Chemoselective ruthenium-catalyzed reduction of acid chlorides to aldehydes with dimethylphenylsilane

Article abstract of DOI:10.1002/adsc.201100693

A variety of aromatic and alkyl acid chlorides can be selectively converted into aldehydes using dimethylphenyl silane (HSiMe₂Ph) as the reducing reagent in the presence of the cationic ruthenium catalyst {Cp[(i-Pr)₃P]Ru(NCMe)₂}⁺ [PF₆] ^-. The reactions proceed under very mild conditions and are tolerant to many functional groups. Copyright

Full text of DOI:10.1002/adsc.201100693

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DOI: 10.1002/adsc.201100693
Chemoselective Ruthenium-Catalyzed Reduction of Acid
Chlorides to Aldehydes with Dimethylphenylsilane
Dmitry V. Gutsulyaka and Georgii I. Nikonova,*
a
Chemistry Department, Brock University, 500 Glenridge Ave., St. Catharines, ON L2S 3A1, Canada
Fax : (+1)-905)-682-9020; phone: (+1)-905-688-5550 ext 3350; e-mail: gnikonov@brocku.ca
Received: September 13, 2011; Revised: November 4, 2011; Published online: February 23, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100693.
Abstract: A variety of aromatic and alkyl acid
chlorides can be selectively converted into alde-
hydes using dimethylphenyl silane (HSiMe2Ph) as
the reducing reagent in the presence of the cationic
lizes a Pd catalyst and fails for electron-poor benzoyl
chlorides and aliphatic acid chlorides.
We have recently reported the Ru-catalyzed hydro-
silylation of carbonyls[14] and developed methods for
the chemoselective hydrosilylation of nitriles[5c] and
pyridines.[15] Here we extend these studies towards
the reduction of acid chlorides by silanes, which is (i)
chemoselective towards the formation of aldehydes,
(ii) works equally well for aromatic and aliphatic sub-
strates, (iii) goes under mild conditions, (iv) shows ex-
cellent tolerance to the presence of several common
functional groups, (v) works “on bench” and (vi) the
catalyst is recyclable.
ruthenium
catalyst
(NCMe)2}+
{CpACTHNGUTERNNU[G (i-Pr)3P]RuACHTGNUTRENNUNG
ACHTUNGTRENNUNG
ditions and are tolerant to many functional groups.
Keywords: acid chlorides; chemoselectivity; hydro-
silylation; ruthenium
+A[PF6]À (1) ef-
Complex {CpACHTNUGTREN[NNUG (i-Pr)3P]RuACHTNURGTEG(NUNN NCCH3)2} CHTNGUTRENNGUN
Reduction of carbonyl substrates is a fundamental or- fectively catalyzes the chemoselective reduction of
ganic reaction allowing for their interconversion (e.g., a variety of acid chlorides to the corresponding alde-
esters to aldehydes) and preparation of a variety of hydes by using HSiMe2Ph as the reducing agent
other organic products (e.g., alcohols from ketones).[1] (Table 1).[16] The use of t-BuCN (10–20% mol) is cru-
Reductions of aldehydes, ketones and imines by si- cial for the success of this reaction, as this nitrile sta-
lanes have been in the spotlight for a long period of bilizes the catalyst without concomitant nitrile hydro-
time, whereas the development of catalytic reduction silylation.[15,17] The reaction proceeds in various sol-
methods for less reactive substrates has received at- vents (CH2Cl2, acetone, acetonitrile[18]), with acetone
tention only recently.[2–6] On the contrary, acid chlor- being the best. Under these conditions, PhCOCl can
ides are very reactive compounds but their transfor- be quantitatively reduced to PhCHO within only 1 h
mation into aldehydes presents a serious chemoselec- at room temperature (Table 1, entry 1). Gratifyingly,
tivity problem. In addition to the classical Rosenmund we observed that aliphatic acyl chlorides (entries 2–8)
reduction by hydrogen, acid chlorides are usually con- also can be reduced as easily as aromatic derivatives
verted to aldehydes by different alumo- and borohy- (entries 1 and 10–13). Chlorine substituents in the a-
drides (such as DIBALH),[1,7] tin hydrides,[8] transition and b-positions are tolerated and the corresponding
metal hydrides,[9] or active metals.[10] These methods aldehydes were obtained with high selectivity (en-
have serious issues of cost, toxicity, safety, sensitivity tries 5–7), but only traces of aldehyde are observed
to reaction conditions and/or incompatibility with for the more reactive bromide derivative
functional groups. In this regard, catalytic reduction BrCH2CH2COCl (entry 8). The less substituted sub-
by silanes, which have little toxicity, are air-stable and strates (entries 2 and 3) react faster than sterically
cheap, is a very attractive but surprisingly little stud- loaded ones (entry 4). The reduction of some sub-
ied alternative.[11–13] Earlier studies with group 9 and strates containing a-protons resulted in the formation
10 metals required harsh conditions,[12] but recently of a mixture of target aldehydes and silyl enols (e.g.,
Maleczka et al. reported the PMHS reduction of entries 2 and 7). But the latter compounds can be
a series of aromatic acid chlorides to aldehydes under easily converted into aldehydes by a simple aqueous
mild conditions.[12e] However, the latter method uti- work-up.[19]
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Dmitry V. Gutsulyak and Georgii I. Nikonov
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Table 1. Catalytic hydrosilylation of acid chlorides with HSiMe2Ph.[a]
Entry
Substrate
Conversion[b] (time)
Products (yield)[b}
1
2
3
4
5
6
7
8
C6H5COCl
CH3COCl
CH3CH2COCl
(CH3)3CCOCl
ClCH2COCl
CH3CHClCOCl
ClCH2CH2COCl
BrCH2CH2COCl
PhCH=CHCOCl
4-BrC6H4COCl
4-O2NC6H4COCl
4-MeOC6H4COCl
100% (1 h)
>90% (2 h)
100% (1 h)
100% (4 h)
100%[c] (1 h)
100% (24 h)
100% (5 h)
80% (24 h)
70%[c] (18 h)
100% (3 h)
90%[c] (5 h)
>90% (24 h)
C6H5CHO (~100%)
CH3CHO (70%); CH2=CHOSiMe2Ph (20%)
CH3CH2CHO (~100%)
ACHTUGNTRENN(UNG CH3)3CCHO (~100%)
ClCH2CHO (90%)
CH3CHClCHO (95%)
ClCH2CH2CHO (80%); ClCH=CHOSiMe2Ph (20%)
BrCH2CH2CHO (traces)
PhCH=CHCHO (18%); PhCH2CH=CHOSiMe2Ph (82%)
4-BrC6H4CHO (~100%)
4-O2NC6H4CHO (85%)
9
10
11
12
4-MeOC6H4CHO (83%); 4-MeOC6H4CH2OSiMe2Ph (17%)
13
14
100% (20 h)
100%[c,d] (3 h)
15
16
>97% (24 h)
100% (24 h)
17
100%[c,d,e] (24 h)
100%[c,f] (20 h)
18
EtOOC-COCl
EtOOCCHO (65%)
[a]
In a general procedure to a solution of acid chloride in acetone-d6 was added 1.5 equiv. of HSiMe2Ph, 10–20 mol% of
t-BuCN, and 5% mol of 1 at room temperature.
Based on H NMR data.
2 equiv. of CH3CN were added instead of t-BuCN.
2.5 equiv. of HSiMe2Ph were added.
Conversion of silane is given.
[b]
[c]
[d]
[e]
[f]
1
Reaction in chloroform.
Chemoselectivity is lost, however, in the case of poor 2,6-pyridinedicarbonyl chloride is converted into
a
conjugated acid chloride, PhCH=CHCOCl a mixture of the corresponding mono- and bis-pyrid-
(entry 9), as a mixture of the corresponding aldehyde
ACHTUNGTRENiNUGN necarboxaldehydes, with the 2,6-bis-pyridinecarboxal-
(13%) and the product of the formal silane 1,4-addi- dehyde becoming the predominant product (~80%)
tion to the aldehyde (57%) is observed. Interestingly, only after 3 h at room temperature (entry 14). In con-
the hydrosilylation of PhCH=CHCHO under similar trast, the reduction of electron-rich furan and thio-
conditions is very sluggish, which suggests that phene derivatives goes to completion after 24 h (en-
PhCH2CH=CHOSiMe2Ph does not stem from the tries 15 and 16). Surprisingly, the course of reduction
direct addition of silane to aldehyde.
of pyridine substrates was sensitive to the position of
Reduction of aromatic acid chlorides with electron- the COCl group in the ring, as the reaction of a 3-sub-
withdrawing groups (entries 10 and 11) proceeds as stituted derivative gave only traces of the aldehyde
smoothly as the reduction of electro-neutral sub- product (entry 17), but this reaction might be compro-
strates (entries 1 and 13), and the reactive nitro group mised by the presence of an excess amount of HCl in
is tolerated (entry 11). In contrast, the reduction of an the reaction mixture. Importantly, the ester function-
electron-rich substrate, with a donating MeO group ality in the reduction of EtOOC-COCl is tolerated
(entry 12), is much slower and some loss of chemose- (entry 18).
lectivity is observed, leading to a mixture of the
target aldehyde (83%) and its hydrosilylation product method, the reaction of 4-bromobenzoyl chloride with
MeOC6H4CH2OSiMe2Ph (17%). HSiMe2Ph was studied in the presence of other poten-
To establish further the selectivity of this reduction
A similar reactivity pattern is observed in the reac- tially reactive compounds, such as alkenes, alkynes,
tions of heteroaromatic acid chlorides. The electron- esters, etc. (Table 2). To our delight, the presence of
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Chemoselective Ruthenium-Catalyzed Reduction of Acid Chlorides to Aldehydes with Dimethylphenylsilane
Table 2. Reduction of p-BrC6H4COCl with HSiMe2Ph in the presence of other substrates.[a]
Entry
Substrate 1
Substrate 2
CH3A(CH2)3CH=CH2
Conversion[b]
1
2
3
4
G
Substrate 1:~100%; Substrate 2: 0%
Substrate 1: 90%; Substrate 2: 0%
Substrate 1: 50%; Substrate 2: 50%
Substrate 1: 50%; Substrate 2: 5%
AcOEt
ꢀ
EtC CEt
ꢀ
PhC CH
[a]
In a general procedure to a solution of acid chloride and substrate 2 in acetone-d6 was added 1.5 equiv. of HSiMe2Ph,
2 equiv. of CH3CN, and 5 mol% of 1.
Based on H NMR data.
[b]
1
hex-1-ene or ethyl ester does not hamper the course thylpiperidine 1-oxyl), a known radical scavenger, did
of reduction of 4-BrC6H4COCl (entries 1 and 2). On not affect the rate of reduction of t-BuCOCl. When
the other hand, the hydrosilylation of an internal a 1:1:1 reaction of 4-BrC6H4COCl, HSiMe2Ph, and
ꢀ
triple bond C C of alkyne proceeds as fast as the 1 was followed by VT NMR, a noticeable reaction
reduction of the COCl group, and a mixture of was observed at À258C with the formation of
(NCCH3)(h2-HSiMe2Ph)}+, a known
EtCH=CACHTUNGTRENNUNG(SiMe2Ph)Et (50%) and 4-BrC6H4CHO {CpAHCTUNRTGEG[NUNN (i-Pr)3P]RuCAHTNUGTRENNGUN
(50%) was observed in the presence of hex-3-yne complex.[14] However, further gentle increase of tem-
(entry 3). Surprisingly, no hydrosilylation of the termi- perature to 08C results in the fast production of alde-
ꢀ
nal triple bond of PhC CH was observed. Unfortu- hyde and no further intermediates were detectable.
nately, the latter compound poisons the catalyst, and To date, the exact mechanism of this catalytic reaction
the reduction of 4-BrC6H4COCl stops at only 50% remains unknown.
conversion (entry 4).
In conclusion, we have discovered a new conven-
In order to check the practicability of this reduction ient and general method for the chemoselective re-
method, a reaction with a representative acid chloride duction of acid chlorides to aldehydes which uses
was performed on a preparative scale. Thus, the reac- a non-toxic silane HSiMe2Ph, operates under mild
tion of 4-BrC6H4COCl (0.5 g) with HSiMe2Ph in the conditions, works equally well for aliphatic, aromatic
presence of 1 (5 mol%) and t-BuCN (10 mol%) af- and heteroaromatic substrates, tolerates several im-
fords the corresponding aldehyde in the 80% isolated portant functional groups, works on bench, is scalable,
yield after recrystallization from hexanes. This proce- and allows for catalyst recycling. Finally, the catalyst
dure does not require a special set-up and can be per- has a good shelf life in air and can be easily assem-
formed on the bench in a flask pre-flushed with inert bled prior to use from commercially available materi-
gas. Importantly, the catalyst is recyclable and can be als.
easily separated from products by precipitation with
hexane. Although there is a slow decomposition of
the catalyst during the reaction, most of it can be re- Experimental Section
cycled at least five times without any significant de-
crease in activity.
All manipulations were carried out using conventional high-
vacuum or nitrogen-line Schlenk techniques. NMR spectra
were recorded on Bruker (1H, 300 MHz; 13C, 75.4 MHz)
and/or Bruker (1H, 600 MHz; 13C, 150.8 MHz) spectrome-
ters. All chemicals were purchased from Sigma–Aldrich,
apart from HSiMe2Ph which was purchased from Gelest.
These reagents were used without purification. NMR sol-
vents were from Cambridge Isotope Laboratories. CDCl3
was dried over CaH2 and acetone-d6 was dried over molecu-
lar sieves (3ꢁ). Other solvents were dried by distillation
from appropriate drying agents or using a Grubbs-type sol-
vent purification system. Complex 1 was prepared according
to the literature procedure.[20]
For the hydrosilylation of pyridine and nitriles cata-
lyzed by 1 we have previously considered an ionic
mechanism, based on nucleophilic abstraction of a sil-
ACHTUNGTRENNUNG
ylium ion by the nitrogen donor.[5c,15] Although acid
chlorides are weaker nucleophiles, we were also puz-
zled by the observation that their reduction proceeds
at rates much slower than the hydrosilylation of nitri-
les,[5c] but nevertheless aldehydes can be obtained in
the presence of acetonitrile. To address this paradox,
we first considered the possibility that the chloride
may be reduced by the initial product of nitrile hydro-
silylation, the imine RHC=NSiMe2Ph. However, the
latter was found to react with acid chlorides to give
acyl imines R’C(O)N=CHR, which then undergo re-
duction of the imine group. Another possibility, a radi-
cal mechanism, was then ruled out as the addition of
General Procedure for Catalytic Reaction of Acid
Chlorides
In a representative procedure, to a solution of acid chloride
stoichiometric amounts of TEMPO (2,2,6,6-tetrame- (0.20 mmol), HSiMe2Ph (0.030 mL, 0.20 mmol), internal
Adv. Synth. Catal. 2012, 354, 607 – 611
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Dmitry V. Gutsulyak and Georgii I. Nikonov
COMMUNICATIONS
standard (TMS or Cp2Fe), and t-BuCN (0.002–0.004 mL, 10–
20 mol%) in acetone-d6 (0.6 mL) was added complex
1 (0.005 g, 5 mol%). The resulting mixture was stirred at
room temperature and the progress of the reaction was
monitored by NMR spectroscopy.
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Representative Example of a Preparative-Scale
Reduction with Catalyst Recycling
To
a solution of 4-BrC6H4COCl (0.500 g, 2.28 mmol),
HSiMe2Ph (0.400 mL, 2.61 mmol), and t-BuCN (0.025 mL,
10 mol%) in CH2Cl2 or acetone (30 mL) was added complex
1 (0.065 g, 5 mol%). The resulting mixture was stirred at
room temperature. Full conversion of the acid chloride was
observed within 3 h (in acetone) or 1 day (in CH2Cl2). To
the resulting mixture was added hexane (30 mL) and the so-
lution was concentrated to 5 mL under vacuum. The prod-
ucts were extracted with hexane (3ꢂ10 mL). The remaining
catalyst was used again (4 times) with the same amounts of
the starting reagents. The aldehyde was recrystallized each
time from hexane solutions at À808C. Yields: ~70% (reac-
tions in CH2Cl2) or ~90% (reactions in acetone).
Acknowledgements
This work was supported by NSERC. We are grateful to the
CFI/OIT for a generous equipment grant.
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