Shuffle off the classic β-Si elimination by Ni-NHC cooperation: Implication for C-C forming reactions involving Ni-alkyl-β-silanes

Article abstract of DOI:10.1039/c1cc14593b

Via a cooperation of NHC, Si substituents and a M center, β-Si elimination was attenuated, revealing a new way to attain a high Ni-β-SiR₃:Ni-σ-SiR₃ ratio. The scope is illustrated by a head-to-tail vinylsilane-α-olefin hydroalkenylation.

Full text of DOI:10.1039/c1cc14593b

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COMMUNICATION
Shuffle off the classic b-Si elimination by Ni-NHC cooperation:
implication for C–C forming reactions involving Ni-alkyl-b-silaneswz
Chun-Yu Ho* and Lisi He
Received 27th July 2011, Accepted 7th November 2011
DOI: 10.1039/c1cc14593b
Via a cooperation of NHC, Si substituents and a M center, b-Si
elimination was attenuated, revealing a new way to attain a high
Ni-b-SiR3 : Ni-r-SiR3 ratio. The scope is illustrated by a head-
to-tail vinylsilane-a-olefin hydroalkenylation.
Catalytic cross-hydroalkenylation (H.A.) of vinylarenes or
dienes with simple olefins is recognized as one of the most
efficient methods for producing olefins of a higher complexity.1
Expanding this process to other widely available and structurally
diverse olefin acceptors is highly desirable, yet remains a daunting
Scheme 1 Hypothetical scenario for a cross-H.A. of 1 and 2 to 3.
challenge due to potential complications. In particular, vinylsilanes
should be able to undergo cross-H.A. using various very efficient
P based Pd/NiH catalysts, but we are unaware of any protocols
may then undergo a chemo- and regioselective alkene insertion
guided by both the NHC and the size of the alkene substituents
that can overcome the competitive b-Si elimination (S.E.)
(H, R1 and SiR3). The cycle closes with a b-H elimination to
form the H.A. products. A hypothetical scenario with high
selectivity for (1) H.A. to b-S.E., (2) head-to-tail (h–t) to h–h or
t–t or t–h, and (3) cross- to homo-product, is outlined in
Scheme 1.
followed by intermolecular insertion of an unactivated olefin.1g,h
Only certain activated p-systems, which have a close-by stabilizing
olefin tether, can be used, possibly because the competition can be
strong even when bidentate ligand is used.2 Large differences in
various SiR3 groups’ reactivity and size may also present inherent
complications that need special attention to obtain better results.3
Thus a new general approach that can shuffle off b-S.E. is
imperative before the full potential of vinylsilanes for preparing
organosilanes by C–C formation can be realized.
As SiMe2Ph is a medium-sized Si group that is frequently
employed as a masked OH in organic syntheses,9 and an
expedient pilot test can be conducted using an easily accessible
in situ generated [IPr-NiH(OTf)],4a (vinyl)SiMe2Ph 1a was first
tested in the presence of the above NHC-NiH with 1-octene 2a.
Our results show that the homoallylsilane 3a can be obtained
catalytically for the first time under optimized conditions.10
This process is scalable and can be done in THF, with a low
level of 1a homo-dimerization and 3a iso-/oligo-/polymerization
(Table 1, entries 1, 2). This is also the first Ni-catalyzed, inter-
molecular a-olefin cross-H.A. that does not use vinylarenes/
dienes as the basis for high chemo- and regioselectivity (i.e. it is
not a benzylic/p-stabilization directed event). Although the
1,2-hydrometallated 1a offers no such stabilization and is less
sterically hindered, the competitive events versus the desired
C–C formation, (e.g. NHC-alkyl reductive or adverse b-H
elimination4,11) are both positively attenuated. Equally signif-
icant for the above H.A. aspects is that this study reveals the
potential of IPr-Ni as a general organosilane preparation tool,
complementing the outcomes of those featuring a b-S.E. key
step from the same set of substrates (Scheme 2).5–7
Continuing our studies aimed at discovering new applications
of NHC in Ni catalysis,4 and with the key precedents for
efficient hydrometallated vinylsilane b-S.E.5–7 and H.A.1 in
mind, we envision a new approach that can shuffle off the
facile b-S.E., with its scope illustrated by vinylsilane–a-olefin
cross-H.A. The strategy involves a bulky NHC-NiH.8 This
combination may concurrently suppress the anticipated b-S.E.
and accommodate the chemo- and regioselectivity problems
associated with the variability in size and polarizability of
vinylsilanes. Under the remarkable influence of NHC steric
bulk, a 1,2- over a 2,1-hydronickelated vinylsilane species is
generated preferentially and the overlap of the Ni–Si orbitals is
positively attenuated (i.e., b-S.E. becomes difficult). This species
Center of Novel Functional Molecules, Department of Chemistry,
The Chinese University of Hong Kong, and CUHK SZRI.
E-mail: jasonhcy@cuhk.edu.hk
w This article is part of the ChemComm ‘Emerging Investigators 2012’
themed issue.
Under those classic conditions with less steric hindrance
(e.g. L = P or small NHC, LM = Ru, Rh, Co, Fe, Pd), the
S
reactions along H.A. pathways cease as a result of C2H4 loss
due to more effective b-S.E. Balancing the sizes of SiR3, R1 and
z Electronic supplementary information (ESI) available: Experimental
details and data for new compounds. See DOI: 10.1039/c1cc14593b
c
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Chem. Commun., 2012, 48, 1481–1483 1481

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Table 1 Group 14 vinylmetalloid–a-olefin h–t cross-hydroalkenylationa
Yield (%)
(3 : 5)
Entry 1, R3M =
2, R1 =
3
1
Me2PhSi
1a n-Hexyl
2a 3a 95; 95b; 84c;
11d; 6e
2f
3
1a —
1b
1c
o5; 10g
Scheme 2 Classic vinylsilane–olefin coupling with b-S.E. key step.
Me3Si
Et3Si
(MeO)3Si
2a 3b 70
3c 86; 93h
4
1d
2a 3d 90 (71 : 29)
95i (77 : 23)
3e 95i (80 : 20)
3f 85i (85 : 15)
2a 3g 95i (80 : 20)
3h 86
(EtO)3Si
(i-PrO)3Si
1e
1f
1g
1h
1i
5
6
Me(MeO)2Si
Me(TMSO)2Si
Me2(EtO)Si
Bu2(BuO)Si
Ph2(EtO)Si
Me2Si(OSiMe2vinyl) 1l
Et3Ge
cis-TES-styrene
2a 3i 81i (84 : 16)
3j 70
1j
1k
3k 56h (83 : 17)
3l 70j
7
8
9
1m
2a 3m 30k,l
2n 3n 24l,m
2o 3o 95
Scheme 3 Vinylsilane labelling experiment.
1n iso-Butyl
1a CH2Ph
CH2CH2Ph
due to more significant 2 consumption and a lower second
insertion energy barrier. The preference for 3 can be retained
by enlarging the SiR3 or lowering the temperature. When we
10
11
12
13
14
15
16
17
18
19
2p 3p 88l
p-CH2(veratrole) 2q 3q 95
2r 3r 72
(CH2)9OMe
CH2OCH2Ph
2s 3s 95n
(CH2)8COOMe 2t 3t 80
5d
used vinylGeEt3 or b-substituted 1, modest yields of 3 were
observed and notably no 4 or 5 resulted (entries 7, 8, 18).
The cross-H.A. scope and chemo- and regioselectivity are
also fine. a-Olefins with alkyl/allyl/aryl/benzyl ethers and the
enolizable ester (2q–t) are all compatible. Attempts to employ 1a
with a-olefins commonly used in vinylsilane couplings failed
(entry 19, o5% 1a conversion),5 possibly due to the anticipated
unfavorable steric effect in both pathways.
1b
1e
1k
1m
2s 3u 75n
2o 3v 95i (75 : 25)
3w 95h (85 : 15)
3x 63k
1a t-Butyl; Cy; Ph
— o5
a
See ESI for a detailed procedure. Yields are an average of at least
1
two runs based on 1; 3 were determined by H NMR and isolation; 5
and 3 : 5 were determined only by 1H NMR because of its limited
stability on isolation. Several entries were analyzed also by GC,
The following experiments were carried out to gain insight
into the proposed b-S.E. suppression mechanism, and to provide
more information on the hydrometallation regioselectivity
achieved by this system. Firstly, sterically fair TMS(2-D-ethenyl)
and allylic cleavage viable 2s were tested as substrates in this
context (Scheme 3, see ESI for detailsz). These yielded the h–t 3
with o5% D/H scrambling according to the NMR spectrum.
This is notable given that b-D/H eliminations can be competitive
if a 2,1-hydronickelation occurs, and ethylene Si-metallation
reversibility is high even at low temperatures.2 Since the oxidative
cyclization/addition or vinylic CD/H activation based on Ni(0),12
or Ir/Ru vinylsilane–styrene/carbonyl coupling mechanisms10,13
may not be generally compatible with allyl ethers 2s, working
hypotheses based on these processes may not be fully accountable.
Secondly, the use of TESOTf means that there are other
mechanistic paradigms that can plausibly explain the results if
the competitive Si–C bond formation is not efficient, such as
one based on a Chalk–Harrod type of silylation.14 We thus
studied the effect of this additive (Scheme 4, i.e. selectivities are
obtained by the action of both TES and IPr rather than
Scheme 1 as shown). A IPr(s-TES)NiH catalyst could arise
from unintended TESH generation and addition to IPr-Ni, so
we attempted to mimic Scheme 4 by adding TESH directly to
IPr-Ni(0). However, this did not result in the formation of any
3a or corresponding hydrogenative dimers under these mild
conditions. We also found that the cross-H.A. is insensitive to
b
see ESI. In THF. 2.5 fold scale. IMes as ligand. PCy3 as ligand.
c
d
e
f
g
h
i
j
1.0 mmol 1a. 45 1C. 40 h. 0 1C. 3l refers to both ends being
h–t reacted cross-product, see ESI for other products observed.
k
l
30 mol% catalyst. Not fully separated, NMR yield only.
3 mmol 2n, 48 h, 3n = (3-iBu-2-Ph)-homoallylTES. 1 mmol 2s.
m
n
L allows a regioselective 2,1-insertion which forms a Si–C bond
and produces internal olefin 4. In our case, a high Ni-b-SiR3 to
s-SiR3 ratio is achieved instead, as implied by the o5% b-Si
transferred product 4 that was observed in the crude NMR
spectrum. The detrimental effect of the combination of either a
smaller L (IMes) with Ni, or IPr with a larger M (Pd) on the
yield, or 3 : 4 ratio, demonstrates that the choice of L–M
combination is strikingly critical (see ESIz).
Next, several samples of 1 with different SiR3 properties
were examined to evaluate the scope of the IPr-Ni-directed
b-S.E. suppression via H.A. (entries 3–6). They all reacted
smoothly with 2a with high H.A. to b-S.E. ratio and chemo-
and regioselectivity. This consistency is notable because the
electronic properties of the Si center can be perturbed strongly
by its substituents, resulting in the reverse polarization of the
proximate vinyl,3 suggesting it is an IPr catalyst-directed
process. If the 1,2-hydronickelation step was hampered by
the choice of 1, (e.g. a small vinylSi(OR)3), then a limited
amount of h–t dimer 5 was also observed. This is presumably
c
1482 Chem. Commun., 2012, 48, 1481–1483
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74, 4565; (d) G. Hilt and D. F. Weske, Chem. Soc. Rev., 2009,
38, 3082; (e) G. Hilt, Synlett, 2011, 1654; (f) J. Joseph,
T. V. RajanBabu and E. D. Jemmis, Organometallics, 2009,
28, 3552; diene–vinylsilane examples: (g) G. Gailiunas,
G. V. Nurtdinova, F. G. Yusupova, L. M. Khalilov,
V. K. Mavrodiev, S. R. Rafikov and V. P. Yurev, J. Organomet.
Chem., 1981, 209, 139; (h) P. Cros, C. Triantaphylides and
G. Buono, J. Org. Chem., 1988, 53, 185.
2 Pd: (a) A. M. LaPointe, F. C. Rix and M. Brookhart, J. Am. Chem.
Soc., 1997, 119, 906; (b) I. Macsari, E. Hupe and K. J. Szabo,
J. Org. Chem., 1999, 64, 9547; (c) X. Wang, H. Chakrapani,
C. N. Stengone and R. A. Widenhoefer, J. Org. Chem., 2001,
66, 1755; Ni: (d) B. Marciniec and H. Maciejewski, J. Organomet.
Chem., 1993, 454, 45.
Scheme 4 Alternative mechanism directed by an IPr(s-TES)NiH.
3 Si dichotomous effect: The Chemistry of Organic Silicon Compounds,
ed. S. Patai and Z. Rappoport, Wiley, Chichester, 1989.
4 (a) C.-Y. Ho and L. He, Angew. Chem., Int. Ed., 2010, 49, 9182;
(b) C.-Y. Ho, Chem. Commun., 2010, 46, 466; (c) C.-Y. Ho, L. He
and C.-W. Chan, Synlett, 2011, 1649.
the size of the silyl triflate. Yet, the most convincing evidence
for excluding (s-TES)Ni species participation is that the
desired H.A. can be performed in silyl triflate free catalyst
generation conditions, such as a AgOTf accelerated anion
exchange with [IPrNi(allyl)Cl]15 (see ESIz).
5 (a) B. Marciniec, New J. Chem., 1997, 21, 815; (b) I. Ojima, Z. Li
and J. Zhu, in The Chemistry of Organic Silicon Compounds, ed.
Z. Rappoport and Y. Apeloig, Wiley, Chichester, 1998, p. 2;
(c) P. Pawluc, W. Prukala and B. Marciniec, Eur. J. Org. Chem.,
2010, 219; Ge: (d) B. Marciniec, H. Lawicka, M. Majchrzak,
M. Kubicki and I. Kownacki, Chem.–Eur. J., 2006, 12, 244.
6 (a) F. Seitz and M. S. Wrighton, Angew. Chem., Int. Ed. Engl.,
1988, 27, 289; (b) Y. Wakatsuki, H. Yamazaki, M. Nakano and
Y. Yamamoto, J. Chem. Soc., Chem. Commun., 1991, 703;
(c) B. Marciniec and C. Pietraszuk, J. Chem. Soc., Chem.
Commun., 1995, 2003; (d) B. Marciniec and C. Pietraszuk, Organo-
metallics, 1997, 16, 4320; (e) L. N. Lewis and N. Lewis, J. Am.
Chem. Soc., 1986, 108, 7228.
Thirdly, on the basis that treating the catalyst mixture with the
selected 1 can provide high Si recovery without dehydrosilylation
even in the equimolar reactions and in the absence of 2, and the
insertion of competitive p-systems into Ni-SiR3/H/b-SiR3 could
be sensitive to the s-SiR3 properties,6,14 describing the results in
terms of a s-SiR3Ni catalyst may be less important. Overall,
these results are consistent with Scheme 1, yet efforts to further
support the hypothesized intermediates using crystals or in situ
NMR failed, thus the precise mechanism awaits further study.
Synthetically, when comparing with the products obtained by
silylation of corresponding alkynes and 2-substituted dienes,
olefin cross-metathesis, or the classic M–H catalyzed vinylsilane–
olefin coupling (vinyl/allylsilanes),5–7,13,14 this work represents
the first general catalytic non-reductive tool to access homo-
allylsilanes featuring a less readily available gem-olefin. Simply
starting with two off-the-shelf chemical feedstocks directly, this
approach lowers the hurdles caused by using 3 as a primary
material in both organosilicon/organic syntheses or in studying
their reactivity. The value of 3 is underscored by its many
synthetic applications, such as in Fleming–Tamao oxidations9
and formal alkene functionalizations (e.g. g-Si alcohols/halides,
b-Si carbonyls), and also the utilization of the Si-activating/
directing a-/b-/g-effect. See ESI for a pendent Si promoted
regioselective Riley oxidation and related examples.
7 With long chain a-olefins: (a) Z. Foltynowicz and B. Marciniec,
J. Organomet. Chem., 1989, 376, 15. Bulky 1 alone cannot inverse
the low M–H : SiR3 ratio: (b) B. Marciniec, J. Waehner, P. Pawluc
and M. Kubicki, J. Mol. Catal. A: Chem., 2007, 265, 25.
8 (a) H. Clavier and S. P. Nolan, Chem. Commun., 2010, 46, 841;
(b) T. Droge and F. Glorius, Angew. Chem., Int. Ed., 2010,
49, 6940.
¨
9 I. Fleming, R. Henning, D. C. Parker, H. E. Plaut and P. E. J.
Sanderson, J. Chem. Soc., Perkin Trans. 1, 1995, 317.
10 The closest advance to our discovery for homoallylsilane appears
to be Ru-catalyzed Michael acceptor–vinylsilane coupling (cross-
H.A. in a broader sense), featuring a trisubstituted electron-
deficient olefin: (a) B. M. Trost, K. Imi and I. W. Davies, J. Am.
Chem. Soc., 1995, 117, 5371; (b) S. Ueno, E. Mizushima,
N. Chatani and F. Kakiuchi, J. Am. Chem. Soc., 2006,
128, 16516; (c) R. Martinez, R. Chevalier, S. Darses and
J.-P. Genet, Angew. Chem., Int. Ed., 2006, 45, 8232. It first involves
a Michael acceptor directed b-CH activation, while here it first
involves a vinylsilane hydrometallation, followed by catalyst direc-
ted unactivated a-olefin insertion.
In summary, the NHC-Ni combination was first recognized as a
potential tool that may attenuate b-S.E.-alkene evolution activity
by a series of highly selective vinylsilane–a-olefin h–t cross-H.A.
examples. Based on the generality and selectivity of this b-S.E.
suppression approach, along with the strategic advantages it
enables, we anticipate that it will find even broader prospects in
other Ni-catalyses involving a b-Si, such as in general C–C forming
reactions or insertions other than those using a-olefins as partners.
Support for this work was provided by CUHK TBF 3120090,
and NSFC project 21102122. We sincerely thank Prof. Tamejiro
Hiyama for advice.
11 N. D. Clement and K. J. Cavell, Angew. Chem., Int. Ed., 2004,
43, 3845.
12 (a) R. Matsubara and T. F. Jamison, J. Am. Chem. Soc., 2010,
132, 6880.; (b) J. Montgomery, Angew. Chem., Int. Ed., 2004,
43, 3890; (c) C.-Y. Ho, K. D. Schleicher, C.-W. Chan and
T. F. Jamison, Synlett, 2009, 2565; (d) S. Ogoshi, T. Haba and
M. Ohashi, J. Am. Chem. Soc., 2009, 131, 10350.
13 (a) B. Marciniec, I. Kownacki and M. Kubicki, Organometallics,
2002, 21, 3263. Ir catalyzed allylic substitution: (b) J. F. Hartwig
and L. M. Stanley, Acc. Chem. Res., 2010, 43, 1461.
14 Catalytic reductive-coupling of 1 and styrene with R3Si–NiH:
(a) H. Maciejewski, B. Marciniec and I. Kownacki,
J. Organomet. Chem., 2000, 597, 175; (b) A. J. Chalk and
J. F. Harrod, J. Am. Chem. Soc., 1965, 87, 16; (c) J. F. Harrod
and A. J. Chalk, J. Am. Chem. Soc., 1965, 87, 1133; (d) D. Troegel
and J. Stohrer, Coord. Chem. Rev., 2011, 255, 1440.
Notes and references
1 (a) T. V. RajanBabu, Chem. Rev., 2003, 103, 2845;
(b) T. V. RajanBabu, Synlett, 2009, 853; (c) H. J. Lim,
C. R. Smith and T. V. RajanBabu, J. Org. Chem., 2009,
15 B. R. Dible and M. S. Sigman, J. Am. Chem. Soc., 2003, 125,
872.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1481–1483 1483