Catalytic intermolecular tail-to-tail hydroalkenylation of styrenes with α olefins: Regioselective migratory insertion controlled by a nickel/n-heterocyclic carbene

Article abstract of DOI:10.1002/anie.201001849

Making head or tail of it: The first single-operation, highly selective intermolecular tail-to-tail hetero-hydroalkenylation from two readily available monosubstituted alkenes is described (see scheme; IPr=1,3-Bis(2,6-di- isopropylphenyl)imidazol-2-ylidene). The reaction is catalyzed by the proposed [(IPr)NiH]OTf species. The method allows the use of more common and structurally diverse α olefins as substrates, which were previously not compatible with known methods. Copyright

Full text of DOI:10.1002/anie.201001849

Communications
DOI: 10.1002/anie.201001849
Olefination
Catalytic Intermolecular Tail-to-Tail Hydroalkenylation of Styrenes
with a Olefins: Regioselective Migratory Insertion Controlled by a
Nickel/N-Heterocyclic Carbene**
Chun-Yu Ho* and Lisi He
New syntheses of alkenes are important in organic chemistry.
Particularly attractive are those methods that use a chemical
feedstock such as ethylene, a-olefins, and styrenes as one of
the components.[1] Major advances in this area include
catalytic olefin cross-metathesis and hydrovinylation, which
join two different alkenes together to provide internal olefins
and a olefins, respectively.[2–4] Although more functionalized
conjugate carbonyls and dienes are used as one of the
partners,[5,6] the direct addition of monoenes to these systems
is also an important advance in the field.[7,8] Recently, some of
these alkene coupling technologies have been developed to
achieve the asymmetric synthesis of biologically important
molecules, with ibuprofen being a representative example.[6e,9]
Nevertheless, all of the above methods are either character-
ized by head-to-head (h-h) or tail-to-head (t-h) C–C bond
Scheme 1. Distinct ligand effect upon nickel hydride mediated hydro-
formation or use ethylene as one of the components; tail-to-
tail (t-t) hetero-hydroalkenylation should provide branched
terminal 1,1-disubstituted alkenes, but such products are
observed only in small amounts, or in intramolecular cases
alkenylation. a) [(R3P)NiH]X-catalyzed t-h hydroalkenylation. If R2 is a
long-chain alkyl group isomerized starting material is observed.
X=OTf or halide. b) {(NHC)NiH}-catalyzed reaction lead to NHC-alkyl
reductive elimination. c) [(IPr)NiH]OTf-catalyzed t-t hydroalkenylation
wherein the geometric constraints of ring closure play a
substantial role. Also, although phosphorus-based nickel(II)
hydride complexes (Scheme 1a) have been shown to achieve
good selectivity and turnover numbers by exploiting various
monodentate P ligands, hemilabile ligands, and counter
ions,[10] successful cases of using long-chain a olefins as
intermolecular hetero-hydroalkenylation substrates are not
reported; as such substrates only isomerize into internal
olefins.[3g,11] N-heterocyclic carbene (NHC) nickel(II) hydride
complexes ({(NHC)NiH}) have been rarely studied for this
purpose, possibly because alkyl reductive elimination of
NHCs can be a very effective process (Scheme 1b), wherein
the alkene hydrometalation is the first step, similar to that of a
typical {PNiH}-catalyzed (P = P ligand) hydrovinylation. We
surmised that {(NHC)NiH}-catalyzed hetero-hydroalkenyla-
tion may be possible by using NHCs having different
structural characteristics relative to those that undergo
described herein. Tf=trifluoromethanesulfonyl.
reductive eliminations under optimized reaction condi-
tions.[12] The change in the regioselective outcome of the
nickel(0)-catalyzed silyltriflate/alkene/aldehyde coupling that
resulted from replacement of the P ligand with NHC encour-
aged us to test {(NHC)NiH} for t-t hydroalkenylation.[13,14]
Herein we describe the first highly selective intermolec-
ular t-t hetero-hydroalkenylation of several types of vinyl-
arenes with unactivated a olefins to selectively yield branched
1,1-disubstituted alkenes with limited isomerization, which is
in contrast to the observations from previous work (Sche-
me 1c). This reaction is also the first NiH-catalyzed hydro-
alkenylation that is not constrained to the use of ethylene/
propene[3g] as one of the reaction partners for the styrene
substrates; the more-common and structurally diverse a ole-
fins that were unable to undergo the hydroalkenylation with
vinylarenes under previous reaction conditions,[3] are now
viable substrates for providing a variety of alkenes directly
from a chemical feedstock in a single operation.[15] The closest
precedent to the transformation that we report herein appears
to be the cobalt-catalyzed 1,4-hydrovinylation and nickel-
catalyzed conjugate addition using a olefins and P ligands
reported by Hilt et al. and Jamison and co-workers, repec-
tively; both methods yielded 1,1-disubstituted olefins in very
high selectivity.[5a,6a,b] An oxidative cyclization-like strategy
involving a metallacycle was proposed to account for the h-t
regioselectivity achieved within those systems, yielding vari-
[*] Dr. C.-Y. Ho, L. He
Center of Novel Functional Molecules, Department of Chemistry,
The Chinese University of Hong Kong
Shatin, NT, Hong Kong SAR (China)
E-mail: jasonhcy@cuhk.edu.hk
[**] Support for this work was provided by the Center of Novel
Functional Molecule, The Chinese University of Hong Kong, and the
Hong Kong General Research Fund (401208). We thank Prof.
Henry N. C. Wong for advice.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001849.
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Chemie
ous linear heterocoupling products, which are in contrast to
the t-t branched products described below that are obtained
from migratory insertion reactions controlled by NHC.
Initally, 1-octene and styrene were selected as substrates
and reacted in the presence of a catalytic amount of in situ
generated [(IPr)NiH]OTf (IPr= 1,3-bis(2,6-di-isopropyl-
phenyl) imidazol-2-ylidene), which was obtained by modify-
ing a procedure reported by Jamison and co-workers (that is,
found to be a possible substrate (entry 7). In addition to
straight-chain 1-octene and a/b-branched a olefins, substrates
with a high tendency to yield stable conjugate alkenes by
isomerization also underwent reaction to give the corre-
sponding products in good yield and selectivity (entries 8–11).
A possible rationale for the formation of a product with a
high t-t selectivity and scope for new substrates is depicted in
Scheme 2. This proposal is largely based on the results from
the most well-studied styrene–ethylene hydrovinylation using
P-based NiH catalysts[3] and the results obtained herein. The
[(IPr)NiH]OTf that was presumably generated in situ could
preferentially add to the vinylarene to form an electronically
more stable benzylic nickel complex in a fashion similar to
that of the typical {PNiH}-catalyzed vinylarene hydrovinyla-
tion. It is thought that the less sterically demanding a olefin
preferentially coordinated to the metal center in a way that
minimized the steric repulsion with the ligand. A selective
migratory insertion then occurred forming a new C–C bond
between the two of the reacting alkenes, thus providing the
high hetero/homo and t-t/t-h selectivity observed. Finally, a
formal syn-b-hydride elimination step regenerates the cata-
lyst.
[(IPr)NiP(OPh)3]-mediated
silyltrilflate/alkene/aldehyde
coupling via an oxanickellacycle intermediate with P(OPh)3
removed), in toluene.[13a] The same catalyst can apparently be
generated from combining IPr and [{(allyl)NiBr}2], the
product of which undergoes anion exchange in a manner
analogous to the {PNiH} catalyst generation protocol.[16] The
Jamison procedure was chosen because of its technical
simplicity (RT generation from a commercial source, no
precatalyst preparation) and because it avoided the use of the
coordinating halide anion, which may have an adverse effect
on catalytic activity.[10,13a]
Although these modifications are simple, we found that
unconventional t-t hetero-hydroalkenylation of styrene and 1-
octene can be achieved at room temperature and under
atmospheric pressure (Table 1, entry 1). Commonly
employed alkyl aluminum halide
additives or co-catalysts used in
related systems were found to be
Table 1: Scope of catalytic t-t hydroalkenylation.
unnecessary.
A
minor product
Entry[a]
R1
R2
Yield
[%][b]
Hetero/Homo
Product
observed for the reaction reported
herein is the homo t-t 1,1-disubsti-
tuted alkene product from the sty-
rene. Other reported systems for
styrene dimerization generally
favor either the t-h or h-h product
as well as polymerization.[5c,17] The
Hetero-hydroalkenylation
1
Ph
Ph
Ph
Ph
n-hexyl
n-hexyl
n-hexyl
n-hexyl
n-hexyl
n-hexyl
n-hexyl
n-hexyl
n-hexyl
n-hexyl
cyclohexyl
cyclohexyl
iBu
82[c]
95
64:36
90:10
47:53
86:14
86:14
88:12
89:11
84:16
90:10
94:6
53:47
69:31
87:13
87:13
95:5
80[d]
91[e]
93
use of
a slightly smaller IMes
2
3
4
5
6
7
8
p-MeC6H4
p-OMeC6H4
p-OAcC6H4
p-FC6H4
p-CH2Cl(C6H4)
2-Napthyl
Ph
90
92
92
(IMes = 1,3-di(2,4,6-trimethyl-
pheny-l)imidazolin-2-ylidene)
ligand also gave the hetero product,
but a fall in the hetero/homo selec-
tivity was observed (90%, 80:20).
Ethereal solvents such as THF can
also be used in this reaction, giving
similar yield and selectivity (96%,
88:12). In light of these interesting
observations, we decided to study
the scope of the reaction. We found
that scaling up the reactions did not
diminish the high yield, and that the
t-t product was generally observed
in all cases examined. The systems
tolerate electron-rich and electron-
deficient styrenes bearing substitu-
ents such as alkyl and ether groups,
as well as OAc and F (entries 2–5,
13–16). Relatively labile function-
alities, such as AcO and benzylic
chloride, remain intact under the
standard conditions (entries 4, 6,
37[f,g]
68[g]
72
Ph
Ph
70[g,h]
90
9
10
11
Ph
Ph
CH2Ph
(CH2)2Ph
81
92
Homo-hydroalkenylation
12
13
14
15
16
Ph
–
–
–
–
–
90
95
–
–
–
–
–
p-MeC6H4
p-OMeC6H4
p-OAcC6H4
p-FC6H4
94
45[g]
35
[a] See Scheme 1c and the Experimental Section for procedures. Reaction conditions: “[(IPr)NiH]OTf”
(5 mol%); for hetero-hydroalkenylation vinylarene/a olefin=1:3, and 2 mmol vinylarene for homo-
hydroalkenylation; toluene (2 mL). Yield and ratio were determined by GC analysis using C6(CH3)6 as a
standard, homo product refers to vinylarene t-t dimer. A limited amount of other regioisomers and other
olefin isomers can be detected by GC analysis; see the Supporting Information. [b] Based on vinylarene,
sum of hetero- and homo t-t products. [c,d] Yield based on 1-octene, styrene/1-octene=1:1 and 3:1,
respectively. [e] A 2.5-fold scale. [f] No homo dimer was observed by GC analysis; potentially a result of
thermal decomposition. Homo dimer can be observed in the NMR spectra of both the crude reaction
mixture and the isolated product. [g] Determined by NMR analysis of the crude reaction mixture.
15). 2-Vinylnaphthalene was also [h] Slow addition of styrene over 5 h at 358C.
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P additives, temperature, and solvent; see the Supporting
Information), and the results support our belief that the
choice of NHC is most responsible for the observed behav-
iors, and that the avoidance of PPh3 helps in terms of
hydroalkenylation efficiency. It has also been reported that
without P(OPh)3 as an additive, a higher stability of [(IPr)-
NiH]OTf towards bases was observed as compared with that
of a P analogue. This stability may explain the high functional
group compatibility observed herein.[13a–c]
Finally, vinylarenes having additional substituents at the
b-position were also found to be good substrates under the
optimized reaction conditions (Scheme 3).[3g,18] Initial
Scheme 2. a) Proposed mechanism. The poor orbital overlap (gray
arrow) slows NHC-alkyl reductive elimination. The size of the n-nexyl
group is less than the Ph group, so hetero-hydroalkenylation is more
favorable. b) Alkene stability. When L is larger than a P ligand (e.g.,
IPr) then isomerization is less likely because the hydrometalation step
is more difficult.
The 1,1-disubstituted alkene products obtained are quite
reactive towards isomerization to the thermodynamically
more stable internal or conjugate olefins. Remarkably, only
limited isomerization and no significant product oligomeriza-
tion were observed. We surmise that the above observations
and the successful use of long-chain a olefins can both be
explained by the steric effect of the IPr ligand. It should be
emphasized that the recent alkene isomerization study using
various {PMH} (M = Pd,Ni) complexes by RajanBabu and co-
workers inspired us to select a bulky NHC ligand (IPr) to
minimize the potential alkene isomerization. The study
showed that alkene isomerization (i.e., hydrometalation and
b-hydride elimination steps involved) is sensitive to the steric
environment of the substrates and the P ligand employed.
Certain 1,1-disubstituted olefin isomerizations were found to
be difficult even when the sterically less hindered PPh3 was
used under more forcing conditions.[11,13e] In this sense, our
products are expected not to isomerize into internal alkenes
when using IPr as a ligand.[12d] Similarly, this hypothesis may
also account for the success of using long-chain a olefins,
which is in contrast to using them with the {PNiH} catalysis.
Notably, removing the P additives (e.g., PPh3, P(OPh)3)
and the careful selection of the NHC are necessary to
promote hydroalkenylation over the elimination processes
observed by Cavell and co-workers, and Jamison and co-
workers, respectively.[12a–c,13a] P ligands were used as acceler-
ators for NHC-alkyl or H-X eliminations from [(NHC)Ni(H
or alkyl)]X complexes. Hypothetical criteria for an effective
NHC-alkyl elimination have been proposed, and the key step
involves orbital mixing between the hydrometallated alkene
and the NHC. We selected a bulky IPr ligand and avoided
P ligands accordingly, with the intention to hinder the possible
orbital mixing step and thus the eliminations; as a result
catalytic hydroalkenylation occured. Several additional
experiments were carried out to elucidate the factors that
may account for the dramatic changes in behavior (e.g.,
Scheme 3. Anethole and indene t-t hetero-hydroalkenylation with an
a olefin. TES=triethylsilyl.
attempts using IPr were not successful, however, with the
operative mechanism in mind, a slightly smaller NHC (IMes)
and a sterically more demanding a olefin were tested. This
combination leads to a longer life time for the benzylic Ni
species and slower isomerization of the a-olefin. Reasonable
yields were observed, with ꢀ 1% homo vinylarene dimers, as
determined by GC analysis.
In summary, a simple, highly regioselective and catalytic
intermolecular t-t hetero-hydroalkenylation of two readily
available types of monoene was achieved. The products may
be useful for the synthesis of more highly substituted products
for metathesis and other conventional olefin functionalization
technologies.[19] Notably, the t-t hetero-hydroalkenylation
currently works only when vinylarenes are used as one of
the components with monoenes, and simple aliphatic internal
alkenes are not compatible substrates.[20] The distinct ligand
effect observed may also facilitate developments in NiH/alkyl
and {(NHC)Ni} chemistry. Our current efforts include inves-
tigating other potential substrates, evaluating the identity of
the active catalyst, and searching for simpler catalyst gen-
eration methods. In gaining some mechanistic insights, a
reaction with a higher turnover and better atom efficiency can
be developed.
Experimental Section
Catalyst generation: [Ni(cod)2] and IPr (0.05 mmol, 5 mol% each)
were added to an oven-dried test tube equipped with a stir bar in a
glove box. After sealing the test tube with a septum, it was removed
from the glove box and connected to a N2 line. The mixture was
dissolved in 2 mL of degassed toluene and stirred at RT for 1 h. 1-
octene (10 mol%), NEt3 (0.3 mmol), para-anisaldehyde (5 mol%),
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Chemie
and TESOTf (10 mol%) were then added sequentially and the
resulting reaction mixture was stirred for 15 mins at RT.
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Hetero-hydroalkenylation (t-t): The a olefin (3.0 mmol) and
vinylarene (1.0 mmol) or amount indicated in Table 1) were added
to the above-described mixture at RT. After stirring the reaction
mixture for 24 h, the mixture was diluted with 4 mL n-hexane, and
then stirred for 30 min (open to the air). The reaction mixture was
then filtered through a short plug of silica gel and rinsed with 75 mL
20% ethyl acetate/n-hexane. The solvent was removed from the
filtrate, and the resulting residue was purified by flash column
chromatography on silica gel (see the Supporting Information) to
afford the products. For b-substituted vinylarenes (Scheme 3), [Ni-
(cod)2] and IMes (0.05 mmol, 10 mol% each) were used, and an
additional 3 equivalents of the a olefin was added after 1 d.
Vinylarene homo-hydroalkenylation (t-t): The above procedure
described above was used, wherein the vinylarene (1 mmol) was used
instead of the 3 mmol of a-olefin.
[11] Isomerization into 2-alkenes using the NiII or PdII catalyst: H. J.
Lim, C. R. Smith, T. V. RajanBabu, J. Org. Chem. 2009, 74, 4565.
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Ref. [13a] may serve as indirect evidence for the NiH generation
at this stage. Coupling using a species generated from Ni/IPr and
allyl bromide was found to be less efficient (< 10% yield),
whereas the use of allyl triflate will result in Friedel–Crafts
reaction. Although the use of NEt3 may deactivate the catalyst, it
provides a mild reaction medium and suppresses the side-
product resulting from a triflic acid impurity. b) C.-Y. Ho, S.-S.
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Chem. Commun. 2010, 46, 466.
Received: March 29, 2010
Revised: August 10, 2010
Published online: September 17, 2010
Keywords: homogeneous catalysis · insertion ·
.
N-heterocyclic carbenes · nickel · olefination
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Communications
2008, 9, 85; f) R. B. Bedford, M. Betham, M. E. Blake, A.
chel, J. Am. Chem. Soc. 1999, 121, 6940. Please see the
Supporting Information for details.
Garces, S. L. Millar, S. Prashar, Tetrahedron 2005, 61, 9799.
[18] Recent advances using ethylene: a) G. Muller, J. I. Ordinas, J.
Mol. Catal. A 1997, 125, 97; b) V. Fassina, C. Ramminger, M.
Seferin, A. L. Monteiro, Tetrahedron 2000, 56, 7403; c) see Ref.
[4d]; d) T. V. RajanBabu, N. Nomura, J. Jin, M. Nandi, H. Park,
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[20] We did not observe the hetero-hydroalkenylation product from
the reaction between cod and styrene. When we tested
norbornene, a linear exo-trans-vinylbenzene was obtained in
27% yield, and a second styrene insertion gave the t-t product in
16%, thus suggesting our system cannot override the strong
steric bias it offers, and b-hydride elimination steps are more
difficult. See the Supporting Information for details.
[19] Hydroboration was selected as a representative example: H.
Laaziri, L. O. Bromm, F. Lhermitte, R. M. Gschwind, P. Kno-
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Angew. Chem. Int. Ed. 2010, 49, 9182 –9186