Branch-selective intermolecular hydroacylation: Hydrogen-mediated coupling of anhydrides to styrenes and activated olefins

Article abstract of DOI:10.1002/anie.200602377

(Chemical Equation Presented) Branching out with hydrogen: Hydrogenation of symmetric or mixed carboxylic anhydrides in the presence of styrenes or activated olefins generates intermolecular hydroacylation products. The use of cationic rhodium catalysts ligated by triphenylarsine (Ph₃As) results in the formation of branched coupling products as single regioisomers in high yields (see scheme; cod = cycloocta-1,5-diene, Ar^F = 3,5-(CF ₃)₂C₆H₃).

Full text of DOI:10.1002/anie.200602377

Angewandte
Chemie
Homogeneous Catalysis
regiocontrol [Eq. (1)]. The significance of these findings is
twofold. First, while catalytic systems for intramolecular
DOI: 10.1002/anie.200602377
Branch-Selective Intermolecular Hydroacylation:
Hydrogen-Mediated Coupling of Anhydrides to
Styrenes and Activated Olefins**
Young-Taek Hong, Andriy Barchuk, and
Michael J. Krische*
hydroacylation involving aldehydes as acyl donors are well
developed,[4] corresponding intermolecular hydroacyla-
tions[5–7] are far more limited in scope because of competitive
decarbonylation of aldehydes.[9] Secondly, prior to the results
reported herein, the catalytic reductive coupling of alkenes to
carbonyl compounds has only been achieved intramolecularly
by cyclization of olefinic aldehydes.[10]
Alkene hydroformylation is the largest volume application of
homogeneous metal catalysis and the prototypical example of
[1]
À
hydrogen-mediated C C bond formation. Remarkably,
while hydroformylation is practiced on a vast scale, systematic
À
efforts toward the development of hydrogenative C C
coupling reactions that extend beyond carbon monoxide
insertion have only recently been described.[2,3] Ideally, it
would be desirable to couple two or more organic molecules
simply through their exposure to gaseous hydrogen in the
presence of a metal catalyst. This goal represents the primary
focus of research in our laboratory.[2]
Our initial studies focused on the reductive coupling of
styrene and benzoic anhydride. Such hydroacylations find
precedent in the reaction of acid chlorides and ethylene
mediated by stoichiometric quantities of [RhH-
(PPh3)3(CO)],[11a] the reductive coupling of dienes and acid
chlorides catalyzed by palladium and mediated by silane,[11b]
and, most importantly, a single report by Miura and co-
workers of the hydrogen-mediated coupling of benzoic
anhydride to styrene catalyzed by [RhCl(cod)]2 (cod = cyclo-
octa-1,5-diene) with (PhO)3P as the ligand. Under these
conditions, a 3:1 mixture of branched and linear products is
obtained in 30% yield.[3b] However, it was found that
increased catalyst loading did little to improve the yield of
the coupling product (Table 1, entries 1 and 2). A survey of
Table 1: Optimization of hydrogen-mediated intermolecular hydroacyla-
tion.[a]
In connection with ongoing efforts toward the develop-
À
ment of hydrogen-mediated C C coupling reactions, we
herewith disclose studies on the hydrogen-mediated reductive
coupling of carboxylic anhydrides, including mixed anhy-
drides, to styrenes and activated olefins, and thus generate a
protocol for intermolecular alkene hydroacylation.[3b,4–7]
Notably, we find that cationic rhodium catalysts ligated by
triphenylarsine (Ph3As)[8] enable formation of branched
hydroacylation products with exceptionally high levels of
Entry
Catalyst (mol%)
L (mol%)
Base Yield [%] Br./Ln.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
[{RhCl(cod)}2] (0.5) PhO3P (2) iPr2NEt
30
45
3:1
3:1
[{RhCl(cod)}2] (5)
[{RhCl(cod)}2] (5)
[{RhCl(cod)}2] (5)
[{RhCl(cod)}2] (5)
[{RhCl(cod)}2] (5)
PhO3P (20) iPr2NEt
Ph3P (20) iPr2NEt
Fur3P (20) iPr2NEt
Ph3As (20) iPr2NEt
Ph3Bi (20) iPr2NEt
15
6:1
22
57
>95:5
>95:5
trace
67
79
63
82
85
84
93
93
[{RhOMe(cod)}2] (5) Ph3As (20) iPr2NEt
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
[*] Y.-T. Hong, A. Barchuk, Prof. M. J. Krische
University of Texas at Austin
[Rh(cod)2]OTf (5)
[Rh(cod)2]OTf (5)
[Rh(cod)2]BF4 (5)
[Rh(cod)2]PF6 (5)
[Rh(cod)2]SbF6 (5)
Ph3As (12) iPr2NEt
Ph3As (12) Li2CO3
Ph3As (12) iPr2NEt
Ph3As (12) iPr2NEt
Ph3As (12) iPr2NEt
Department of Chemistry and Biochemistry
1 University Station – A5300
Austin, TX 78712-1167 (USA)
Fax: (+1)512-471-8696
E-mail: mkrische@mail.utexas.edu
[Rh(cod)2]BArF (5) Ph3As (12) iPr2NEt
4
[Rh(cod)2]BArF (2) Ph3As (4.4) iPr2NEt
4
[**] Acknowledgment is made to the Research Corporation Cottrell
Scholar Award (CS0927), the Alfred P. Sloan Foundation, the
Dreyfus Foundation, Eli Lilly, Johnson & Johnson, the NIH-NIGMS
(RO1-GM69445), and the Robert A. Welch Foundation for partial
support of this research.
[a] Cited yields are of isolated material. For the experiments above,
400 mol% of styrene and 100 mol% of Bz2O were used. However, under
the conditions cited in entry 13, but using 100 mol% of styrene and
200 mol% of Bz2O, an 87% yield of the branched reductive coupling
product was obtained. See the Supporting Information for detailed
experimental procedures.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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monodentate ligands proved more fruitful. Whereas Rh
nitro groups are not subject to reduction under the conditions
of hydrogen-mediated coupling. Simple aliphatic alkenes
gave diminished yields of the reductive coupling product and
exhibit incomplete levels of regioselection.[13] To further
assess the scope of the reaction, different carboxylic anhy-
drides were hydrogenated in the presence of styrene. Aro-
matic, heteroaromatic, and a,b-unsaturated anhydrides
couple in good to excellent yield and with complete branch-
selective regiocontrol (Table 2, bottom). Notably, a,b-un-
saturated coupling products 14–17, are not subject to over-
reduction under the conditions of hydrogen-mediated cou-
pling. Aliphatic anhydrides, such as acetic anhydride, provide
diminished yields of the coupling product.[13] As demon-
strated by the formation of 19–24 (Table 3), norbornene also
catalysts ligated by PPh3 gave only a 15% yield of the
coupling product, more electron-deficient ligands (2-Fur)3P
(Fur= furanyl) and Ph3As provided the branched coupling
product as a single regioisomer and, for the latter case, in
substantially improved yield (Table 1, entries 3–5). Given
these results, the ligand Ph3As was screened against a series of
RhI sources (Table 1, entries 7–13). It was found that cationic
RhI complexes were especially effective precatalysts. Indeed,
upon use of [Rh(cod)2]BArF as precatalyst (ArF = 3,5-
4
(CF3)2C6H3), the coupling product is obtained in 93% yield
as a single regioisomer (Table 1, entry 13). Enhanced reac-
tivity conferred through the use of noncoordinating counter-
ions, and in particular BArF À, has been noted for cationic
4
RhI- and IrI-based hydrogenation catalysts.[12] Catalyst load-
ing could be reduced to 2 mol% without any decline in the
yield (Table 1, entry 14). Notably, the present second-gener-
ation catalytic system (Table 1, entries 13 and 14) provides
better yields and better regioselectivities at less than half the
catalyst loading than the original catalytic system described in
the pioneering study by Miura and co-workers (Table 1,
entries 1 and 2).
Various substrate combinations were explored under
these optimized conditions. It was found that substituted
styrenes and related vinylarenes reductively couple to benzoic
anhydride in good to excellent yield with complete regiocon-
trol favoring the branched product (Table 2, top). As dem-
onstrated by the formation of product 7, arenes containing
Table 3: Hydrogen-mediated coupling of assorted carboxylic anhydrides
to norbornene.[a]
19, 85%
22, 69%
20, 93%
23, 74%
21, 91%
24, 61%
[a] Cited yields are of isolated material. See the Supporting Information
for detailed experimental procedures.
couples readily to heteroaromatic and a,b-unsaturated car-
boxylic anhydrides. Of greater interest, gaseous ethylene
participates in the coupling. For example, the 2-carboxyindole
anhydride shown in Equation (2) (chosen because of the low
Table 2: Hydrogen-mediated coupling of Bz2O to different vinylarenes (top)
and hydrogen-mediated coupling of different anhydrides to styrene (bottom).[a]
1, 93%
4, 71%
7, 82%
2, 78%
5, 81%
8, 75%
3, 75%
6, 70%
9, 74%
volatility of the product) is converted into the corresponding
ethyl ketone in an unoptimized 44% yield simply by using a
balloon containing roughly equal volumes of hydrogen and
ethylene gas.
——————————————————————————————
Coupling to mixed anhydrides would be desirable for
more highly functionalized carboxylic acid precursors.
Accordingly, mixed anhydrides derived from trimethylacetic
acid (pivalic acid) and various a,b-unsaturated acids were
prepared and subjected to the optimized conditions for
coupling to styrene and norbornene. Gratifyingly, transfer of
the a,b-unsaturated acyl moiety was observed exclusively.
Complete levels of branch regioselectivity were also observed
in coupling reactions to styrene (Table 4).
10, 67%
13, 84%
11, 63%
14, 73%
12, 77%
15, 93%
16, 74%
17, 85%
18, 92%
To gain insight into the catalytic mechanism, the coupling
of benzoic anhydride and styrene was conducted in a
deuterium atmosphere. Deuterium is incorporated primarily
at the b position, but the extent of incorporation is base-
[a] Cited yields are of isolated material. In all cases, >95:5 regioselection is
observed. See the Supporting Information for detailed experimental proce-
dures.
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Chemie
Table 4: Hydrogen-mediated coupling of mixed anhydrides to styrene
tion may direct the formation of the branched alkyl rhodium
and norbornene.[a]
intermediate, as this would allow the resulting coordinatively
unsaturated Rh center to interact with the adjacent arene.
However, the regio-determining hydrometalation of mecha-
nism A, which occurs prior to any interaction with the
anhydride, is inconsistent with the fact that different acyl
donors, such as the O-2-pyridyl ester, exhibit different levels
of regioselection. It is known that low-valent rhodium
complexes undergo oxidative addition to carboxylic anhy-
drides under mild conditions to afford acyl metal carbox-
ylates.[14] Hence, catalytic mechanism B, which involves
oxidative addition of an anhydride followed by insertion of
styrene and hydrogenolytic cleavage of the rhodium–carbon
bond is herewith proposed to account for the changes in the
regiochemistry arising from the use of different acyl deriva-
tives.
In summary, we have reported a regioselective intermo-
lecular hydroacylation of vinylarenes in which symmetric and
mixed carboxylic anhydrides are used as acyl donors. High
levels of branch selectivity are promoted through the use of
cationic rhodium catalysts ligated by triphenylarsine. Future
studies will focus on the hydrogen-mediated coupling of
simple a-olefins to aliphatic carboxylic anhydrides and acid
chlorides, as well the development of enantioselective var-
iants involving chirally modified triphenylarsine ligands.
Entry
1
Mixed anhydride
Product
Yield[%]
71
2
3
62
70
4
5
6
7
86
80
84
73
[a] Cited yields are of isolated material. In all cases involving styrene,
>95:5 regioselection is observed. See the Supporting Information for
detailed experimental procedures.
Received: June 13, 2006
Revised: August 8, 2006
Published online: September 22, 2006
dependant. When iPr2NEt or Li2CO3 are used as the base, 0.4
and 0.8 deuterium atoms are incorporated, respectively.
These data are consistent with the catalytic mechanism A
(Scheme 1) initially proposed by Miura and co-workers[3b]
which involves heterolytic activation of hydrogen by way of
the dihydride. Incomplete incorporation of deuterium may
result from reversible coordination and hydrometalation of
styrene. Furthermore, dehydrogenation of iPr2NEt may
compete with hydrogen activation, thus contributing further
to the incomplete incorporation of deuterium. Dissociation of
the weakly coordinating ligand Ph3As prior to hydrometala-
Keywords: alkenes · homogeneous catalysis · hydroacylation ·
.
hydrogenation · rhodium
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Scheme 1. Plausible catalytic mechanism as supported by deuterium labeling studies. Bz=benzyl.
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Communications
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À
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optimum reported conditions, a 66% conversion was deter-
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