Mild partial deoxygenation of esters catalyzed by an oxazolinylborate-coordinated rhodium silylene

Article abstract of DOI:10.1039/c5dt02844b

An electrophilic, coordinatively unsaturated rhodium complex supported by borate-linked oxazoline, oxazoline-coordinated silylene, and N-heterocyclic carbene donors [{κ³-N,Si,C-PhB(Ox^Me2)(Ox^Me2SiHPh)Im^Mes}Rh(H)CO][HB(C₆F₅)₃] (2, Ox^Me2 = 4,4-dimethyl-2-oxazoline; Im^Mes = 1-mesitylimidazole) is synthesized from the neutral rhodium silyl {PhB(Ox^Me2)₂Im^Mes}RhH(SiH₂Ph)CO (1) and B(C₆F₅)₃. The unusual oxazoline-coordinated silylene structure in 2 is proposed to form by rearrangement of an unobserved isomeric cationic rhodium silylene species [{PhB(Ox^Me2)₂Im^Mes}RhH(SiHPh)CO][HB(C₆F₅)₃] generated by H abstraction. Complex 2 catalyzes reductions of organic carbonyl compounds with silanes to give hydrosilylation products or deoxygenation products. The pathway to these reactions is primarily influenced by the degree of substitution of the organosilane. Reactions with primary silanes give deoxygenation of esters to ethers, amides to amines, and ketones and aldehydes to hydrocarbons, whereas tertiary silanes react to give 1,2-hydrosilylation of the carbonyl functionality. In contrast, the strong Lewis acid B(C₆F₅)₃ catalyzes the complete deoxygenation of carbonyl compounds to hydrocarbons with PhSiH₃ as the reducing agent.

Full text of DOI:10.1039/c5dt02844b

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Mild partial deoxygenation of esters catalyzed by
an oxazolinylborate-coordinated rhodium silylene†
Cite this: Dalton Trans., 2015, 44,
15897
Songchen Xu, Jeffery S. Boschen, Abhranil Biswas, Takeshi Kobayashi, Marek Pruski,
Theresa L. Windus and Aaron D. Sadow*
An electrophilic, coordinatively unsaturated rhodium complex supported by borate-linked oxazoline, oxazo-
line-coordinated silylene, and N-heterocyclic carbene donors [{κ3-N,Si,C-PhB(OxMe2)(OxMe2SiHPh)ImMes}-
Rh(H)CO][HB(C6F5)3] (2, OxMe2 = 4,4-dimethyl-2-oxazoline; ImMes = 1-mesitylimidazole) is synthesized
from the neutral rhodium silyl {PhB(OxMe2)2ImMes}RhH(SiH2Ph)CO (1) and B(C6F5)3. The unusual oxazoline-
coordinated silylene structure in 2 is proposed to form by rearrangement of an unobserved isomeric
cationic rhodium silylene species [{PhB(OxMe2)2ImMes}RhH(SiHPh)CO][HB(C6F5)3] generated by H abstrac-
tion. Complex 2 catalyzes reductions of organic carbonyl compounds with silanes to give hydrosilylation
products or deoxygenation products. The pathway to these reactions is primarily influenced by the degree
of substitution of the organosilane. Reactions with primary silanes give deoxygenation of esters to ethers,
amides to amines, and ketones and aldehydes to hydrocarbons, whereas tertiary silanes react to give
1,2-hydrosilylation of the carbonyl functionality. In contrast, the strong Lewis acid B(C6F5)3 catalyzes the
complete deoxygenation of carbonyl compounds to hydrocarbons with PhSiH3 as the reducing agent.
Received 25th July 2015,
Accepted 28th July 2015
DOI: 10.1039/c5dt02844b
www.rsc.org/dalton
alcohols through C–O bond cleavage, whereas hydrodeoxy-
genation to alkanes through complete C–O hydrogenolysis is
Introduction
Partial reductions are an important part of an overall strategy obtained under more forcing conditions. Alternatively, the con-
for conversions of oxygenated materials and are germane to version of esters to ethers through selective partial reduction
conversions of biorenewables. Such transformations can be could be valuable in synthetic schemes and in biorenewable
considered the counterpart to selective partial oxidation of conversions.
hydrocarbons typically sought for petrochemical conversions.
The conversion of cyclic esters to ethers may be accom-
Selectivity for partial oxidation or partial reduction products plished through stoichiometric or multistep methods, while
is a key longstanding challenge in catalysis. Organometallic transformations of acyclic esters have been more challenging.7–9
complexes and homogeneous catalytic systems can provide The seminal report of mid-transition-metal-catalyzed partial
insight into active sites to guide downstream applications and ester reduction involves hydrosilylation and gives a mixture of
strategies for the development of mild conversions. In this ethers and alkoxysilanes.10 Group 8-based multimetallic carbo-
regard, complete deoxygenation vs. partial deoxygenation of nyl compounds were more recently reported to catalyze ether
sugars and protected sugars into hydrocarbons may be formation using trialkylsilanes (Ru) or tetramethyldisiloxane
affected by secondary vs. tertiary silanes, respectively, in the (Fe).11 These catalysts require heat or photochemical activation
presences of strong Lewis acid catalysts.1 Similar control in for catalytic conversion. Other metal precursors divert the pro-
metal-catalyzed hydrosilylation and hydrodeoxygenation cata- ducts to the more common ester reductive cleavage12 or partial
lysis has focused on catalyst site design2–5 or tailored oxygenated reduction to aldehydes.13 Titanium complexes also catalyze
substrates.6
hydrosilylations to give ester cleavage or partial reduction of
The ester group, as a common functionality in biorenew- lactones to lactols, depending on catalytic conditions and
able feedstocks, fuels, chemicals, and materials, typically silane reductant.14 Alternatively, gallium bromide15 and
reacts under reducing conditions to give alcohols or protected indium bromide16 Lewis acid catalysts employ tertiary silanes
or disiloxanes for conversions of esters to ethers, whereas
B(C6F5)3 catalyzes hydrosilylation to give silyl acetals.17
Catalytic intermediates in the above transition-metal
systems are ambiguous, whereas the Lewis acid-based silane
U.S. Department of Energy Ames Laboratory and Department of Chemistry, Iowa
State University, 1605 Gilman Hall, Ames, Iowa 50011, USA
†Electronic supplementary information (ESI) available: Experimental descrip-
activations have been established by kinetics and isolation of
tion of catalysis, solution-phase and solid-state NMR spectra, and computational
models of proposed intermediates.17b,18 Organometallic-like
results. See DOI: 10.1039/c5dt02844b
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intermediates in the above pathways may include metal silane
X-ray quality single crystals of 2 are not yet available.
compounds19 and metal silylene compounds.20 Metal-co- In addition, attempts to generate 2 as [B(C6F5)4]− or
ordinated silylenes are known to have Lewis acid character21 [B{C6H3(CF3)2}4]−-counterion containing complexes from reac-
and are proposed as catalytic intermediates in the hydrosilyl- tions of 1 and [Ph3C][B(C6F5)4] or [(Et2O)2H][B(C6H3(CF3)2)4]
ation of carbonyls and olefins.20,22 In addition, iridium com- did not provide pure samples. Despite this, the characteri-
pounds with bridging silylenes [Ir(μ-SiEt2)(H)2SiHEt2]2,23 and zation of 2 as an unusual oxazoline-coordinated silylene
iridium-coordinated N-heterocyclic silylenes24 are catalysts for complex of rhodium is supported by extensive and unambigu-
silane-mediated reduction of amides to amines. The related ous spectroscopic and computational analysis, as described
catalytic reactions were not reported with rhodium. Migrations below. The 1H NMR spectra of 2 in bromobenzene-d5, tetra-
of chloride from rhodium to a transient unsaturated silicon hydrofuran-d8, chloroform-d1 or methylene chloride-d2 provide
center (as well as hydride and methyl migrations from silicon evidence for the same species, and solid state NMR spectro-
to rhodium) are proposed in formation of rhodium disilacyclo- scopy also supports the assignment. The discussion below
butanes.25 However, dirhodium silylene dimers were ruled out focuses on data acquired in bromobenzene-d5.
1
as intermediates in silane redistribution and dehydrocoupling
The H NMR spectrum of the isolated product 2 contained
reactions,26 and the presence of a donor as a base-stabilized resonances assigned to one RhH and one SiH, as well as a
silylene or in the form of a bimetallic compound can diminish pattern of PhB(OxMe2)2ImMes signals that indicated overall C1
reactivity. Nonetheless, the Lewis acid character of silylenes symmetry for the product and a set of phenyl resonances from
may provide a mechanistic connection to the B(C6F5)3, GaBr3, the silyl moiety. In the isolated material, the rhodium hydride
and InBr3 conversions. For example, rhodium-coordinated sily- resonance was observed as
a doublet of doublets at
3
lenes are invoked as intermediates in a cationic carbene-oxazo- −12.92 ppm (1JRhH = 20.0 Hz, JHH = 4.0 Hz). Silicon satellite
line rhodium(I)-catalyzed carbonyl hydrosilylation.20a,27,28 signals were not detected for the rhodium hydride resonances
However, the dearth of isolated rhodium-coordinated silylenes in either 1 or 2, and from this we rule out Rh↽H–Si or Rh–
is notable.24,29
H⇀Si interactions. A virtual triplet at 5.15 ppm (J = 4.0 Hz)
We recently synthesized a zwitterionic rhodium(III) silyl was assigned to the single SiH, with the splitting resulting
3
2
hydride containing the mixed carbene–oxazoline borate ligand from JHH and JRhH coupling of similar magnitude. The
{PhB(OxMe2)2ImMes}RhH(SiH2Ph)CO (1, OxMe2 = 4,4-dimethyl- assignment of the former coupling is supported by a crosspeak
2-oxazoline; ImMes = 1-mesitylimidazole).30 Neither 1 nor its in a COSY experiment. The SiH signal was further downfield
synthetic precursor {PhB(OxMe2)2ImMes}Rh(CO)2 show any cata- and exhibited a greater 1JSiH coupling constant (1JSiH = 234 Hz)
lytic hydrosilylation activity in the presence of ethyl acetate than the diastereotopic SiHs in the rhodium starting material
and phenylsilane. Therefore, we attempted to enhance the 1, which were observed as doublets at 4.23 and 4.68 ppm
1
reactivity of 1 by reaction with B(C6F5)3 to form a cationic (2JHH = 6.0 Hz, JSiH = 170 and 188 Hz, 1 H each). The increase
complex. The resulting isolated oxazoline-coordinated in one-bond coupling constant is typically attributed to greater
rhodium silylene catalyzes the deoxygenation of esters to s-character in the Si–H bond, which results from the interaction
ethers, amides to amines, and ketones to alkanes in the pres- of the silicon with a substituent of greater effective electro-
ence of primary organosilanes as the reducing agent and negativity than H. An EXSY experiment did not reveal cross-
oxygen-acceptor. These catalytic dexoygenations contrast the peaks between RhH and SiH signals.
expected addition chemistry of silanes and carbonyls typically
In addition to the hydride, the rhodium coordination
observed with rhodium complexes.31 Herein we describe the sphere of 2 includes carbene and carbonyl ligands. The 13C
characterization of the internal base-stabilized silylene catalyst {1H} NMR spectrum provided unambiguous evidence for
and its reactivity in carbonyl deoxygenation.
coordination of these ligands through two doublets at 186.33
(1JRhC = 51 Hz, NHC) and 169.23 ppm (1JRhC = 37.5 Hz, CO).
The rhodium coordination sphere also includes a silicon-
1
based ligand, assigned on the basis of the JRhSi value of 42
Hz. The 29Si NMR chemical shift of 6.5 ppm is consistent with
a four-coordinate silicon center. To validate the chemical shift
measured in solution, we measured a 29Si NMR spectrum of 2
in the solid state, using 1H → 29Si cross-polarization under
magic angle spinning (CPMAS), which similarly featured a
single resonance centered at 6 ppm (see Fig. S6 in the ESI†).
Results and discussion
The reaction of {PhB(OxMe2)2ImMes}RhH(SiH2Ph)CO (1) and
B(C6F5)3 in benzene at room temperature produces a dark oily
precipitate 2 (eqn (1)), and this material is purified to a black
solid by washing with benzene and pentane.
1
The MAS H NMR spectrum also contained an upfield signal
at ∼−13 ppm assigned to the RhH.
The fifth group bonded to rhodium is an oxazoline. Corre-
1
lations between 15N and H NMR resonances assigned to 4,4-
ð1Þ
dimethyl-2-oxazoline provided two 15N NMR chemical shifts of
−199 and −167 ppm for the two inequivalent groups. These
chemical shifts indicate that both oxazolines are coordinated
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in 2, because the chemical shift of non-coordinated 4,4- through low temperature NMR acquisition gave broad
dimethyl-2-oxazoline is −128 ppm. In the precursor 1, 15N signals, further hindering analysis by 13C, 15N, and 29Si NMR
NMR chemical shifts of −161 and −172 ppm were observed for spectroscopic methods. In the absence of added CO,
rhodium-coordinated oxazolines trans to SiH2Ph and hydride, [{PhB(OxMe2)2ImMes}Rh(H)SiHPh(CO)2]+ is not detected in the
respectively. One of these signals shifts 30 to 40 ppm upfield 1H NMR spectra of 2. Thus, we conclude that the CO transfer
in the cationic compound 2. Significantly, a correlation in 2 is a feature specific to the mass spectral analytical method.
1
between the upfield 15N signal at −199 ppm and the H NMR
Contrary to the parent ion signal for 2 in bromobenzene,
signal assigned to the SiH indicates that a N–Si bond has the mass spectrum of 2 dissolved in chloroform provided a
formed in 2. The remaining oxazoline that appeared at signal at 797.28 m/z. This mass is consistent with the formu-
−167 ppm was assigned to the fifth ligand on rhodium. With lation [{PhB(OxMe2)2ImMes}RhSiH2(C6F5)(CO)]+ resulting from
the exception of the oxazoline, the coordination of H, SiHPh, C6H5/C6F5 exchange under MS data acquisition conditions.
1
NHC, and CO to rhodium is unambiguously established by There is no evidence for this rearrangement in H, 19F, or 11B
1JRhE coupling, and silylene insertion into rhodium-hydride, NMR spectra of 2 in chloroform-d1, and mass spectrum
-carbonyl or -N-heterocyclic carbene donor, rather than the acquired in negative ion mode contained signals from only
1
oxazoline, are ruled out. At the same time, these NMR data [HB(C6F5)3]−. Moreover, the H NMR spectra of isolated 2 dis-
indicate that the four groups bonded to the silicon center are solved in chloroform-d1, methylene chloride-d2, and bromo-
hydrogen, phenyl, rhodium, and an oxazoline.
benzene-d5 contained similar signals in terms of chemical
Characterization of 2 by 11B and 19F NMR spectroscopy pro- shifts and coupling constants, and all three solvents provided
vided additional support for hydride abstraction. Two reson- catalytically active species. The NMR signals of a bromo-
ances were observed in the 11B NMR spectrum at −8 and benzene solution of 2 are maintained at 80 °C for 72 h, although
−25 ppm assigned to the PhB(OxMe2)2ImMes ligand and [HB- a small amount of black precipitate and aliphatic signals associ-
(C6F5)3]−, respectively. The latter signal was broad (400 Hz ated with decomposition products were observed. In addition,
1
peak width at half-height), and the JBH was not resolved. The solutions of 2 persist at room temperature for greater than
chemical shift of −25 ppm is consistent with a four-coordinate 1 month.
anionic borate center.32 Three 19F NMR resonances at −132.08,
DFT calculations of 15N and 29Si NMR chemical shifts for
−162.79, and −165.72 ppm were distinguished from those of gas-phase model compounds 1-A, a plausible intermediate
B(C6F5)3 in bromobenzene-d5, which appeared at −126.93, [{PhB(OxMe2)2ImMes}Rh(vSiHPh)(H)CO]+ (B), and oxazoline-
−141.38, and −158.88 ppm. Moreover, a similar ortho-F signal coordinated silylene (2-A) provide additional support for the
for [HB(C6F5)3]− was reported for [Cp*2ZrH][HB(C6F5)3].33 The proposed structure of 2 and a possible pathway for its for-
formation of [HB(C6F5)3]− was further supported by mass spec- mation via the coordinatively unsaturated rhodium silylene
tral analysis in negative ion mode, which provided spectra (Fig. 1). The calculations also provide additional evidence
dominated by a peak at 512.9 m/z in bromobenzene or ruling out an unsaturated silicon center in 2. The calculated
chloroform.
atomic coordinates (1-A) and the coordinates of 1 determined
Mass spectrometry of 2 in bromobenzene, in positive by single crystal X-ray diffraction are in good agreement, and
ion mode, provided a spectrum with the major peak at the calculated configuration of 1-A maintains the trans dis-
707.21 m/z, which corresponds to the calculated mass of posed carbene and carbonyl ligands. The calculated 15N NMR
[{PhB(OxMe2)2ImMes}RhSiH2PhCO]+. In addition, peaks with chemical shifts of 1 are well reproduced in 1-A: −150 (trans to
M+
27.99 m/z were observed, corresponding to SiH2Ph, cf. experimental −161 ppm) and −158 ppm (trans to
[{PhB(OxMe2)2ImMes}RhSiH2Ph(CO)2]+ and [{PhB(OxMe2)2ImMes}- H, cf. experimental −172 ppm). The gas phase calculated 29Si
RhSiH2Ph]+, which may come from gas-phase reactions invol- NMR chemical shift of 1-A is 14 ppm, in reasonable agreement
ving loss or gain of CO. There was also a peak at 601.18 m/z with the experimental value of −21 ppm (see Tables S1 and
that corresponds to the mass of [{PhB(OxMe2)2ImMes}RhH(CO)]+, S2† for a comparison of calculated and experimental NMR
resulting from loss of a SiHPh group.
chemical shifts).
Some evidence for the appearance of the dicarbonyl species
[{κ3-N,Si,C-PhB(OxMe2)(OxMe2SiHPh)ImMes}Rh(H)CO]+ (2-A)
through a gas-phase reaction in the mass spectrometer comes was calculated as a gas-phase model for 2. Satisfyingly, the cal-
from independent generation of [{PhB(OxMe2)2ImMes}Rh(H)- culated 15N NMR chemical shifts of the oxazolines in 2-A
SiHPh(CO)2]+ through the addition of an atmosphere of CO to mirror the experimental pattern, with the chemical shifts of
a solution of 2. [{PhB(OxMe2)2ImMes}Rh(H)SiHPh(CO)2]+ gave the oxazoline N bonded to Si calculated to be −191 ppm and
an identical m/z for the parent ion as the minor signal the N coordinated to Rh at −167 ppm. The calculated 29Si
observed for 2. The 1H NMR spectrum of this dicarbonyl NMR chemical shift of 2-A is 73 ppm, which is ∼65 ppm down-
species contained a new RhH at −13.12 ppm (1 H, doublet of field compared to the experimental value. The difference in
doublets) and a new SiH at 5.53 ppm (1 H). The coordination calculated vs. experimental 29Si NMR chemical shifts may be
of CO to 2 indicates that there is an open coordination site in related to the cationic charge in the gas phase vs. a solvated
the complex. Unfortunately, [{PhB(OxMe2)2ImMes}Rh(H)SiHPh ionic species.
(CO)2]+ is too short-lived at room temperature for isolation or
detailed NMR characterization. Attempts to extend its lifetime rhodium silylene [{PhB(OxMe2)2ImMes}Rh(vSiHPh)(H)CO]+ (B)
For comparison, the calculated geometry of a putative
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Fig. 1 Calculated structures of [{PhB(OxMe2)2ImMes}Rh(SiH2Ph)(H)CO] (1-A), [{PhB(OxMe2)2ImMes}Rh(vSiHPh)(H)CO]+ (B), and [{κ3-N,Si,C-PhB(OxMe2)-
(OxMe2SiHPh)ImMes}Rh(H)CO]+ (2-A). H atoms on Si and Rh are illustrated, and all other H atoms are not included in the rendered depiction for
clarity.
was optimized from a few starting configurations to the
stationary point with carbene and carbonyl ligands trans, a
hydride and oxazoline trans, and the SiHPh and oxazoline
trans. Moreover, the calculated 29Si NMR chemical shift for B
is 370 ppm as expected for a coordinatively unsaturated silicon
center, and this is 364 ppm downfield from the experimental
value for 2 and 280 ppm shift downfield from 2-A. Thus, the
terminal silylene structure is unambiguously ruled out by
experimental and computed 29Si NMR chemical shift values.
The optimized structure of 2-A is 17 kcal mol−1 lower in energy
than the silylene isomer B, suggesting that the terminal un-
saturated silylene isomer is unlikely to be accessed. In line
with this, compound 2 is also formed from reactions of 1 and
B(C6F5)3 in THF as a competitive donor rather than the THF-
stabilized silylene. In addition, identical chemical shifts for 2
were observed upon treatment with carbonyl compounds such
as acetophenone and ethyl acetate. That is, there is no appar-
ent substitution of the oxazoline for other 2-electron donor
groups. In addition, there is no detectable interaction between
2 and an added equivalent of B(C6F5)3.
Despite the lack of observable interaction between 2 and
carbonyls, complex 2 is a catalyst for the selective partial
reduction of esters to ethers (see Table 1), silyl-protected
acetals, or silyl-protected alcohols. Notably, the pathway for
conversion of the ester to ether or to hydrosilylation products
depends on the silane, with primary silanes giving selective
deoxygenation and tertiary silanes giving 1,2-addition. In an
initial set of experiments, phenylsilane, methylphenylsilane,
and benzyldimethylsilane were tested as representative
primary, secondary, and tertiary silanes in the partial
reduction of ethyl acetate. The primary silane PhSiH3 gives
diethyl ether as the partial deoxygenation product. Compound
2 is highly reactive and selective for this transformation, giving
full conversion and similar yields with 0.1–1 mol% catalyst in
30 min.
Table 1 Catalytic deoxygenation or hydrosilylation of ethyl acetate
using 2
mol%
cat.
Time Yield TOF
Conversion
Solvent (h)
(%)
(h−1
)
1
1
0.2
C6D5Br 24
55
75
70
2.3
150
700
CDCl3
CDCl3
0.5
0.5
1
1
C6D5Br 12
CDCl3 24
77
0
6.4
n.a
1
C6D5Br 12
88
7.3
In contrast, the tertiary silane BnMe2SiH gives the hydro-
silylation product and a small amount of ester cleavage
product benzyl(ethoxy)dimethylsilane. The yield of ester clea-
vage increases with 8 equiv. of BnMe2SiH. Interestingly, the
secondary silane reagent PhMeSiH2 gives a mixture of both
products (eqn (2)). In this case, the ester cleavage product is
not formed, as determined by comparison of the 1H NMR
spectrum of the reaction to an authentic sample prepared by
zinc-catalyzed dehydrocoupling of PhMeSiH2 and EtOH.34 The
relative amounts of diethyl ether and disiloxane byproduct
from reduction with PhMeSiH2 are ∼1 : 1 as expected by
stoichiometry, while PhSiH3 gives unquantified, presumably
oligomeric, siloxanes as byproducts.
ð2Þ
Although the deoxygenation reaction occurs both in bromo-
benzene-d5 and chloroform-d1, the reaction is significantly
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a
faster in chloroform. However, all scaled-up reactions were per-
formed in methylene chloride and equivalent yields are
obtained in the two chlorinated solvents. There is no evidence
that 2 and methylene chloride react, and chloroform and
methylene chloride may provide identical active catalytic sites.
However, we cannot rule out the possible involvement of
solvent, especially one as reactive as chloroform, in the cata-
lytic cycle. Indeed, solvent effects on catalysis are observed. For
example, bromobenzene is a superior solvent to chloroform
for hydrosilylation using the tertiary silane BnMe2SiH, as the
latter solvent gives no detectable conversion. Interestingly both
chloroform-d1 and bromobenzene-d5 are equally effective sol-
vents for conversions involving the secondary silane
PhMeSiH2, which gives a mixture of both hydrosilylation and
deoxygenation products as noted above.
Table 2 Reactions of esters and PhSiH3 catalyzed by 2 or B(C6F5)3
Ester
Cat. (mol%)
°C
h
Products
Yield (%)b
2 (0.1)
25 2.5 MeOEt
54
B(C6F5)3 (1.0) 25 0.5 C2H6 + CH4
2 (0.1) 25 25
B(C6F5)3 (1.0) 25 0.5
100c
56 (49)
95
2 (0.1)
25 0.5 Et2O
70
B(C6F5)3 (1.0) 25 0.5 C2H6
100c
2 (1.0)
25
6
87 (62)
94
B(C6F5)3 (1.0) 25 0.5 C16H34 + CH4
These optimized conditions for diethyl ether synthesis were
applied to the deoxygenation of a variety of ester substrates in
the presence of 2.0 equiv. of PhSiH3 in chloroform or methyl-
ene chloride, and the results are summarized in Table 2. Typi-
cally, 1 mol% catalyst is required for conversion of most esters
to ethers in Table 2, and the lower catalyst loading (0.1 mol%)
determined for ethyl acetate above is effective for less sterically-
demanding substrates such as ethyl formate and exo-2-norbor-
nyl formate. Substrates with long alkyl chains such as methyl
palmitate, or substituents with heteroatoms, such as 6-bromo-
hexanoate and 2-thiopheneacetate, require 1 mol% catalyst
loading and elevated temperature to give the ether products.
The low yield for the deoxygenation of ethyl formate is probably
due to the volatility of the gaseous methyl ethyl ether product.
The catalysis product of exo-2-norbornyl formate is exo-2-
methoxybicyclo[2.2.1]heptane (Table 2), indicating that the R–O
bond in R′CO2R is not broken during reduction. The isolated
yield of the deoxygenation is comparable to the yield from exo-
norborenol and iodomethane using sodium hydride via the
Williamson ether method,35 suggesting this deoxygenation
method has potential synthetic utility. Olefins are unaffected
by catalytic ester deoxygenation conditions. Methyl oleate and
methyl trans-3-pentenoate are effectively and selectively deoxy-
genated to their corresponding ethers in good yield, although
elevated temperature is required. The stereochemistry of the
CvC double bond is maintained as suggested by the 13C{1H}
NMR spectrum of the isolated products. The cyclic ester
γ-butyrolactone is deoxygenated quickly at room temperature
2 (1.0)
80 72
82 (75)
B(C6F5)3 (1.0) 25 0.5 C6H13Br + C2H6 95
2 (1.0)
80 24
54 (50)
93d
B(C6F5)3 (1.0) 25 0.5
2 (1.0)
2 (1.0)
60 72
80 24
86 (62)
65
2 (1.0)
2 (2.0)
25
25
8
6
60 (55)
50
a See ESI for experimental conditions for catalytic reactions and
product characterization. b Spectroscopic yields from micromolar scale
experiments, isolated yields given in parenthesis. c Substrate conversion.
d Trace 2-ethylthiophene observed.
with moderate yield of the product, while elevated tempera- results indicated that the precatalyst contains [HB(C6F5)3]−,
tures result in ring-opening. Both carbonyl groups in tri- and no B(C6F5)3 was detected by 11B or 19F NMR spectroscopy,
fluoroacetic anhydride are deoxygenated with 2.0 mol% of the a small amount of that Lewis acid could form during catalysis.
catalyst loading at room temperature.
Tris(perfluorophenyl)borane is
For these reasons, the interaction of PhSiH3 and esters in the
catalyst for carbonyl presence of catalytic B(C6F5)3 was tested as a possible back-
a
hydrosilylation and C–O bond cleavage in the presence of ter- ground reaction, and the results are given in Table 2 for com-
tiary and secondary silanes,1 and might be involved in the parison to the catalytic action of 2. Interestingly, the
present deoxygenation since InBr3 and GaBr3 are known cata- combination of PhSiH3 and B(C6F5)3 with esters provides
lysts with tertiary silanes.15,16 However to the best of our hydrocarbons (complete deoxygenation) through C–O bond
knowledge, B(C6F5)3-catalyzed reactions of esters and primary cleavage rather than the selective deoxygenation to ethers
silanes are not yet reported,17a and the above studies demon- observed with 2. These experiments indicate that trace
strate a significant effect of silane upon the product of ester B(C6F5)3, as a unilateral catalytic actor, is not responsible for
conversion. Although the 11B NMR and mass spectrometry selective conversion of esters to ethers.
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As noted above, the common pathway for reactions of esters
The combined results from the catalytic reactions using
and hydridic reagents involves reductive cleavage of the C–O B(C6F5)3 or 2 as catalysts (Tables 2 and 3) shed some light on
linkage. In contrast, deoxygenation of amides to amines is the the pathways and active species involved in the transform-
typical pathway, observed with metal hydride reagents.36 Cata- ations. First, B(C6F5)3 catalyzes the C–O bond cleavage reac-
lytic hydrosilylation reactions of esters and amides also tend tions of ketones and amides in the presence of phenylsilane to
to follow the pathways established for metal hydrides, with give the same products that are observed with 2. However, the
ester hydrosilylation giving reductive cleavage and amide relative rates observed from the two catalysts vary significantly
hydrosilylation providing amines. Thus, catalytic reactions of with the substrates, indicating that the reactive species in the
phenylsilane with amides, employing 2 or B(C6F5)3 as catalysts, two systems are not equivalent. That is, compound 2 is not
provide further insight into the pathway for ester deoxygenation. providing free B(C6F5)3 as the catalyst, nor is the reaction of
Interestingly, both catalysts mediate amide deoxygenation PhSiH3 with B(C6F5)3 or 2 affording an equivalent activated
to tertiary amines (Table 3). These experiments were then organosilane species that interacts with the carbonyl substrate.
extended to ketones and aldehydes, which generally give com- This point is further emphasized by the distinct products
plete deoxygenation to alkanes. Both 2 and B(C6F5)3 catalyze observed from reaction of PhSiH3 and esters or aldehydes, as
the conversion of acetophenone to ethylbenzene, with the catalyzed by 2 or B(C6F5)3.
B(C6F5)3-catalyzed reaction occurring in
a
shorter time.
A possible pathway for the catalytic processes involves step-
However, the dimethoxy-substituted electron-rich ketone is wise carbonyl hydrosilylation to give a silyl-protected acetal fol-
deoxygenated with similar reaction times with 2 or B(C6F5)3, lowed by C–O bond cleavage to provide either two silyl ethers
while the 2,4,6-trimethoxybenzaldehyde is selectively deoxyge- or the ether/disiloxane products (Scheme 1).
nated by 2 but is unreactive under B(C6F5)3-catalyzed con-
A few points emerge from the effect of the silane the
ditions at room temperature. Furthermore, the dimethoxy- product selectivity. First, the addition of PhSiH3 to a mixture
substituted aldehyde decomposes under the B(C6F5)3-catalyzed of the silyl-protected acetal BnMe2SiOCHMeOEt and
conditions, but is selectively deoxygenated in the presence of BnMe2SiH increases the yield of the ester cleavage product
2. The neutral hydrosilyl hydridorhodium compound, which BnMe2SiOEt. This likely occurs through formation of
might be envisioned to catalyze ketone hydrosilylation via an PhH2SiOEt-type intermediates that undergo silyl exchange
oxidative addition and insertion pathway,31 is not reactive with BnMe2SiH, as suggested by transient OEt resonances in
1
under the tested catalytic conditions.
the H NMR spectrum of the reaction mixture. This ester clea-
vage reaction may be responsible for the difference between
conversion and yield of ethers from deoxygenation. In
addition, the silyl protected acetal is also an intermediate that
leads to diethyl ether. There is no ethyl acetate remaining in
the in situ reaction mixture of Scheme 2, yet diethyl ether is
observed upon addition of PhSiH3. Thus, 1,2-hydrosilylation is
the first step of both deoxygenation and ester cleavage path-
ways. However, the ether yield is lower when BnMe2SiOCH-
Table 3 Catalytic deoxygenation of aldehydes, ketones, and amides
using 1 mol% 2 or B(C6F5)3 as catalysta
Reaction
Cat.
°C
h
Yieldb (%)
2
80
80
3
3
85
88
B(C6F5)3
2
80
80
24
18
86 (80)
84
B(C6F5)3
2
25
25
25
7
0.5
18
65 (58)
87
B(C6F5)3
1
n.d.c
2
25
25
24
18
57 (55)
65
Scheme 1 Two C–O bond cleavage pathways for the carbonyl hydro-
silylation product.
B(C6F5)3
2
25
25
7
18
65 (48)
n.d.c
B(C6F5)3
2
25
25
4
4
77
B(C6F5)3
n.d.d
a See ESI for experimental condition for catalytic reactions and product
characterization. b Spectroscopic yields from micromolar scale
experiments, isolated yields given in parenthesis. c Not detected
(starting material recovered). d Unidentified product mixture.
Scheme 2 Effect of PhSiH3 on ester cleavage vs. deoxygenation
pathways.
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MeOEt is allowed to react with PhSiH3 in comparison to the tion and tertiary silanes give hydrosilylation products, while
reaction of EtOAc and PhSiH3 described in Table 1 (24 vs. secondary silanes give mixtures of both processes. This reactiv-
75%). The difference in ether yield may result from steric ity pattern is also followed with B(C6F5)3-catalyzed reactions of
effects due to the large BnMe2Si group, or the effect of added silanes and carbonyls, with primary silanes giving complete
PhSiH3 also may reflect a different pathway accessible to SiH- deoxygenation and tertiary silanes giving addition chemistry.17
substituted acetals vs. tertiary silyl substituted acetals. A However, the selectivity, rates, and in many cases, products
related experiment with acetophenone shows that the hydro- of 2-catalyzed conversions are inequivalent from those of
silylation product PhMeHCOSiH2Ph reacts to give ethyl- B(C6F5)3, suggesting that the unsaturated rhodium-silyl moiety
benzene in the presence of excess PhSiH3 and catalytic is central to the conversions observed here. Catalytic hydro-
quantities of 2.
silylation chemistry involving tertiary silanes does not appar-
ently involve Si–C bond cleavage because the tertiary silane is
incorporated into the product. This observation rules out silyl-
ene transfer as the mode of action for the hydrosilylation of
esters and other carbonyls by tertiary silanes. Currently, we are
Conclusions
The reaction of rhodium silyl {PhB(OxMe2)2ImMes}RhH(SiH2Ph) studying related systems with kinetic and isotopic experiments
CO (1) and B(C6F5)3 affords the zwitterionic complex 2, which to understand the effects of silane and catalyst on selective
contains a SiHPh group bonded to both rhodium and the partial deoxygenation pathways. The tunability of the latter,
nitrogen of an oxazoline donor. This species is formulated as a through systematic ligand variation, will be key to controlling
rhodium-bonded silylene that is coordinated by a two-electron selectivity in both activation reactions involving metal-silicon
donor (a donor-stabilized silylene), leaving a coordinatively and metal-hydrogen bonds or Lewis acid sites.
unsaturated rhodium site. The preference for oxazoline-silicon
vs. oxazoline-rhodium coordination suggests that the putative
silylene is more electrophilic than the rhodium center in 2.
Also, note that the five-coordinate rhodium silyl [{κ3-N,N,
C-PhB(OxMe2)2ImMes}Rh(SiH2Ph)CO]+ isomer, which might be
anticipated based on established α-H-migration chemistry,21,37
is not observed. The isomer resulting from silylene insertion
into the Rh–C of the more nucleophilic N-heterocyclic carbene
is also not observed. NHC-coordination to an unsaturated
silicon center might also be expected based on the stabili-
zation of silanone derivatives and zero-valent silicon centers
with NHCs.29,38 In addition, a series of dichlorosilylene com-
pounds coordinated by metal centers (V, Fe, Co, Ni, Rh) and
N-heterocyclic carbenes have related connectivity ([M]–SiX2–
NHC) to the unobserved isomer of 2.39 Despite this precedent,
compound 2 apparently does not isomerize to the carbene-co-
ordinated rhodium silylene, likely as a result of the inert
rhodium–carbene interaction.
Acknowledgements
Dr Bruce Fulton is thanked for assistance with solution-phase
15N NMR measurements. This research was supported by the
U.S. Department of Energy, Office of Basic Energy Sciences,
Division of Chemical Sciences, Geosciences, and Biosciences
through the Ames Laboratory Catalysis and Chemical Physics
projects (Contract no. DE-AC02-07CH11358).
Notes and references
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Compound 2 is an excellent catalyst for the deoxygenation
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deoxygenation pathway is unusual for esters, which are more
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