Switch in stereoselectivity caused by the isocyanide structure in the rhodium-catalyzed silylimination of alkynes

Article abstract of DOI:10.1021/ja202881y

The reaction of terminal alkynes with hydrosilanes and tert-alkyl isocyanides in the presence of Rh₄(CO)₁₂ gives (Z)-β-silyl-α,β-unsaturated imines in good yields. On the other hand, the use of aryl isocyanides in place of tert-alkyl isocyanides leads to the formation of E isomers.

Full text of DOI:10.1021/ja202881y

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Switch in Stereoselectivity Caused by the Isocyanide Structure in the
Rhodium-Catalyzed Silylimination of Alkynes
Yoshiya Fukumoto,* Motoyuki Hagihara, Fuyuko Kinashi, and Naoto Chatani
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
S Supporting Information
b
Scheme 1. Rh4(CO)12-Catalyzed Reactions of Alkynes with
Hydrosilanes and Isocyanides
ABSTRACT: The reaction of terminal alkynes with hydro-
silanes and tert-alkyl isocyanides in the presence of Rh4-
(CO)12 gives (Z)-β-silyl-R,β-unsaturated imines in good
yields. On the other hand, the use of aryl isocyanides in place
of tert-alkyl isocyanides leads to the formation of E isomers.
ecent advances have resulted in an expanded use of vinylsi-
R
lanes as reagents in electrophilic substitution1 and transition-
metal-catalyzed cross-coupling reactions.2 The regio- and stereo-
selective addition of silicon compounds (e.g., SiꢀE with E = H,
Si, Ge, Sn, B, P, S, Se, Te, CN, etc.) to CtC triple bonds in the
presence of Lewis acids or transition-metal complexes is an
efficient strategy for preparing stereodefined vinylsilanes.3 A
high degree of stereochemical control has been attained in the
hydrosilylation of alkynes for selective production of both cis and
trans addition products, depending on the reaction conditions
used.4 Other reactions usually proceed in a cis fashion, except for
the trans carbosilylation of alkynes catalyzed by Lewis acids.5 The
reaction mechanism involves syn addition of either a SiꢀM bond
or a MꢀE bond to the alkyne, followed by reductive elimination
after oxidative addition of the SiꢀE bond to the transition metal
to form a SiꢀMꢀE species. Treatment of alkynes with a
combination of a hydrosilane and carbon monoxide also gives
mainly cis addition products as the result of the addition of silyl
and formyl groups across the CtC triple bonds. This reaction is
known as silylformylation of alkynes.6
corresponding aldehyde 9a without loss in the product yield or
E/Z ratio (Scheme 2). Whereas HSiEt2Me also reacted to give
(Z)-2a0 stereoselectively (E/Z = 12/88) in 67% yield, the desired
product was not obtained when HSi(OMe)3 was used under the
present reaction conditions.
In examining isocyanides other than tert-octyl isocyanide, we
found that the stereoselectivity of the product could be switched
depending on the substituent on the isocyanide nitrogen. Iso-
cyanides bearing tert-alkyl groups, such as 1-adamantyl and tert-
butyl isocyanides, also reacted with 1a and HSiMe2Ph to
predominantly give Z isomers, although the stereoselective out-
come was decreased slightly in comparison with that of tert-octyl
isocyanide (entries 2 and 3). The use of cyclohexyl and butyl
isocyanides provided 5a and 6a, respectively, in good yields with
a roughly equal mixture of (E)- and (Z)-imines (entries 4 and 5).
In contrast, high E selectivity was attained when 2,6-dimethyl-
phenyl isocyanide (xylyl isocyanide) or 2,6-diethylphenyl iso-
cyanide was employed in the catalytic reaction (entries 6 and 7).
In addition, hydrolysis of 7a gave 9a in 82% isolated yield with
retention of the E selectivity (E/Z = 98/2).
Several types of alkynes were subjected to the catalytic
reaction with HSiMe2Ph and either tert-octyl isocyanide or xylyl
isocyanide in order to evaluate the scope and limitations of the
present transformation, and the results are summarized in
Table 2. Functional groups such as methoxy (1c), ester (1d),
and trifluoromethyl (1e) groups at the para position of the
aromatic ring were tolerated in the reaction, although electron-
withdrawing substituents retarded the reaction rate (entries
5ꢀ10). A switch in E/Z selectivity for the product imines was
also observed in reactions of other aryl-substituted alkynes with
the same product ratio as was found for 1a. The vinyl-substituted
alkyne 1f was reacted under each set of reaction conditions to
Isocyanides, which are isoelectronic with CO, are often
employed as synthetic surrogates of CO in the production of
imine derivatives under acid or transition-metal catalysis.7 Unlike
CO, both the electronic and steric nature of isocyanides permit
improvements in both product yield and selectivity and occa-
sionally allow the course of the reaction to be controlled.8,9
Herein we report on the rhodium-catalyzed reaction of alkynes
with hydrosilanes and isocyanides to afford β-silyl-R,β-unsatu-
rated imines. Interestingly, silylimination with aryl isocyanides
leads to the formation of E isomers, while the use of tert-alkyl
isocyanides gives Z isomers (Scheme 1).10
Treatment of phenylacetylene (1a, 1 mmol) with dimethyl-
phenylsilane (2 mmol) and 1,1,3,3-tetramethylbutyl isocyanide
(tert-octyl isocyanide, 2 mmol) in the presence of Rh4(CO)12
(0.015 mmol) as a catalyst in tetrahydrofuran (THF) at 70 °C for
30 min gave imine 2a in 91% yield with 93% Z selectivity, as
determined by 1H NMR analysis of the reaction mixture
(Table 1, entry 1). An attempt to isolate 2a by bulb-to-bulb
distillation under reduced pressure resulted in a decrease in the
yield to 53%. Therefore, the imine was hydrolyzed to give the
Received: March 30, 2011
Published: June 06, 2011
r
2011 American Chemical Society
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Table 1. Rh4(CO)12-Catalyzed Silylimination of 1a with
HSiMe2Ph and Various Types of Isocyanidesa
Table 2. Rh4(CO)12-Catalyzed Silylimination of Alkynes
with HSiMe2Ph and tert-Octyl Isocyanide or Xylyl
Isocyanidea
a Reaction conditions: 1a (1 mmol), HSiMe2Ph (2 mmol), isocyanide (2
mmol), and Rh4(CO)12 (0.015 mmol) in THF (8 mL) at 70 °C.
b Product yields and E/Z ratios were determined by 1H NMR spectros-
c
copy using 1,3,5-trimethoxybenzene as an internal standard. Isolated
yield was 53% (E/Z = 10/90) as the imine.
Scheme 2. Hydrolysis of Imines 2a and 7a to Aldehyde 9a
a Reaction conditions: alkyne (1 mmol), HSiMe2Ph (2 mmol), isocya-
nide (2 mmol), and Rh4(CO)12 (0.015 mmol) in THF (8 mL) at 70 °C.
b Product yields and E/Z ratios were determined by 1H NMR spectros-
copy using 1,3,5-trimethoxybenzene as an internal standard. c Rh4-
e
(CO)12 (0.02 mmol). d THF (1 mL) at 40 °C. The hydrosilylation
product was also obtained in 30% yield.
selectivity (E/Z = 30/70), but this was improved to 12/88 by
decreasing the reaction temperature to 40 °C (entries 13 and
14). However, this protocol was not applicable to sterically
demanding alkynes [e.g., tert-butylacetylene (1i)] or internal
alkynes [e.g., diphenylacetylene (1j)] (entries 18ꢀ21).
give (Z)-2f (entry 11) and (E)-7f (entry 12). The reaction
of 1-octyne (1g) with HSiMe2Ph and tert-octyl isocyanide
under the original reaction conditions afforded 2g with moderate
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To examine the reaction mechanism and especially to gain
information regarding the Z-to-E isomerization process, some
additional experiments were carried out. The reaction mechan-
ism for silylimination, which would be expected to be closely
related to that for silylformylation, should essentially afford Z
isomers. Thus, isolated Z-rich 7a (E/Z = 14/86) was exposed to
the standard reaction conditions in place of an alkyne in order to
study the stability of the products. However no isomerization
occurred, and 100% of the starting material was recovered. E-rich
7a (E/Z = 81/19) also remained intact under the same reaction
conditions (Scheme 3). These results suggest that Z-to-E iso-
merization likely occurs during the silylimination catalytic
cycle, for example, via the formation of a zwitterionic carbene-
metal11 or a metallacyclopropene12 intermediate derived from
the (Z)-β-silylvinylmetal complex or via direct anti addition of
the silylmetal species to an alkyne.13
The reaction of 1a in the presence of equimolar amounts of
tert-octyl isocyanide and xylyl isocyanide was next examined to
estimate the variation in the distribution of products. To our
surprise, E-rich 7a (E/Z = 85/15) was obtained as a major
product along with Z-rich 2a (E/Z = 13/87), indicating that the
stereoselective preferences for both 2a and 7a in the reactions of
1a with tert-octyl and xylyl isocyanide were maintained (Scheme 4).
This observation indicates that the isomerization likely occurs
after the formation of the iminometal species via the insertion of
the isocyanide into the RhꢀC bond.
A mechanistic rationale for the present catalytic reaction is
proposed in Scheme 5. Formation of the RhꢀSi species by the
reaction of Rh4(CO)12 with a hydrosilane, followed by syn addi-
tion of the RhꢀSi bond to an alkyne affords the (Z)-β-silylvi-
nylrhodium species Z-I. Subsequent insertion of an isocyanide
into the RhꢀC bond forms the iminorhodium intermediate Z-II,
which subsequently reacts with a hydrosilane to give the Z isomer
with regeneration of the RhꢀSi species when tert-alkyl isocya-
nides are employed. In case of the reaction with aryl isocyanides
leading to the formation of E isomers, we propose that the
zwitterionic intermediate A is formed in the isomerization of Z-II
to E-II, with stabilization of the anion on the nitrogen by the
electron-withdrawing aryl group.14 It would appear that the
reasonably good E selectivities in the reactions of aryl- and
vinyl-substituted terminal alkynes with aryl isocyanides can be
attributed to the stabilization of the vinyl intermediate A by con-
jugation with the unsaturated substituents. However, this does
not account for the results obtained when primary- and second-
ary-alkyl-substituted isocyanides are used at the present stage.
In the competition experiment shown in Scheme 4, a mixture
of (E)- and (Z)-7a was obtained as the main product, although
the reaction with xylyl isocyanide alone was slower than that with
tert-octyl isocyanide (Table 2, entries 1 and 2). One possible
explanation for the results is that tert-octyl isocyanide is a better
ligand than xylyl isocyanide in terms of accelerating the reaction,
while on the other hand, the migration of the β-silylvinyl group in
Z-I to the carbon atom of xylyl isocyanide coordinated on the
rhodium center to give Z-II occurs more readily than in the case
of tert-octyl isocyanide because of the electron-withdrawing
nature of the aryl group on the isocyanide nitrogen.
Scheme 3. E/Z Stability of 7a
Scheme 4. Silylimination of 1a in the Presence of Equimolar
Amounts of tert-Octyl Isocyanide and Xylyl Isocyanide
In conclusion, we have reported on the discovery of an
interesting stereochemical switch in the rhodium-catalyzed sily-
limination of alkynes to give vinylsilane derivatives. A new reaction
Scheme 5. Plausible Reaction Mechanism
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pathway involving the production of a zwitterionic species has
been proposed to explain the formation of E isomers. Further
investigations dealing with the relationship between the sub-
stituent on the isocyanide nitrogen and the product stereoselec-
tivity, details of the reaction mechanism, and the synthetic utility
of this stereoselective reaction are currently underway.
T.; Berthet, J.-C.; Ephritikhine, M.; Eisen, M. S. Organometallics 2008,
27, 3103–3112 and references cited therein.
(8) For example, see: Tobisu, M.; Kitajima, A.; Yoshioka, S.; Hyodo,
I.; Oshita, M.; Chatani, N. J. Am. Chem. Soc. 2007, 129, 11431–11437.
(9) For recent reviews of isocyanides in organic synthesis, see: (a)
Lygin, A. V.; de Meijere, A. Angew. Chem., Int. Ed. 2010, 49, 9094–9124.
(b) El Kaim, L.; Grimaud, L. Tetrahedron 2009, 65, 2153–2171. (c)
D€omling, A. Chem. Rev. 2006, 106, 17–89. (d) Suginome, M.; Ito, Y. Sci.
Synth. 2004, 19, 445–530.
(10) Ojima et al. reported silylformylation of alkynes catalyzed by
(tBuNdC)4RhCo(CO)4. However, no formation of the correspond-
ing imines was described. See: Ojima, I.; Donovan, R. J.; Eguchi, M.;
Shay, W. R.; Ingallina, P.; Korda, A.; Zeng, Q. Tetrahedron 1993, 49,
5431–5444.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
characterization data for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) Ojima, I.; Clos, N.; Donovan, R. J.; Ingallina, P. Organometallics
1990, 9, 3127–3133.
’ AUTHOR INFORMATION
(12) Tanke, R. S.; Crabtree, R. H. J. Chem. Soc., Chem. Commun.
1990, 1056–1057.
(13) Sridevi, V. S.; Fan, W. Y.; Leong, W. K. Organometallics 2007,
26, 1157–1160.
Corresponding Author
fukumoto@chem.eng.osaka-u.ac.jp
(14) Many studies have found that a silyl group can slightly stabilize a
cation on an R-carbon. For a review, see: Lambert, J. B. Tetrahedron
1990, 46, 2677–2689.
’ ACKNOWLEDGMENT
This work was supported by the Ministry of Education,
Culture, Sports, Science, and Technology, Japan. Thanks are
also given to the Instrumental Analysis Center, Faculty of Engi-
neering, Osaka University, for assistance with HRMS and
elemental analyses.
’ REFERENCES
(1) For reviews, see: (a) Curtis-Long, M. J.; Aye, Y. Chem.—Eur. J.
2009, 15, 5402–5416. (b) Oshima, K. Sci. Synth. 2002, 4, 713–756. (c)
Fleming, I.; Barbero, A.; Walter, D. Chem. Rev. 1997, 97, 2063–2192. (d)
Fleming, I.; Dunoguꢀes, J.; Smithers, R. Org. React. 1989, 37, 57–575.
(2) For recent reviews, see: (a) Denmark, S. E. J. Org. Chem. 2009,
74, 2915–2927. (b) Denmark, S. E.; Regens, C. S. Acc. Chem. Res. 2008,
41, 1486–1499. (c) Denmark, S. E.; Baird, J. D. Chem.—Eur. J. 2006,
12, 4954–4963. (d) Hiyama, T.; Shirakawa, E. Top. Curr. Chem. 2002,
219, 61–85.
(3) For general reviews, see: (a) Suginome, M.; Matsuda, T.;
Ohmura, T.; Seki, A.; Murakami, M. In Comprehensive Organometallic
Chemistry III; Crabtree, R. H., Mingos, M. P., Eds.; Elsevier: Amsterdam,
2007; Vol. 10, Chapter 16, pp 725ꢀ787. (b) Beletskaya, I.; Moberg, C.
Chem. Rev. 2006, 106, 2320–2354. (c) Suginome, M.; Ito, Y. Chem. Rev.
2000, 100, 3221–3256. (d) Beletskaya, I.; Moberg, C. Chem. Rev. 1999,
99, 3435–3461.
(4) For recent reviews, see: (a) Ball, Z. T. In Comprehensive
Organometallic Chemistry III; Crabtree, R. H., Mingos, M. P., Eds.;
Elsevier: Amsterdam, 2007; Vol. 10, Chapter 17, pp 789ꢀ813. (b)
Trost, B. M.; Ball, Z. T. Synthesis 2005, 853–887.
(5) (a) Yoshikawa, E.; Kasahara, M.; Asao, N.; Yamamoto, Y.
Tetrahedron Lett. 2000, 41, 4499–4502. (b) Yoshikawa, E.; Gevorgyan,
V.; Asao, N.; Yamamoto, Y. J. Am. Chem. Soc. 1997, 119, 6781–6786.
(c) Asao, N.; Yoshikawa, E.; Yamamoto, Y. J. Org. Chem. 1996,
61, 4874–4875. (d) Yeon, S. H.; Han, J. S.; Hong, E.; Do, Y.; Jung,
I. N. J. Organomet. Chem. 1995, 499, 159–165. For a review, see:
(e) Asao, N.; Yamamoto, Y. Bull. Chem. Soc. Jpn. 2000, 73, 1071–1087.
(6) For preceding studies, see: (a) Ojima, I.; Ingallina, P.; Donovan,
R. J.; Clos, N. Organometallics 1991, 10, 38–41. (b) Matsuda, I.; Ogiso,
A.; Sato, S.; Izumi, Y. J. Am. Chem. Soc. 1989, 111, 2332–2333. For a
review, see: (c) Aronica, L. A.; Caporusso, A. M.; Salvadori, P. Eur. J. Org.
Chem. 2008, 3039–3060.
(7) For recent reports on imine syntheses using isocyanide as a C1
source, see: (a) Tobisu, M.; Imoto, S.; Ito, S.; Chatani, N. J. Org. Chem.
2010, 75, 4835–4840. (b) Park, S.; Shintani, R.; Hayashi, T. Chem.
Lett. 2009, 38, 204–205. (c) Zhang, W.-X.; Nishiura, M.; Hou, Z.
Angew. Chem., Int. Ed. 2008, 47, 9700–9703. (d) Barnea, E.; Andrea,
10017
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