tion Fund, and a grant to RCAST at Doshisha University from
the Ministry of Education, Japan.
Notes and references
†
Typical procedure for reduction of acetals by phenylsilane in the
presence of RhCl(PPh3)3: in a 20 mL Schlenk tube was dissolved
RhCl(PPh3)3 (23 mg, 0.025 mmol) in THF (1 mL). To this solution were
added 2-phenyl-1,3-dioxane (1) (0.5 mmol) and PhSiH3 (2) (0.19 mL, 1.5
mmol), and the solution was stirred at room temperature for 48 h. Dibenzyl
(91 mg, 0.5 mmol, as an internal standard), THF (5 mL), and 1 M NaOH aq.
(5 mL) were added, and the mixture was stirred for 2 h. A mixture of the
products and dibenzyl was obtained by extraction with diethyl ether
followed by concentration. The yield was determined by 1H NMR
spectroscopic analysis using an internal standard method.
reductant. In order to getting high selectivity for 4, the highest
active catalyst RhH(CO)(PPh3)3 was used, and several reaction
conditions were changed. When the amount of catalyst was
decreased to 2.0 mol% from 5.0 mol%, conversion was not
changed but the yield of 4 increased to 79% from 20%. Finally,
effect of the addition of phosphines for this reduction was
investigated (Table 2).
Procedure for reduction of trimethyl orthobenzoate (3) by
RhH(CO)(PPh3)3 in the presence of P(4-CH3OC6H4)3: in to a 20- mL
Schrenk tube were added RhH(CO)(PPh3)3 (0.005 mmol), phosphine (0.025
mmol), trimethyl orthobenzoate (3) (0.5 mmol), and THF (1.0 mL). Then,
to this mixture were added dibenzyl (91 mg, 0.5 mmol) and PhSiH3 (1.5
mmol), and the mixture was stirred at 60 °C for 24 h. After the reaction, THF
(5 mL) and 1 M NaOH aq. (5 mL) were added, and the mixture was stirred
for 2 h. The mixture was extracted, and the organic layer was dried over
Na2SO4 followed by concentration to give crude product. This mixture was
analyzed by 1H NMR measurement.
Table 2 The effect of additional phosphines on the reduction of trimethyl
orthobenzoate (3)
Entry
Phosphine–3 (mol%)
Conv. (%)b
1j (%)b
4 (%)b
1
2
3
4
6d
7d,e
None
PPh3 (10)
P(4-C6H4F)3 (10)
P(4-C6H4OMe)3 (10)
P(4-C6H4OMe)3 (5)
None
68
87
81
100
99
34
59
80
70
91c
93
29
7
7
8
9
7
4
1 S. A. Godleski, in Comprehensive Organic Synthesis, ed. B. M. Trost and
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2 K. Tamao, in Comprehensive Organic Synthesis, ed. B. M. Trost and I.
Fleming, Pergamon, Oxford, 1991, vol. 3, , 435; D. W. Knight, in
Comprehensive Organic Synthesis, ed by B. M. Trost and I. Fleming,
Pergamon, Oxford, 1991, vol. 3, , 481.
a A mixture of RhH(CO)(PPh3)3 (0.005 mmol), PhSiH3 (1.50 mmol),
phosphine (0.025–0.100 mmol), trimethyl orthobenzoate (3) (0.5 mmol) in
THF (1.0 mL) was stirred at 60 °C for 48 h. b Determined by 1H NMR.
c Contains benzaldehyde (4%). d RhH(CO)(PPh3)3 (0.0025 mmol) was
used. e 96 h.
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72, 799; K. Nagayama, F. Kawataka, M. Sakamoto, I. Shimizu and A.
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Adding 10 mol% of PPh3 to the reaction of 3 with 3 eq. of
PhSiH3 in the presence of 1 mol% of RhH(CO)(PPh3)3 at 60 °C
for 48 h increased the conversion of 3 by 19% with a 21%
increase of 4 compared with no additive. Then various
phosphines were tested as an additive for this reaction. Addition
of P(4-CH3OC6H4)3 showed better result than PPh3 and P(4-
C6H4F)3, while addition of other monophosphines (PMePh2,
P(o-Tol)3) and bidentate phosphines (dppe, dppb, BINAP)
resulted in lower conversions. Using 0.5 mol% of
RhH(CO)(PPh3)3 with 5 mol% of this phosphine resulted in
almost quantitative conversion with 93% of 4 and 7% of 5
formed. This illustrates the dramatic effect of adding phosphine
compared with no addition of phosphine (only 34% conversion
for twice as long reaction time). Although the reason is not clear
yet, addition of phosphine may accelerate the reductive
elimination step in the catalytic cycle.
The rhodium–silane reduction system is effective for the
reductive cleavage of sp3-carbon sp3-oxygen bonds. Acetals
were reduced to ethers in up to 98% yield using PhSiH3 in the
presence of RhCl(PPh3)3. On the other hand, trimethyl
orthobenzoate was reduced to benzaldehyde dimethyl acetal in
93% yield by the same silane in the presence of 0.5 mol% of
RhH(CO)(PPh3)3 and 5 mol% of P(4-CH3OC6H4)3. This is the
first example of homogenous catalytic reduction of acetals and
orthoesters using hydrosilane as a reducing reagent in the
presence of a rhodium complex. Consequently, the method in
this report is thought to be a new route for the synthesis of
ethers, acetals and aldehydes under mild conditions.
We are grateful to Dr Takayuki Yamashita for helpful
discussions during the course of this work. This work was
partially supported by Doshisha University’s Research Promo-
9 T. Ohta, M. Kamiya, K. Kusui, T. Michibata, M. Nobutomo and I.
Furukawa, Tetrahedron Lett., 1999, 40, 6963.
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