the Zn(2-EH)2-CH3MgCl system (entry 7) could
produce zinc alkoxide from the Grignard methylation of
the carboxylate moiety; ZnEt2-diamine systems (entries
14-15) can also alkylate carbonyl substrates and form
catalytically active alkoxides species.20
The zinc hydride-zinc alkoxide interchange can also
readily be used to interpret the dehydrogenative silyla-
tion of alcohols by PMHS, as shown in Scheme 3. Like
alkali hydrides, zinc hydride species [D] readily react
with alcohols to give zinc alkoxides with H2 evolution,39
presumably through the four-membered intermediate [J ],
and zinc alkoxides add to PMHS to give the four-
membered intermediate [L] from which the polymethyl-
siloxane [M] is formed and the initial zinc hydride species
is regenerated. The driving force of polysilyl ether forma-
tion from PMHS and alcohols in the presence of ZnH
catalysts overrides the reduction of carbonyl compounds
when these are carried out in protic solvents.
alkylation of aldehydes by dialkylzinc in the presence of
chiral amino alcohols.35 The coordinative ability of zinc
toward esters probably explains why Zn(BH4)2 reduces
esters while NaBH4 is usually inefficient.15
We have illustrated here the striking properties of
soluble zinc carboxylates activated by NaBH4 as catalysts
for the hydrosilylation of carbonyl compounds. Although
we have not been able to characterize the involved active
species, we assume that soluble zinc hydride complexes
should be involved in the catalytic cycle. Ashby et al. have
prepared a variety of zinc hydride species from the
reaction of dialkylzinc and zinc salts with hydride reduc-
ing agents such as LiH, NaH, NaBH4, LiAlH4, etc.36 and
we have previously shown that ZnCl2 reduced by 4 equiv
of LiH is an efficient catalyst, while insoluble ZnH2 is
inactive (Table 1). The isolated zinc hydride complexes
such as 2 and 3 were found less active than the in situ
formed Zn(2-EH)2 + NaBH4 combination, presumably
because of their oligomeric nature preventing association
with SiH bonds. To our knowledge, there are only two
zinc hydride derivatives characterized by X-ray structure,
[TptBu]ZnH37 and complex 3 (Table 1).23
A plausible mechanism for the PMHS hydrosilylation
of carbonyl compounds in the presence of zinc hydride
catalysts is illustrated by Scheme 2. We assume that, like
alkali metal hydrides,31a zinc hydride species [D] are able
to coordinate to PMHS to form adduct [E] in which SiH
bonds are activated in a reactive pentavalent dihydrido-
silicate associated to the zinc Lewis acid center. This four-
membered adduct [E] is reminiscent of LiAlH4 reductions
in which Li+ activates the CdO group and Al-H bonds
act as hydride donors38 and of Zn(BH4)2 in which Zn
activates the carbonyl group and B-H bonds are the
hydride donors. As for LiAlH4, hydrogen may be trans-
ferred in a concerted way to the CdO bond coordinated
to zinc in six-membered ring transition state [F ]. The
(34) Mu¨ller, B.; Ruf, M.; Vahrenkamp, H. Angew. Chem., Int. Ed.
Engl. 1994, 33, 2089.
(35) Noyori, R.; Kitamura, M. Angew. Chem., Int. Ed. Engl. 1991,
30, 49 and references therein.
(36) (a) Ashby, E. C.; Watkins, J . J . Inorg. Chem. 1973, 12, 2493.
(b) Watkins, J . J .; Ashby, E. C. Inorg. Chem. 1974, 13, 2350. (c) Ashby,
E. C.; Watkins, J . J . Inorg. Chem. 1977, 16, 2070, 2075.
(37) Looney, A.; Han, R.; Gorell., I. B.; Cornebise, M.; Yoon, K.;
Con clu sion
We have developed an economical, safe, and industri-
ally important method for the hydrosilylation-reduction
Parkin, G.; Rheingold, A. L. Organometallics 1995, 14, 274. TptBu
bis(3-tert-butylpyrazolyl)hydroborato.
)
(38) (a) Noyori, R.; Tomino, I.; Tanimoto, Y.; Nishizawa, M. J . Am.
Chem. Soc. 1984, 106, 6709. (b) Boone, J . R.; Ashby, E. C. Top.
Stereochem. 1979, 11, 53.
(39) Bell, N. A.; Kassyk, A. L. J . Organomet. Chem. 1988, 345, 245;
Inorg. Chim. Acta 1996, 250, 345.

Notes
J . Org. Chem., Vol. 64, No. 7, 1999 2589
to literature procedures. Sodium borohydride was supplied by
Morton Inc. Zinc, iron, manganese, tin, and cobalt 2-ethylhex-
anoates (metallic soaps) were commercially available from OMG-
Vasset but can also easily be prepared from the reaction of metal
oxide, carbonate, or acetate with 2-ethylhexano¨ıc acid in toluene
at 120 °C, with azeotropic distillation of water or acetic acid.
All compounds were characterized by GC-MS and 1H and 13C
NMR.
Gen er a l Red u ction P r oced u r e. Solid NaBH4 (2 mol %) was
added to a solution of zinc 2-ethylhexanoate (2 mol %) in
diisopropyl ether (DIPE). The mixture was stirred at room
temperature until all hydrogen has been evolved (ca. 15 min).
The substrate (1 molar equiv) was added, and the mixture was
heated at 75 °C (reflux DIPE). Then 1.1 molar equiv (for
aldehydes, ketones or epoxides) or 2.2 molar equiv (for esters
and lactones) PMHS was added in ca. 1 h. The reaction was
followed by GC injection of the organic phase obtained after
hydrolysis of aliquot samples by 10% NaOH. The mixture was
maintained under reflux until all the substrate has been
consumed and then was cooled at room temperature and slowly
pumped onto an aqueous 30%NaOH or 45%KOH solution (1.2
equiv of OH-/mol of initial PMHS, carefully avoiding the reverse
hydrolysis). The two-phase mixture was stirred for 1 h at 40 °C
until two clear phases were formed. The organic phase was
concentrated in vacuo, and the resulting alcohol was bulb-to-
bulb distilled in vacuo.
of aldehydes, ketones, esters, triglycerides, and epoxides
to alcohols by PMHS in the presence of various metal
hydride catalysts among which zinc hydrides, best gener-
ated from the reaction of soluble metal carboxylates with
sodium borohydride, proved to be the most effective
catalysts. The procedure is simple and highly selective
for a large variety of carbonyl substrates and affords a
cheap and safe alternative to the use of stoichiometric
hazardous LiAlH4 reagents. Zinc hydride also efficiently
catalyzes the dehydrogenative hydrosilylation of a variety
of alcohols using a simple procedure. The hydrosilylation
presumably occurs via an interchange between zinc
hydride and zinc alkoxide activating PMHS to pentaco-
ordinated hydridosilicates. Further work is being carried
out in order to isolate the active species and extend the
scope of these reactions to enantioselective reduction.40
Exp er im en ta l Section
Gen er a l. Chemicals described in the study were all reagent
grade purchased from Fluka or available in bulk quantities from
Firmenich S.A. PMHS used in this study (Mw ) 2200; specific
gravity at 25 °C ) 1; 1.55 wt % H) was obtained from Rhone
Poulenc (trade name ) Rhodorsil Hydrofugeant 68), but PMHS
from other suppliers (Aldrich, Hu¨ls, Bayer, GE, Dow-Corning)
was found suitable as well. The molecular weight of PMHS was
taken to be 65 for calculations of stoichiometry. The complexes
Zn(BH4)2,41 [HZnOC2H4NMe2]2,42 [PhZnH‚Py]3,43 [HZnN(Me)-
C2H4NMe2]2,44 and (PhZnH)2‚TMEDA22 were prepared according
Ack n ow led gm en t. The author wishes to thank M.
M. J oe¨l Pastori and Hung Hai Vo for their invaluable
technical assistance, and Dr. J ean Paul Leresche (Fir-
menich S.A), Dr. Lucca Giannini, and Prof. Carlo
Floriani (University of Lausanne) for helpful discus-
sions.
Su p p or tin g In for m a tion Ava ila ble: Su p p or tin g In -
for m a tion Ava ila ble: Table 10 (dehydrogenative silylation
of alcohols by PMHS) and full Experimental Section (synthesis
(40) Mimoun, H. International Application PCT/1B98/01378 (Sept
3, 1998) to Firmenich S.A.
(41) Crabbe, P.; Garcia, G. A.; Rius, C. J . Chem. Soc., Perkin Trans.
1 1973, 810.
(42) (a) Bell, N. A.; Coates, G. E. J . Chem. Soc. A 1968, 823. (b)
Goeden, G. V.; Caulton, K. G. J . Am. Chem. Soc. 1981, 103, 7354.
(43) De Koning, A. J .; Boersna, J .; Van Der Kerk, G. J . M. J .
Organomet. Chem. 1980, 195, 1.
(44) Bell, N. A.; Moseley, P. T.; Shearer, H. M. M.; Spencer, C. B. J .
Chem. Soc., Chem. Commun. 1980, 359.
1
of 4b, 18b, 23b, 31b, 37-43, reduction of peanut oil; H and
13C NMR spectra of new compounds 37-43 and jojoba alcohols;
MS spectra of C18:1, C20:1, C22:1, and C24:1). This material is
available free of charge via the Internet at http://pubs.acs.org.
J O982314Z