Reactivity of a polar silene toward terminal alkynes: Preference for C-H insertion over cycloaddition

Article abstract of DOI:10.1021/om100756p

A variety of terminal alkynes were added to Mes₂Si=CHCH ₂t-Bu, 4, a naturally polarized silene. Three different modes of reactivity were observed: addition across the acetylenic C-H bond to give silylacetylenes 6a-i and 12, cycloaddi

Full text of DOI:10.1021/om100756p

5972 Organometallics 2010, 29, 5972–5981
DOI: 10.1021/om100756p
Reactivity of a Polar Silene toward Terminal Alkynes: Preference for
C-H Insertion over Cycloaddition
Kaarina K. Milnes, Laura C. Pavelka, and Kim M. Baines*
Department of Chemistry, University of Western Ontario, London, Ontario, Canada, N6A 5B7
Received August 2, 2010
A variety of terminal alkynes were added to Mes2SidCHCH2t-Bu, 4, a naturally polarized silene.
Three different modes of reactivity were observed: addition across the acetylenic C-H bond to give
silylacetylenes 6a-i and 12, cycloaddition to give silacyclobutenes 7 and 9, and ene-addition to give
vinylsilane 8. The reactivity of the naturally polarized silene 4 toward terminal alkynes is compared
to that of nonpolar silenes.
Introduction
silacyclobutenes; however, when the alkyne or the silene
contains an R-hydrogen, the formation of ene-adducts is also
possible.5,6 Both concerted5b,c and diradical6e,g pathways for
silacyclobutene formation have been proposed. We have
recently presented unambiguous evidence for the formation of
a diradical intermediate during the addition of terminal
alkynes to Brook silenes (Scheme 1).7-9 Although these results
provided evidence for the nature of the intermediate formed
during the reaction and an understanding of the factors govern-
ing the regioselectivity of the reaction, Brook silenes are a
special class of relatively nonpolar silenes, and thus, a general-
ization of the mechanism of addition of alkynes to all silenes
cannot be made. Thus, we were interested in studying the
mechanism of alkyne addition to naturally polarized silenes.
After a thorough review of the literature, we realized that
relatively little research has been reported on the addition of
alkynes to polar silenes in comparison to the relatively large
body of work reported on the addition of alkynes to the atypical,
nonpolar Brook silenes.2 The published work has focused
on the addition of alkynes to three transient silenes: the highly
reactive Wiberg silene Me2SidC(SiMe3)2, 1,1-dimethylsilene,
Since the pioneering work of Gusel’nikov and Flowers in
1967,1 silene (R2SidCR2) chemistry has matured significant-
ly;2 much is now known regarding the properties and reactivity
of these fundamentally important compounds, and applica-
tions of silene chemistry are now being developed. For exam-
ple, silenes have been used as monomers in the synthesis of new
inorganic polymers3 and as reagents in organic synthesis.4 To
take full advantage of the chemistry of silenes, it is necessary to
gain a better understanding of the scope and mechanisms of
the reactions of silenes.
We have been interested in the addition of alkynes to Brook
silenes, (Me3Si)2SidCR(OSiMe3), a well-known regioselective
reaction of these relatively nonpolar silenes that typically yields
*Corresponding author. E-mail: kbaines2@uwo.ca.
(1) Gusel’nikov, L. E.; Flowers, M. C. J. Chem. Soc., Chem. Commun.
1967, 864.
(2) For reviews see: (a) Gusel’nikov, L. E.; Nametkin, N. S. Chem.
Rev. 1979, 79, 529. (b) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419.
(c) Brook, A. G.; Baines, K. M. Adv. Organomet. Chem. 1986, 25, 1. (d)
Brook, A. G.; Brook, M. A. Adv. Organomet. Chem. 1996, 39, 71. (e)
M€uller, T.; Ziche, W.; Auner, N. The Chemistry of Organic Silicon
Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley & Sons: New York,
1998; Vol. 2, Chapter 16. (f) Morkin, T. L.; Owens, T. R.; Leigh, W. J.
The Chemistry of Organic Silicon Compounds; Rappoport, Z.; Apeloig, Y.,
Eds.; Wiley & Sons: New York, 2001; Vol. 3, Chapter 17. (g) Ottosson, H.;
(5) (a) Brook, A. G.; Harris, J. W.; Lennon, J.; El Sheikh, M. J. Am.
Chem. Soc. 1979, 101, 83. (b) Brook, A. G.; Baumegger, A.; Lough, A. J.
Organometallics 1992, 11, 3088. (c) Lassacher, P.; Brook, A. G.; Lough,
A. J. Organometallics 1995, 14, 4359.
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Eklof, A. M. Coord. Chem. Rev. 2008, 252, 1287.
(6) (a) Naka, A.; Ishikawa, M.; Matsui, S.; Ohshita, J.; Kunai, A.
Organometallics 1996, 15, 5759. (b) Naka, A.; Ishikawa, M. Organome-
tallics 2000, 19, 4921. (c) Naka, A.; Ishikawa, M J. Organomet. Chem.
2000, 611, 248. (d) Naka, A.; Ikadai, J.; Shingo, M.; Yoshizawa, K.; Kondo,
Y.; Kang, S.-Y.; Ishikawa, M. Organometallics 2002, 21, 2033. (e) Yoshizawa,
K.; Kondo, Y.; Kang, S.-Y.; Naka, A.; Ishikawa, M. Organometallics 2002,
21, 3271. (f) Naka, A.; Ishikawa, M. Chem. Lett. 2002, 3, 364. (g) Naka, A.;
Ohnishi, H.; Miyahara, I.; Hirotsu, K.; Shiota, Y.; Yoshizawa, K.; Ishikawa,
M. Organometallics 2004, 23, 4277. (h) Naka, A.; Ohnishi, H.; Ohshita, J.;
Ikadai, J.; Kunai, A.; Ishikawa, M. Organometallics 2005, 24, 5356.
(i) Ohshita, J.; Ohnishi, H.; Naka, A.; Senba, N.; Ikadai, J.; Kunai, A.;
Kobayashi, H.; Ishikawa, M. Organometallics 2006, 25, 3955. (j) Naka, A.;
Motoike, S.; Senba, N.; Ohshita, J.; Kunai, A.; Yoshizawa, K.; Ishikawa, M.
Organometallics 2008, 27, 2750.
(7) Milnes, K. K.; Jennings, M. C.; Baines, K. M. J. Am. Chem. Soc.
2006, 128, 2491.
(8) Milnes, K. K.; Baines, K. M. Can. J. Chem. 2009, 87, 307.
(9) (a) Milnes, K. K.; Gottschling, S. E.; Baines, K. M. Org. Biomol.
Chem. 2004, 2, 3530. (b) Gottschling, S. E.; Grant, T. N.; Milnes, K. K.;
Jennings, M. C.; Baines, K. M. J. Org. Chem. 2005, 70, 2686.
(3) (a) Pavelka, L. C.; Milnes, K. K.; Baines, K. M. Chem. Mater.
2008, 20, 5948. (b) Bravo-Zhivotovskii, D.; Melamed, S.; Molev, V.; Sigal,
N.; Tumanskii, B.; Botoshansky, M.; Molev, G.; Apeloig, Y. Angew. Chem.,
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(4) (a) Batsanov, A. S.; Clarkson, I. M.; Howard, J. A. K.; Steel, P. G.
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J. A. K.; Leech, M. A.; Steel, P. G.; Yufit, D. S. J. Chem. Soc., Perkin Trans.
1 1999, 3645. (c) Berry, M. B.; Griffiths, R. J.; Sanganee, M. J.; Steel, P. G.;
Whelligan, D. K. Tetrahedron Lett. 2003, 44, 9135. (d) Berry, M. B.;
Griffiths, R. J.; Sanganee, M. J.; Steel, P. G.; Whelligan, D. K. Org. Biomol.
Chem. 2004, 2, 2381. (e) Sanganee, M. J.; Steel, P. G.; Whelligan, D. K. Org.
Biomol. Chem. 2004, 2, 2393. (f) Sellars, J. D.; Steel, P. G.; Turner, M. J.
Chem. Commun. 2006, 2385. (g) Sellars, J. D.; Steel, P. G. Org. Biomol.
Chem. 2006, 4, 3223. (h) Ottosson, H.; Steel, P. G. Chem.-Eur. J. 2006, 12,
1576. (i) Sellars, J. D.; Steel, P. G. Eur. J. Org. Chem. 2007, 3815. (j)
Hughes, N. J.; Pullin, R. D. C.; Sanganee, M. J.; Sellars, J. D.; Steel, P. G.;
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J. D.; Steel, P. G. Org. Biomol. Chem. 2007, 5, 3201. (l) Bower, J.; Box,
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pubs.acs.org/Organometallics
Published on Web 10/22/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 22, 2010 5973
Scheme 1. Addition of Alkynes to Brook Silenes
Scheme 3. Proposed Route to Silacyclobutene Formation from a
Mixture of Trichlorovinylsilane, t-BuLi, and a Disubstituted
Alkyne
Scheme 2. Pyrolysis of 1,1-Dimethyl-1-silacyclobutane, 1, in the
Presence of Acetylene, Propyne, or 2-Butyne
alkyne (Scheme 3). The structures of two silacyclobutenes,
R=SiMe3; R1=Ph and R=SiMe3; R1=cyclohex-1-enyl,
were determined by X-ray crystallography.12a,d The silene
substrate was proposed to form by the addition of t-BuLi to
trichlorovinylsilane, in the presence of the disubstituted alkyne,
to form an R-silyl anion, which then eliminates LiCl. Formal
[2þ2] cycloaddition between the silene and the alkyne gives the
silacyclobutene. Alternatively, the observed silacyclobutene
may be formed by direct reaction of the R-silyl anion with
the alkyne. It is difficult to determine unambiguously if the
silene is an actual intermediate in these studies; however, the
products obtained are readily explained by the intermediacy of a
silene. Other related silenes with different substituents on the
silicon or the R-carbon were also examined.12b The 1,1-dialkoxy-
substituted analogue behaved in the same manner as 1,1-
dichlorosilene 3; however, no reaction was observed between
disubstituted alkynes and 1,1-diorgano-substituted silenes.
The addition of acetylene to Cl3Si-CH2Li has been exam-
ined by density functional theory.13 Two possible reaction
pathways for the addition were identified: pathway A, where
the silene, Cl2SidCH2, is formed prior to addition of acet-
ylene to give 1,1-dichlorosilacyclobut-3-ene; and pathway B,
where the R-silyl anion, Cl3Si-CH2Li, adds directly to acet-
ylene followed by cyclization and elimination of LiCl to yield
the same silacyclobutene (Scheme 4). Pathway B was calcu-
lated to be 29.9 kcal mol-1 lower in energy than pathway A.13
Thus, dichlorosilacyclobutene is likely formed by pathway
B, bypassing the formation of the silene. Of course, the
energetics of the addition reaction will be influenced by the
reaction conditions (i.e., solvent, temperature) and the sub-
stituents on the silene. The pathway involving the deproton-
ation of acetylene by the lithio species was not considered,
although presumably it is lower in energy than either path-
way A or B.13
and 1,1-dichloroneopentylsilene, studied extensively by Auner
and co-workers; as well as one sterically crowded stable silene.
The work is briefly summarized here.
The Wiberg silene, Me2SidC(SiMe3)2, does not react with
diphenylacetylene or bis(trimethylsilyl)acetylene;10 the addition
of terminal alkynes to Me2SidC(SiMe3)2 has not been exam-
ined. 1,1-Dimethyl-1-silacyclobutane, 1, was pyrolyzed to
afford 1,1-dimethylsilene and ethylene in the presence of
acetylene, propyne, or 2-butyne (Scheme 2).11 The reaction
of 1,1-dimethylsilene with acetylene yielded 1,1-dimethyl-1-
silacyclobutene and 1,1,3,3-tetramethyl-1,3-disilacyclobutane,
2, the product of silene dimerization. The reaction of the
silene with propyne yielded the analogous silacyclobutene,
silene dimer 2, an allene (the product of formal ene-addition),
and a propargylsilane, an isomerization product of the afore-
mentioned allene. Pyrolysis of silacyclobutane 1inthepresenceof
2-butyne formed the corresponding allene and propargylsilane;
however, no silacyclobutene was observed. Evidently, internal
alkynes do not readily cycloadd to this silene and, as a conse-
quence, the ene-addition pathway becomes more dominant.
The addition of disubstituted alkynes to the putative 1,1-
dichloroneopentylsilene, 3, has been well studied by Auner
and co-workers.12 Silacyclobutenes are the major products
formed; however, vinylsilanes, from a formal ene-addition,
may also be obtained depending on the structure of the
More recently, the reaction between a sterically crowded
1-hydrosilene (bearing a 2,4,6-tris[bis(trimethylsilyl)methyl]-
phenyl and a xanthenyl substituent) and diphenylacetylene
was reported to yield a silacyclobutene.14
In summary, there are few studies of the addition of alkynes
to silenes, particularly where the intermediacy of the silene
was proven unambiguously. The predominant reaction
(10) Wiberg, N.; Preiner, G.; Schieda, O. Chem. Ber. 1981, 114, 3518.
(11) Conlin, R. T.; Kwak, Y.-W.; Huffaker, H. B. Organometallics
1983, 2, 343.
(12) (a) Auner, N.; Seidenschwarz, C.; Herdtweck, E. Angew. Chem.,
€
Int. Ed. 1991, 30, 1151. (b) Auner, N.; Heikenwalder, C.-R.; Wagner, C.
€
Organometallics 1993, 12, 4135. (c) Auner, N. Organosilicon Chemistry,
From Molecules to Materials; Auner, N.; Weis, J., Eds.; Weinheim: New
York, 1994; p 103. (d) Auner, N.; Grasmann, M.; Herrschaft, B.; Hummer, M.
Can. J. Chem. 2000, 78, 1445.
(13) Muller, T.; Bendikov, M.; Auner, N.; Apeloig, Y. Organometal-
lics 2001, 20, 598.
(14) Ozaki, S.; Sasamori, T.; Tokitoh, N. Organometallics 2008, 27,
2163.

5974 Organometallics, Vol. 29, No. 22, 2010
Milnes et al.
Scheme 5. Addition of Aromatic Alkynes to 4
Scheme 4. Theoretical Investigation of the Addition of Acetylene
to an r-Silyl Anion
pathway is regioselective cycloaddition of the alkyne to give a
silacyclobutene; however, ene-addition also occurs. Silenes
appear to be less reactive toward internal alkynes in compar-
ison to terminal alkynes.
To explore the scope of the reactivity of naturally polarized
silenes toward alkynes, we selected the stable 1,1-dimesitylneo-
pentylsilene, Mes2SidC(H)CH2t-Bu, 4,15 as a substrate that is
relatively simple to prepare in near quantitative yield. The
addition of a variety of terminal alkynes with varying electronic
properties to silene 4 was examined.
Scheme 6. Addition of Aliphatic Alkynes to 4
Results
Silene 415 was prepared by the addition of t-BuLi to a
pentane solution of fluorovinylsilane 5 at -78 °C followed by
warming to room temperature. It was necessary to add <1
equivalent of t-BuLi to fluorosilane 5 when forming silene 4 to
prevent polymerization of the silene.3a As such, silene 4 was
always contaminated with residual fluorosilane 5. After
warming to room temperature, the pentane was removed and
replaced with C6D6; the presence of 4 was confirmed by 1H
NMR spectroscopy before addition of the alkyne.
When phenylacetylene was added to silene 4, a mixture
of silylacetylene 6a, silacyclobutene 7, and vinylsilane 8
1
(55:36:9) was produced as determined by H NMR spectro-
scopy (Scheme 5). In contrast, the addition of 1-ethynyl-
4-(trifluoromethyl)benzene or 4-ethynylanisole to 4 pro-
duced only silylacetylenes 6b,c, respectively (Scheme 5).
The ratio of silacyclobutene 7 to silylacetylene 6a was quite
variable; the relative amount of 7 ranged from 0 to 30% of
the product mixture. Vinylsilane 8 was consistently produced
in minor amounts. No one compound could be isolated from
the mixture of 5, 6a, 7, and 8 by chromatography. In one
experimental run, 6a was obtained contaminated only by the
fluorovinylsilane. Silylacetylene 6c could be separated
from unreacted fluorovinylsilane 5 by chromatography; how-
ever, 6a,b could not. Although, after treatment of the mixtures
(6a/5 or 6b/5) with aqueous NaOH in THF, silylacetylenes 6a,
b could both be separated from the corresponding vinylsilanol,
presumably formed from hydrolysis of the fluorosilane.
Addition of ethoxyacetylene to 4 produced a mixture of
silacyclobutene 9 and silylacetylene 6d (48:52) as determined
phenylacetylene, the ratio of silacyclobutene 9 to silylacetylene
6d was quite variable; the relative amount of 9 ranged from 48
to 100% of the product mixture. Silylacetylene 6d could not be
separated from 9 or unreacted fluorovinylsilane 5 by chroma-
tography; however, treatment of 9 contaminated only with 5
with aqueous NaOH in THF allowed for isolation of 9 from
the corresponding vinylsilanol.
The addition of cyclopropyl alkynes 10a-c to silene 4
produced silylacetylenes 6g-i quickly and quantitatively
(Scheme 7). Silylacetylene 6g could not be separated from
unreacted 5 by chromatography; however, treatment of the
mixture with aqueous NaOH in THF allowed for isolation of
6g from the corresponding vinylsilanol. Silylacetylenes 6h,i
were separated from unreacted fluorovinylsilane 5 by chro-
matography; however, 6i remained contaminated with the
starting alkyne, 10c.
1
by H NMR spectroscopy (Scheme 6). Silene 4 gave only
silylacetylenes 6e,fwhen treated with other nonaromatic alkynes
(tert-butylacetylene and trimethylsilylacetylene) (Scheme 6); silyl-
acetylenes 6e,f could be purified by chromatography. As with
Silylacetylenes 6a-i, silacyclobutenes 7 and 9, and vinylsilane
8 were identified by IR, 1H, 13C, gCOSY, 1H-13C gHSQC and
1
gHMBC, H-29Si gHMBC NMR spectroscopy, and mass
spectrometry. The spectral features of 6a-i are very similar,
and thus, only the spectral data of silylacetylene 6g will be
discussed in detail. Two multiplets in the 1H NMR spectrum of
(15) (a) Delpon-Lacaze, G.; Couret, C. J. Organomet. Chem. 1994,
480, C14. (b) Delpon-Lacaze, G.; de Battisti, C.; Couret, C. J. Organomet.
Chem. 1996, 514, 59.

Article
Organometallics, Vol. 29, No. 22, 2010 5975
Scheme 7. Addition of Alkynes 10a-c to Silene 4
Chart 1
Since vinylsilane 8 was produced in only minor amounts
and could not be separated from 6a and 7, it was difficult to
identify all the signals corresponding to 8 in the 1D and 2D
NMR spectra of the mixture. One pair of doublets observed
in the 1H NMR spectrum of the mixture (6.22 and 6.32 ppm,
J =18 Hz) was assigned to the vinylic hydrogens on the
Si-CHdCH-t-Bu trans-oriented double bond. The chemical
shifts and coupling constants of a second pair of doublets,
obscured by the signals attributed to the phenyl substituent
silylacetylene 6g at 1.65-1.69 and 1.50-1.54 ppm were assigned
to an AA0XX0 spin system attributable to an X-CH2CH2-Y
moiety. The two substituents on the CH2CH2 moiety are (1) a
t-Bu group, as seen by correlations between the multiplets
1
assigned to the two CH2 units in the H dimension and the
signal assigned to the quaternary 13C of the t-Bu substituent in
the 13C dimension of the 1H-13C gHMBC NMR spectrum of
6g, and (2) a Mes2RSi substituent, as revealed by correlations
between the same multiplets in the 1H dimension and the 29Si
1
of 6a or 7, were estimated from the H-29Si gHMBC spec-
trum of the mixture containing 6a, 7, and 8 (7.05 and 7.16 ppm,
J = 18 Hz). All the vinylic 1H signals assigned to 8 correlated
to a 29Si resonance at -21.7 ppm.
1
signal at -29.6 ppm in the 29Si dimension of the H-29Si
Deuterated alkyne 11, prepared by the treatment of the
conjugate base of 10c with D2O, was added to a solution of
silene 4 in C6D6; the only product obtained was the deuter-
ated silylacetylene 12 (Chart 1). The 1H NMR chemical shifts
of the deuterated silylacetylene 12 were essentially the same
as those of 6i; however, the splitting pattern and multiplicity
of the signals attributable to the CHDCH2 moiety reflected
the incorporation of the deuterium. The 2H NMR spectrum
of 12 revealed one broad signal at 1.47 ppm.
Alkyne 10c was methylated by treatment with BuLi in the
cold (THF) followed by the addition of MeI to give a mixture
of alkyne 13 and cyclopropene 14, contaminated with trans,
trans-2-methoxy-1-methyl-3-phenylcyclopropylethene, 15,
in a ratio of 61:33:6, respectively, as determined by 1H NMR
spectroscopy (Chart 1).16 The desired alkyne 13 was readily
separated from cyclopropene 14 by chromatography, yield-
ing a mixture of alkyne 13 and alkene 15 in a ratio of 92:6,
respectively, as determined by GC analysis.16
gHMBC NMR spectrum of 6g. The presence of an alkyne
functional group was confirmed by the observation of two
signals in the 13C NMR spectrum of 6g at 110.94 and 82.65
ppm, which fall in the typical chemical shift range for alkynyl
13C’s, and by the absorption observed in the IR spectrum of 6g
at 2161 cm-1. On the basis of the 1H NMR spectrum of 6g and
the correlations observed in the 1H-13C gHMBC NMR spec-
trum of 6g, the cyclopropyl ring is still intact.
The NMR spectroscopic data of 7 and 9 are consistent
with the proposed structures. A multiplet at 3.31 ppm (dt,
J = 9.6, 1.5 Hz) and a doublet at 6.88 ppm (J = 1.2 Hz) or a
multiplet at 2.66 ppm (dt, J = 10, 1.8 Hz) and a doublet at 5.90
ppm (J = 2.4 Hz) were observed in the 1H NMR spectra of 7
and 9, respectively, assigned to the saturated CH in the ring and
the vinylic hydrogen of each compound. The gCOSY NMR
spectrum of 7 showed correlations between the multiplet at 3.31
ppm and signals at 1.73 and 1.84 ppm assigned to the CH2t-Bu
1
hydrogens as well as to the vinylic H signal at 6.88. Similar
Alkyne 13 was added to a solution of silene 4 dissolved in
C6D6 (Scheme 8). The disappearance of the signal assigned
to the vinylic 1H of 4 was monitored by 1H NMR spectros-
copy over the course of 6 days. New signals in the vinyl
region of the 1H NMR spectrum of the reaction mixture were
observed; however, the relative amount of alkyne 13 com-
pared to residual fluorosilane 5 remained unchanged. Chro-
matographic separation of the crude product mixture yielded a
mixture of vinylsilane 16 and disiloxane 17 (in a ratio of 83:17,
respectively) as well as unreacted alkyne 13 and fluorosilane 5,
as determined by 1H NMR spectroscopy. Vinylsilane 163a and
disiloxane 1715b were identified by comparison of the 1H, 13C,
and 29Si chemical shifts with the literature data.17
correlations were observed in the gCOSY NMR spectrum of 9.
The alkene functional group was confirmed by the presence of
two signals in the 13C NMR spectra of 7 and 9 within the typical
chemical shift range for alkenyl carbons. For 7, the two signals
resonated at 135.68 and 160.70 ppm, while in 9, the adjacent
OEt group influences the chemical shifts of the vinylic carbons:
one carbon is shielded (119.55 ppm) and the other is deshielded
(164.82 ppm). The 29Si gHMBC NMR spectrum of 7 revealed
that the 1H signals at 3.31 and 6.88 ppm correlated to a signal at
-13.0 ppm in the 29Si dimension. This 29Si resonance also
showed correlations to 1H signals assigned to the Mes-H (6.72
and 6.73 ppm) and the CH2t-Bu hydrogens. The 29Si signal at
-3.5 ppm in the 29Si gHMBC NMR spectrum of 9 showed
similar correlations. Thus, the signals at -13.0 and -3.5 ppm
were assigned to the silacyclobutene silicon of 7 and 9, respec-
tively. The 29Si chemical shift data are consistent with the known
data for other similar silacyclobutenes.12 The 1H chemical shift
of the signal assigned to the saturated CH in the ring and the
correlations between the 29Si signal and both the saturated CH
Determination of the Regiochemistry of Silacyclobutenes.
1
Silacyclobutenes 7 and 9 were examined by H 1-D ROE
(16) The synthesis of (trans,trans-2-methoxy-1-methyl-3-phenyl-
cyclopropyl)ethyne, 10c, produces (trans,trans-2-methoxy-1-methyl-3-
phenylcyclopropyl)ethene, 15, as a byproduct. Alkyne 10c is always
contaminated with <10% alkene 15. See ref 9.
1
and vinylic signals in the H dimension of the 29Si gHMBC
(17) A mixture of vinylsilane 16 and disiloxane 17 was isolated from
the reaction mixture of alkyne 13 and silene 4. Disiloxane 17: high-
resolution EI-MS for C48H69OSi2 (Mþ - H) (m/z) calcd 717.4887, found
717.4907.
NMR spectrum of 7and 9provide strong evidence for silacyclo-
butenic structures. Determination of the regiochemistry of 7and
9 will be presented below.

5976 Organometallics, Vol. 29, No. 22, 2010
Milnes et al.
Scheme 9. Chemical Degradation of Silacyclobutene 7
Scheme 8. Reaction of Alkyne 13 and Silene 4
doublet assigned to the CH2t-Bu (1.99 ppm, 2 H, J = 7.2 Hz),
and a singlet assigned to the SiCH2 (2.70 ppm, 2 H) were
present in the 1H NMR spectrum of 18. In the 1H-29Si
gHMBC NMR spectrum of 18, a strong correlation was
observed between the singlet at 2.70 ppm and the silicon signal
at -1.6 ppm, which also correlated to the 1H signal assigned to
the methoxy group of 18 (3.04 ppm, 3 H). There were no
correlations between the silicon signal at -1.6 ppm and any of
the 1H signals attributed to the phenyl substituent of 18. On the
basis of these correlations, the silicon signal at -1.6 ppm was
assigned to the silicon in 18. In addition, this silicon chemi-
cal shift is similar to that of Mes2Si(Ot-Bu)CH2CH2t-Bu (1.3
ppm), prepared from the addition of tBuOH to silene 4.15b
From the 1H and 1H-29Si gHMBC NMR spectroscopic data,
it is apparent that the structure contains the Mes2Si(OMe)CH2
moiety. The singlet at 2.70 ppm assigned to the SiCH2 also
showed correlations to signals at 124.87, 137.70, and 143.34
spectroscopy to determine the regiochemistry of the addi-
tion. Irradiation of both mesityl o-CH3 signals (2.46 and 2.56
ppm) in 7 led to enhancement of the signal at 6.88, assigned
to the vinylic 1H. Also, enhancement of the signal assigned to
the ortho hydrogen of the phenyl substituent was observed
after irradiation of the signals at 3.31 ppm (saturated ring
SiCH) and 1.84 ppm (one of the CH2t-Bu hydrogens); no
enhancement of the vinylic 1H signal was observed in these
cases. These results suggest that the vinylic 1H is attached to
the unsaturated ring carbon adjacent to silicon.
Upon examination of the 1H 1-D ROE spectroscopic data
for ethoxy-substituted silacyclobutene 9, some key differ-
ences were noted in comparison to 7. Enhancement of the
1
1
ppm in the 13C dimension of the H-13C gHMBC NMR
signal assigned to the vinylic H (5.90 ppm) was observed
after irradiation of the signal at 2.66 ppm assigned to the
saturated ring SiCH and the signal at 1.29 ppm assigned to
one of the CH2t-Bu hydrogens. There was no enhancement
of the vinylic 1H signal after irradiation at either of the
signals assigned to the mesityl o-CH3 groups. Furthermore,
irradiation of the signal at 1.07 ppm assigned to the OCH2-
CH3 group caused enhancement of the signals assigned to the
mesityl o-CH3 hydrogens. These spectroscopic data lead to
the conclusion that the regiochemistry of 9 is reversed from
spectrum, which correspond to the two vinylic carbons and the
ipso carbon of the phenyl substituent, respectively. The two
vinylic carbon signals (124.87, 137.70) also correlated to the
doublet at 1.96 ppm in the 1H spectrum, assigned to the CH2t-
Bu hydrogens. Thus, the SiCH2 must be connected to a
C(Ph)dC(H)CH2t-Bu moiety.
The structure assigned to 18, where the phenyl substituent
is attached to the carbon two bonds from silicon, is the only
possibility that is consistent with the NMR spectroscopic
data. Thus, the phenyl-substituted silacyclobutene 7 is de-
rived from the terminal alkynyl carbon adding to the silicon
of silene 4. The original regiochemistry, with respect to
alkyne addition to silene 4, should be retained in the ring-
opened product. The methoxide anion is presumed to selec-
tively attack at the silicon, which leads to cleavage of the
former silenic Si-C bond; similar behavior has been ob-
served when other silacyclobutenes were treated with base.18
Alternative ring-opened products derived from a silacyclo-
butene with the phenyl substituent adjacent to silicon were
considered; however, none of these structures were consis-
tent with the spectroscopic data obtained.
1
that of 7; the vinyl H is bonded to the unsaturated ring
carbon two bonds from silicon.
The 1H(vinylic)-29Si coupling constants in 7 and 9 were also
examined to support the assigned structures. The regiochem-
istry of silacyclobutenes can be assigned on the basis of
1
the magnitude of the H(vinylic)-29Si coupling constants; in
general, the magnitude of the two-bond coupling constant
(8-12 Hz) is significantly smaller than the magnitude of the
trans three-bond coupling constant (17-30 Hz).7 The mag-
1
nitude of the H(vinylic)-29Si coupling constant for 9 was
obtained from the 29Si satellites of the vinylic 1H signal in the
1H NMR spectrum, while that for 7 was extracted from the
1H-29Si gHMBC spectrum since the 29Si satellites of the
vinylic signal in the 1H NMR spectrum were obscured. The
values were found to be 6 Hz for 7 and 22 Hz for 9,
corresponding to a two-bond and a three-bond coupling,
respectively, in agreement with the previously assigned re-
giochemistry.
The reduced reactivity of 9 toward NaOMe in comparison
to 7 is also consistent with the ethoxy substituent being
located on the carbon adjacent to silicon. Presumably, the
intermediate anion would be less stable; the added instability
may be sufficient to make the addition of NaOMe unfavor-
able (Chart 2).
Finally, the structures of 7 and 9 were implicated by
chemical degradation. Compounds 7 and 9 were treated
with NaOMe in refluxing THF (Scheme 9). Only silacyclo-
butene 7 was observed to react with NaOMe; the ethoxy-
substituted ring 9 was unreactive toward NaOMe under
these conditions.
Discussion
Silylacetylenes 6a-i and 12 are derived from addition of
the acetylenic C-H(D) bond across the SidC of 4. While
addition of the deuterated alkyne 11 to silene 4 did not
qualitatively reduce the rate of the reaction, the formation of
deuterated silylacetylene 12 does provide evidence that the
acetylenic hydrogen becomes bound to the former silenic
1
The ring-opened product, 18, was identified by H, 13C,
1
1
gCOSY, H-29Si gHMBC, H-13C gHSQC, and gHMBC
NMR spectroscopy and mass spectrometry. There are several
features in the NMR spectral data of 18 that provide unam-
biguous evidence for the structure of the product. A triplet
1
assigned to the vinylic H (5.35 ppm, 1 H, J = 7.5 Hz), a
(18) Mohseni-Ala, J.; Auner, N. Inorg. Chim. Acta 2006, 359, 4677.

Article
Organometallics, Vol. 29, No. 22, 2010 5977
Chart 2
Scheme 10. Proposed Mechanism of the Addition of an Alkyne to
Silene 4
carbon in compounds 6a-i. Insertion of a silene into the
terminal CH bond of an alkyne has not previously been
observed. The possibility that silylacetylenes 6a-i and 12
were formed by reaction of the silene precursor, an R-silyl
anion, with the alkynes was considered; however, the spec-
troscopic evidence clearly indicates that silene 4 is being
formed prior to alkyne addition and, thus, rules out this
possibility.
Scheme 11. Proposed Mechanism of Ethoxyacetylene Cycload-
dition
Analogous products have been isolated from the addition
of phenylacetylene to a phosphasilene (SidP).19 The forma-
tion of the observed addition products to the phosphasilene
was attributed to the highly polar nature of the double
bond, i.e., Siþ-P-. A C-H insertion product has also been
observed in the reaction between an ylide-like silylene and
terminal alkynes;20 the preference for C-H insertion over
cycloaddition with the CtC bond has similarly been attrib-
uted to the polar nature of the compound.
seems probable that an initial oxonium complex could form,
in addition to the complex with the triple bond, in the
reaction between ethoxyacetylene and silene 4 (Scheme 11).
The coordination of oxygen-containing donor molecules,
such as ethers, to naturally polarized silenes has been studied
extensively.2 The THF complex of the polar silene Me2SidC-
(SiMe3)(SiMet-Bu2) is quite stable and has been character-
ized by X-ray crystallography.24 Oxonium complexes have
long been proposed and since confirmed as intermediates in
the addition of alcohols to silenes.25 Evidence for the revers-
ible formation of an acid-base complex between naturally
polarized silenes and carbonyl compounds has been pro-
vided by Leigh and co-workers.26,27 Interestingly, the lifetime
of the Lewis acid-base complex formed in the reaction between
silene 4 and trans-2-phenylcyclopropanecarbaldehyde was suf-
ficient to allow ester formation via the Lewis acid-catalyzed
aldehyde dimerization (Tishchenko reaction).21 Hence, the
initial formation of a complex in the addition of ethoxyacety-
lene to 4 is well precedented. After complexation to the ether
oxygen, cyclization likely occurs via nucleophilic attack of the
silenic carbon on the terminal alkynyl carbon of another
ethoxyacetylene-silene complex, followed by attack of the
substituted alkynyl carbon on silicon, forming silacyclobutene
9, and thereby releasing the coordinated ethoxy group and an
equivalent of silene 4. Thus, the two products formed in the
addition of ethoxyacetylene to silene 4 can be understood in
terms of the formation of two competing Lewis acid-base
complexes between the silene and the alkyne.
On the basis of electronegativities, the SidC bond of silene
4 is expected to be more polar than the SidP of the phos-
phasilene, and thus, silylacetylenes 6a-i may possibly be
formed by silene 4 acting as a Brønsted base, abstracting the
alkynyl proton. However, given an average pKa of 25 for
alkynes, it is difficult to believe that the silene is a strong
enough base to remove the weakly acidic proton of an alkyne.
We have shown that silene 4 is a strong Lewis acid,21 and thus,
we propose that the alkyne first complexes to the silenic silicon,
which would make the silenic carbon more basic. Abstraction
of a proton from a second equivalent of alkyne (or, perhaps,
from another complexed alkyne) followed by displacement of
the complexed alkyne by the conjugate base (Scheme 10)
would result in the formation of the silylacetylene. We have
proposed a similar mechanism to account for the insertion of
silene 4 into the CH bond of acetonitrile.22 Yoshizawa and co-
workers have examined the addition of methylacetylene to the
Brook-type silene (H3Si)2SidC(OSiH3)CH3 computationally.
The triple bond of the alkyne complexes to the silenic silicon in
the transition state; however in this case, the transfer of electron
density is proposed to flow from the silene to the alkyne.6e
Interestingly, phenylacetylene and ethoxyacetylene were
the only alkynes to give cycloadducts with silene 4. The
regiochemistry of silacyclobutene 7 is typical for the addition
of terminal alkynes to silenes;5,11,23 however, silacyclobutene
9 has the opposite regiochemistry.
The addition of phenylacetylene to silene 4 also produces a
silacyclobutene (7) in addition to silylacetylene 6a and small
Given that oxygen is a stronger nucleophile than the CtC
triple bond and the well-known oxaphilicity of silicon, it
€
(24) (a) Wiberg, N.; Wagner, G.; Muller, G.; Riede, J. J. Organomet.
Chem. 1984, 271, 381. (b) Wiberg, N.; Wagner, G.; M€uller, G. Angew.
Chem., Int. Ed. Engl. 1985, 24, 229. (c) Wiberg, N.; Wagner, G.; Reber, G.;
Riede, J.; Mu€ller, G. Organometallics 1987, 6, 35.
(25) Morkin, T. L.; Leigh, W. J. Acc. Chem. Res. 2001, 34, 129.
(26) (a) Leigh, W. J.; Bradaric, C. J.; Sluggett, G. W. J. Am. Chem.
Soc. 1993, 115, 5332. (b) Bradaric, C. J.; Leigh, W. J. Organometallics 1998,
17, 645.
(19) Driess, M.; Pritzkow, H.; Rell, S.; Winkler, U. Organometallics
1996, 15, 1845.
€
(20) Yao, S.; van Wullen, C.; Sun, X.-Y.; Driess, M. Angew. Chem.,
Int. Ed. 2008, 47, 3250.
(21) Milnes, K. K.; Baines, K. M. Organometallics 2007, 26, 2392.
(22) Hardwick, J. A.; Baines, K. M. Organometallics 2010, 29, 1305.
(23) Apeloig, Y.; Bravo-Zhivotovskii, D.; Zharov, I.; Panov, V.;
Leigh, W. J.; Sluggett, G. W. J. Am. Chem. Soc. 1998, 120, 1398.
(27) Owens, T. R.; Grinyer, J.; Leigh, W. J. Organometallics 2005, 24,
2307.

5978 Organometallics, Vol. 29, No. 22, 2010
Milnes et al.
amounts of vinylsilane 8. In contrast, the reaction of silene 4
with 1-ethynyl-4-(trifluoromethyl)benzene or 4-ethynylanisole
resulted only in the formation of a silylacetylene. At this time, it
is difficult to understand the differences in reactivity observed
for the three aryl alkynes, particularly with regards to the
formation (or lack thereof) of a cycloadduct. If a zwitterionic
intermediate is formed, presumably the trifluoromethyl- or the
alkoxy-substituted aryl alkyne (depending on the regiochem-
istry of the zwitterion) would (at least) equally favor cycload-
duct formation. If a diradical intermediate was formed during
the course of cycloaddition, as has been shown in the addition
of alkynes to Brook silenes,7,8 the three aryl alkynes are
expected to be (at least) equally capable of stabilizing the
intermediate. However, no cycloadduct was observed with the
substituted aryl alkynes. Further studies are required to under-
stand the reactivity differences.
We attempted to investigate the nature of a putative
intermediate during the formation of a silacyclobutene by
use of alkynes 10a-c as mechanistic probes.9 However,
addition of these alkynes gave only the CH insertion prod-
ucts 6g-i; no addition to the carbon-carbon triple bond
was observed and, as a consequence, no mechanistic infor-
mation regarding the cycloaddition could be deduced in
these experiments.
We believe the difference in reactivity between the natu-
rally polarized silene 4 and Brook silenes is best understood
in terms of the Lewis acidities of the two silenes: naturally
polarized silenes are more Lewis acidic, and complexation
with an alkyne increases the basicity of the silenic carbon
(and the acidity of the alkynyl hydrogen), leading to CH
insertion being favored over cycloaddition. With the non-
polar Brook silenes, cycloaddition via a diradical intermedi-
ate predominates.7,8 This is a clear example of how the
polarity of the SidC bond of the silene can have a profound
influence on the structure of the product formed in a given
reaction. The formation of silacyclobutene 9 is best ex-
plained by proposing the formation of a complex with the
ethereal oxygen rather than the triple bond.
At this time, the factors controlling the regioselectivity of
the addition of phenylacetylene to silene 4 and the observed
differences in reactivity between the various substituted
aryl alkynes are not well understood. Studies using compu-
tational methods may allow us to gain further insights into
the mechanistic details of alkyne additions to polar silenes
and may help us to understand these important reactions
more completely.
Experimental Section
The formation of vinylsilane 8 can be understood in terms
of a formal ene-addition between phenylacetylene and silene
4 where the alkyne acts exclusively as the enophile. This type
of behavior has previously been reported between 4 and
aldehydes or ketones.15b,21 Notably, the same relative regio-
chemistry is observed in 8 as in the phenyl-substituted
silacyclobutene 7. Phenylacetylene was the only alkyne that
gave any ene-addition products upon reaction with silene 4.
Evidently, the other alkynes either are poorer enophiles (too
electron rich) or are better substrates for C-H addition
across the SidC bond, which renders the ene-addition path-
way less favorable.
The addition of alkyne 13 to silene 4 did not afford any
products; evidently unpolarized internal alkynes do not
add to this silene. This is consistent with previous work on the
reactivity of disubstituted alkynes with 1,1-diorgano-substi-
tuted polar silenes.10,11,12b Although no addition occurred, in
the presence of alkyne 13 silene 4 did slowly convert to
vinylsilane 16 and disiloxane 17. Vinylsilane 16 is likely formed
by a hydrogen transfer in silene 4. The formation of vinylsilane
16 from silene 4 has been observed previously,3a even though
silene 4 has been reported to be stable in solution for extended
periods of time (weeks).15 Presumably disiloxane 17 is formed
by the addition of adventitious water to silene 4, the product of
which then reacts with a second equivalent of 4.15b
General Experimental Details. All reactions were performed
in flame-dried Schlenk tubes, or NMR tubes sealed with a
septum, under an inert atmosphere of argon. Benzene-d6 was
˚
distilled from LiAlH4, stored over 4 A molecular sieves,
and degassed prior to use. Pentane was purged with N2 and passed
through alumina prior to use. t-BuLi and BuLi were purchased
from Aldrich Chemical Co., and methyl iodide was purchased
from Fisher Chemical Inc. Phenylacetylene, 1-ethynyl-4-
(trifluoromethyl)benzene, 4-ethynylanisole, ethoxyacetylene,
trimethylsilylacetylene, and tert-butylacetylene were purchased
˚
from Aldrich Chemical Co. and stored over 4 A molecular
sieves. (trans-2-Phenylcyclopropyl)ethyne, 10a,9b (trans,trans-
2-methoxy-3-phenylcyclopropyl)ethyne, 10b,9b (trans,trans-
2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 10c,9b and
1,1-dimesitylneopentylsilene, 4,15 were prepared according to
the previously reported procedures. The NMR standards used
are as follows: residual C6D5H (7.15 ppm) for 1H NMR spectra,
C6D6 central transition (128.0 ppm) for 13C NMR spectra,
1
Me4Si as an external standard (0 ppm) for H-29Si gHMBC
spectra, and CF3Ph as an external standard (-63.9 ppm against
CFCl3) for 19F NMR spectra. IR spectra were recorded (cm-1
)
from thin films on a Bruker Tensor 27 FT-IR spectrometer.
Electron impact mass spectra were obtained using a MAT model
8400 mass spectrometer using an ionizing voltage of 70 eV. Mass
spectral data are reported in mass-to-charge units, m/z.
General Procedure for the Addition of Alkynes to 1,1-Di-
mesitylneopentylsilene, 4. A pentane solution of fluorodimesityl-
vinylsilane, 5 (80 mg, 0.25 mmol), was converted to 1,1-dimesityl-
neopentylsilene, 4. The pentane was removed in vacuo, yielding
an orange residue. The residue was dissolved in C6D6 (0.7 mL)
and then added to a septum-sealed NMR tube. The ratio of
silene 4 to fluorosilane 5 was determined by 1H NMR spectros-
copy. Excess alkyne was added to the orange solution; the color
faded to yellow upon addition of the alkyne. The ratio of products in
the crude reaction mixture was determined by 1H NMR spectros-
copy; the mixture was usually contaminated with residual alkyne
and fluorosilane 5. The solvent was removed by rotary evaporation,
yielding a pale yellow residue. The products were separated from
fluorosilane 5 by chromatography or by treatment with aqueous
NaOH followed by chromatography. Specific experimental details
can be found in the Supporting Information.
Conclusions
We have examined the addition of cyclopropyl alkynes
10a-c, 11, and simple alkynes (RCtCH; R = Ph, p-CF3C6H4,
p-MeOC6H4, OEt, t-Bu, SiMe3) to Mes2SidC(H)CH2t-Bu
(4). The addition of an alkynyl CH bond across the SidC
bond of 4 is clearly favored over cycloaddition or an ene
reaction. This is in stark contrast to the behavior of Brook-type
silenes with alkynes, where cycloaddition is the preferred
reaction pathway. Given the predominance of CH insertion
in the studied reactions, it is surprising that CH insertion
was not observed in the reaction between 1,2-dimethylsilene
and acetylene or propyne.11
6a: colorless, waxy solid; IR (cm-1) 624 (s), 690 (s), 756 (s),
829 (s), 848 (m), 882 (m), 1026 (m), 1066 (m), 1158 (m), 1217 (m),

Article
Organometallics, Vol. 29, No. 22, 2010 5979
1236 (m), 1363 (m), 1410 (m), 1450 (m), 1488 (m), 1548 (w),
1605 (s), 2157 (s), 2865 (m), 2955 (s), 3024 (m); 1H NMR (C6D6)
δ 0.89 (s, 9 H, C(CH3)3), 1.56-1.59 (XX0 portion of an AA0XX0
spin system, 2 H, SiCH2), 1.70-1.73 (AA0 portion of an AA0XX0
spin system, 2 H, CH2t-Bu), 2.09 (s, 6 H, Mes p-CH3), 2.62 (s, 12
H, Mes o-CH3), 6.74 (s, 4 H, Mes-H), 6.86-6.90 (m, 3 H, Ph m,
p-H), 7.37-7.38 (m, 2 H, Ph o-H); 13C NMR (C6D6) δ 15.94
(SiCH2), 20.99 (Mes p-CH3), 24.47 (Mes o-CH3), 29.05
(C(CH3)3), 31.24 (C(CH3)3), 39.17 (CH2t-Bu), 96.18 (Mes2Si-
CtC), 107.83 (Mes2Si-CtC), 124.15 (Ph i-C), 128.47, 128.51
(Ph m,p-C), 129.94 (Mes m-C), 131.76 (Mes i-C), 131.87 (Ph
o-C), 138.91 (Mes p-C), 144.12 (Mes o-C); 29Si NMR (C6D6)
δ -28.6 (Mes2Si); high-resolution EI-MS for C32H40Si (Mþ)
m/z calcd 452.2899, found 452.2888.
1.64-1.67 (AA0 portion of an AA0XX0 spin system, 2 H, CH2t-
Bu), 2.08 (s, 6 H, Mes p-CH3), 2.57 (s, 12 H, Mes o-CH3), 6.72 (s,
4 H, Mes-H); 13C NMR (C6D6) δ 16.07 (SiCH2), 20.98 (Mes p-
CH3), 24.46 (Mes o-CH3), 28.57 (CtCC(CH3)3), 29.12
(CH2C(CH3)3), 30.44 (CtCC(CH3)3), 31.21 (CH2C(CH3)3),
39.19 (CH2t-Bu), 84.29 (Mes2Si-CtC), 117.57 (Mes2Si-CtC),
129.83 (Mes m-C), 132.31 (Mes i-C), 138.61 (Mes p-C), 144.04
(Mes o-C); 29Si NMR (C6D6) δ -29.5 (Mes2Si); high-resolution
EI-MS for C30H44Si (Mþ) m/z calcd 432.3212, found 432.3199.
6f: colorless oil; IR (cm-1) 623 (m), 768 (s), 846 (s), 1026 (w),
1065 (w), 1249 (m), 1363 (m), 1410 (m), 1451 (m), 1605 (m), 2865
1
(m), 2957 (s); H NMR (C6D6) δ 0.11 (s, 9 H, SiMe3), 0.90
(s, 9 H, C(CH3)3), 1.48-1.51 (XX0 portion of an AA0XX0 spin
system, 2 H, SiCH2), 1.66-1.69 (AA0 portion of an AA0XX0 spin
system, 2 H, CH2t-Bu), 2.06 (s, 6 H, Mes p-CH3), 2.57 (s, 12 H,
Mes o-CH3), 6.70 (s, 4 H, Mes-H); 13C NMR (C6D6) δ -0.43
(SiMe3), 15.83 (SiCH2), 20.97 (Mes p-CH3), 24.47 (Mes o-CH3),
29.05 (C(CH3)3), 31.20 (C(CH3)3), 39.10 (CH2t-Bu), 116.30
(Mes2Si-CtC), 116.84 (Mes2Si-CtC), 129.88 (Mes m-C),
131.53 (Mes i-C), 138.89 (Mes p-C), 144.10 (Mes o-C); 29Si NMR
(C6D6) δ -19.3 (Me3Si), -30.0 (Mes2Si); high-resolution EI-MS
for C29H44Si2 (Mþ) m/z calcd 448.2982, found 448.2976.
6b: colorless, waxy solid; IR (cm-1) 625 (m), 654 (m), 705 (m),
806 (m), 843 (s), 884 (m), 1019 (m), 1069 (s), 1107 (m), 1133 (s),
1170 (s), 1236 (m), 1263 (m), 1325 (s), 1365 (m), 1408 (m), 1453
(m), 1608 (s), 2162 (m), 2867 (m), 2959 (s); 1H NMR (C6D6) δ
0.90 (s, 9 H, C(CH3)3), 1.55-1.58 (XX0 portion of an AA0XX0
spin system, 2 H, SiCH2), 1.67-1.70 (AA0 portion of an AA0XX0
spin system, 2 H, CH2t-Bu), 2.09(s, 6 H, Mes p-CH3), 2.60 (s, 12 H,
Mes o-CH3), 6.75 (s, 4 H, Mes-H), 7.03-7.05 (m, 2 H,
C6H4CF3), 7.10-7.12 (m, 2 H, C6H4CF3); 13C NMR (C6D6) δ
15.81 (SiCH2), 20.98 (Mes p-CH3), 24.44 (Mes o-CH3), 29.02
(C(CH3)3), 31.25 (C(CH3)3), 39.18 (CH2t-Bu), 99.28 (Mes2Si-
CtC), 105.99 (Mes2Si-CtC), 124.6 (q, CF3, 1JCF = 255 Hz),28
125.34 (q, Ar m-C, 3JCF = 3 Hz), 127.50 (Ar i-C), 130 (Ar p-C),28
130.01 (Mes m-C), 131.25 (Mes i-C), 131.99 (Ar o-C), 139.21
(Mes p-C), 144.06 (Mes o-C); 29Si NMR (C6D6) δ -28.3; 19F
NMR (C6D6) δ -62.7; high-resolution EI-MS for C33H39SiF3
(Mþ) m/z calcd 520.2773, found 520.2780.
6g: colorless solid; IR (cm-1) 623 (m), 697 (m), 848 (m), 1028
(m), 1363 (m), 1410 (m), 1456 (m), 1605 (s), 2161 (s), 2864 (m),
2956 (s), 3028 (w); 1H NMR (C6D6) δ 0.85 (ddd, 1 H, CH2, J =
4.6, 6.2, 8.6 Hz), 0.90 (s, 9 H, C(CH3)3), 1.15 (ddd, 1 H, CH2, J =
4.8, 5.6, 8.8 Hz), 1.38 (ddd, 1 H, SiCtCCH, J = 4.4, 5.6, 8.8
Hz), 1.50-1.54 (XX0 portion of an AA0XX0 spin system, 2 H,
SiCH2), 1.65-1.69 (AA0 portion of an AA0XX0 spin system, 2 H,
CH2t-Bu), 2.09 (s, 6 H, Mes p-CH3), 2.20 (ddd, 1 H, PhCH, J =
4.6, 6.2, 8.8 Hz), 2.60 (br s, 12 H, Mes o-CH3), 6.65-6.69 (m, 2
H, Ph o-H), 6.74 (s, 4 H, Mes-H), 6.95-7.00 (m, 3 H, Ph m,p-H);
13C NMR (C6D6) δ 13.12 (SiCtCCH), 16.04 (SiCH2), 17.82
(CH2), 20.99 (Mes p-CH3), 24.48 (Mes o-CH3), 26.49 (PhCH),
29.07 (C(CH3)3), 31.22 (C(CH3)3), 39.14 (CH2t-Bu), 82.65 (Si-
CtC), 110.94 (Si-CtC), 126.10 (Ph o-C), 126.30 (Ph p-C),
128.53 (Ph m-C), 129.89 (Mes m-C), 132.17 (Mes i-C), 138.74
(Mes p-C), 140.65 (Ph i-C), 144.00 (Mes o-C); 29Si NMR (C6D6)
δ -29.6 (Mes2Si); high-resolution EI-MS for C35H44Si (Mþ)
m/z calcd 492.3212, found 492.3198.
6c: colorless, waxy solid; IR (cm-1) 623 (m), 764 (m), 831 (m),
847 (m), 882 (w), 1034 (m), 1171 (m), 1249 (s), 1292 (m), 1363
(w), 1410 (w), 1466 (m), 1507 (s), 1605 (s), 2154 (s), 2864 (m),
2955 (s); 1H NMR (C6D6) δ 0.91 (s, 9 H, C(CH3)3), 1.58 - 1.61
(XX0 portion of an AA0XX0 spin system, 2 H, SiCH2), 1.73-1.76
(AA0 portion of an AA0XX0 spin system, 2 H, CH2t-Bu), 2.10 (s,
6 H, Mes p-CH3), 2.65 (s, 12 H, Mes o-CH3), 3.10 (s, 3 H, OCH3),
6.49-6.51 (XX0 portion of an AA0XX0 spin system, 2 H,
C6H4OMe), 6.75 (s, 4 H, Mes-H), 7.34 - 7.36 (AA0 portion of
an AA0XX0 spin system, 2 H, C6H4OMe); 13C NMR (C6D6) δ
16.03 (SiCH2), 20.99 (Mes p-CH3), 24.49 (Mes o-CH3), 29.07
(C(CH3)3), 31.25 (C(CH3)3), 39.20 (CH2t-Bu), 54.63 (OCH3),
94.39 (Mes2Si-CtC), 108.07 (Mes2Si-CtC), 114.23 (Ar m-C),
116.34 (Ar i-C), 129.92 (Mes m-C), 132.04 (Mes i-C), 133.43 (Ar
o-C), 138.81 (Mes p-C), 144.13 (Mes o-C), 160.19 (Ar p-C); 29Si
NMR (C6D6) δ -28.8; high-resolution EI-MS for C33H42SiO
(Mþ) m/z calcd 482.3005, found 482.3013.
6h: colorless solid; IR (cm-1) 625 (m), 700 (s), 849 (m), 1028 (m),
1118 (m), 1232 (m), 1409 (m), 1451 (s), 1605 (s), 2160 (s), 2864
(m), 2956 (s), 3026 (m); 1H NMR (C6D6) δ 0.90 (s, 9 H,
C(CH3)3), 1.50-1.53 (XX0 portion of an AA0XX0 spin system,
2 H, SiCH2), 1.64-1.66 (AA0 portion of an AA0XX0 spin system,
2 H, CH2t-Bu), 1.89 (dd, 1 H, SiCtCCH, J = 3.3, 6.3 Hz), 2.09
(s, 6 H, Mes p-CH3), 2.26 (t, 1 H, PhCH, J = 6.6 Hz), 2.58 (s,
12 H, Mes o-CH3), 2.82 (s, 3 H, MeO), 3.47 (dd, 1 H, MeOCH,
J = 3.3, 6.9 Hz), 6.74 (s, 4 H, Mes-H), 6.98-7.07 (m, 5 H, Ph-H);
13C NMR (C6D6) δ 16.03 (SiCH2), 17.29 (SiCtCCH), 20.99
(Mes p-CH3), 24.49 (Mes o-CH3), 29.07 (C(CH3)3), 31.22
(C(CH3)3), 33.34 (PhCH), 39.15 (CH2t-Bu), 57.80 (MeO),
66.71 (MeOCH), 84.04 (Si-CtC), 108.68 (Si-CtC), 126.45
(Ph p-C), 128.129 (Ph m-C), 128.55 (Ph o-C), 129.90 (Mes
m-C), 132.08 (Mes i-C), 135.75 (Ph i-C), 138.81 (Mes p-C),
144.00 (Mes o-C); 29Si NMR (C6D6) δ -29.6 (Mes2Si); high-
resolution EI-MS for C36H46OSi (Mþ) m/z calcd 522.3318,
found 522.3325.
6d: contaminated with 9 (∼2:1 mixture of 9 to 6d); 1H NMR
(C6D6) δ 0.81 (t, 3 H, OCH2CH3, J = 7.2 Hz), 0.90 (s, 9 H,
C(CH3)3), 1.50-1.53 (XX0 portion of an AA0XX0 spin system,
2 H, SiCH2), 1.64-1.67 (AA0 portion of an AA0XX0 spin system,
2 H, CH2t-Bu), 2.09 (s, 6 H, Mes p-CH3), 2.61 (s, 12 H, Mes
o-CH3), 3.49 (q, 2 H, OCH2CH3, J = 7.2 Hz), 6.74 (s, 4 H,
Mes-H); 13C NMR (C6D6) δ 16.58 (SiCH2), 20.98 (Mes p-CH3),
24.47 (Mes o-CH3), 29.09 (C(CH3)3), 31.18 (C(CH3)3), 39.21
(CH2t-Bu), 111.629 (Mes2Si-CtC), 129.82 (Mes m-C), 133.11
(Mes i-C), 138.44 (Mes p-C), 143.87 (Mes o-C) (not all signals
from 6d were visible); 29Si NMR (C6D6) δ -28.3 (Mes2Si); high-
resolution EI-MS for C28H40SiO (Mþ) m/z calcd 420.2849,
found 420.2845.
6i: contaminated with alkyne 10c (25%); 1H NMR (C6D6) δ
0.92 (s, 9 H, C(CH3)3), 1.16 (s, 3 H, CtC-CCH3), 1.49-1.52
(XX0 portion of an AA0XX0 spin system, 2 H, SiCH2), 1.66-1.69
(AA0 portion of an AA0XX0 spin system, 2 H, CH2t-Bu), 2.09 (s,
6 H, Mes p-CH3), 2.43 (d, 1 H, PhCH, J = 7.2 Hz), 2.59 (s, 6 H,
Mes o-CH3), 2.60 (s, 6 H, Mes o-CH3), 2.99 (s, 3 H, MeO), 3.53
(d, 1 H, MeOCH, J = 7.2 Hz), 6.74 (s, 4 H, Mes-H), 7.01-7.03
(m, 1 H, Ph p-H), 7.10-7.12 (m, 2 H, Ph m-H), 7.35-7.36
(m, 2 H, Ph o-H); 13C NMR (C6D6) δ 12.32 (CtC-CCH3),
16.06 (SiCH2), 18.02 (CtC-CCH3), 20.97 (Mes p-CH3), 24.47
(Mes o-CH3), 29.11 (C(CH3)3), 31.22 (C(CH3)3), 33.95 (PhCH),
39.25 (CH2t-Bu), 58.46 (MeO), 68.35 (MeOCH), 82.16 (Si-CtC),
6e: colorless oil; IR (cm-1) 622 (m), 768 (m), 847 (m), 1027 (m),
1064 (m), 1252 (m), 1362 (m), 1410 (m), 1454 (m), 1605 (m),
2153 (m), 2192 (w), 2865 (m), 2958 (s); 1H NMR (C6D6) δ 0.92
(br s, 9 H, CH2C(CH3)3), 1.12 (s, 9 H, SiCtCC(CH3)3),
1.46-1.49 (XX0 portion of an AA0XX0 spin system, 2 H, SiCH2),
(28) The chemical shift and J value were estimated from the 1H-13C
gHMBC spectrum.
(29) Chemical shift estimated from the 1H-13C gHMBC spectrum.

5980 Organometallics, Vol. 29, No. 22, 2010
Milnes et al.
114.91 (Si-CtC), 126.60 (Ph p-C), 128.22 (Ph m-C), 129.90 (Mes
m-C), 130.92 (Ph o-C), 132.21 (Mes i-C), 134.74 (Ph i-C), 138.76
(Mes p-C), 144.04 (Mes o-C); 29Si NMR (C6D6) δ -29.6
(Mes2Si); high-resolution EI-MS for C37H48OSi (Mþ) m/z calcd
536.3474, found 536.3460.
Addition of (trans,trans-2-methoxy-1-methyl-3-phenylcyclo-
propyl)[2H]ethyne, 11, to 1,1-Dimesitylneopentylsilene, 4. Alkyne
11 was added to silene 4 as described in the general alkyne
addition procedure. Specific experimental details can be found
in the Supporting Information. 12: 2H NMR (C6H6) δ 1.47 (br s,
SiCHD).
7: as a mixture with 6a and 8; 1H NMR (C6D6) δ 0.90 (s, 9 H,
C(CH3)3), 1.73 (dd, 1 H, SiCHCH2, J = 14, 10 Hz), 1.84 (dd,
1 H, SiCHCH2, J = 14, 1.0 Hz), 2.07 (s, 3 H, Mes p-CH3), 2.13
(s, 3 H, Mes p-CH3), 2.46 (s, 6 H, Mes o-CH3), 2.56 (s, 6 H, Mes
o-CH3), 3.31 (dt, 1 H, SiCH, J = 10, 1.5), 6.72 (s, 2 H,
Mes-H), 6.73 (s, 2 H, Mes-H), 6.8830 (d, 1 H, SiCHdCPh,
J = 1.2 Hz,31 JH-Si = 6 Hz32), 7.10-7.12 (m, 1 H, Ph p-H),
Preparation of 1-(trans,trans-2-Methoxy-1-methyl-3-phenyl-
cyclopropyl)-1-propyne, 13. A solution of alkyne 10c (200 mg,
1.07 mmol) in THF (3 mL) was cooled to -78 °C. BuLi (0.7 mL,
1.6 M in hexanes) was added dropwise to the cold solution.
Upon completion of the addition, the reaction mixture was
allowed to stir in the cold for 1 h, after which time, the reaction
mixture appeared dark brown in color. The solution was
allowed to warm to RT and then quenched with excess MeI.
An aqueous solution of NaOH (15%) was added to the reaction
mixture followed by the addition of Et2O. The aqueous phase
was extracted with Et2O (3 ꢀ 2 mL); the combined organic
layers were dried over MgSO4, the solids were removed by
gravity filtration, and the solvent was removed by rotary evapo-
7.20-7.22 (m, 2 H, Ph m-H), 7.58-7.60 (m, 2 H, Ph o-H); 13
C
NMR (C6D6) δ 21.07 (Mes p-CH3), 24.11 (Mes o-CH3), 25.14
(Mes o-CH3), 29.89 (C(CH3)3), 31.24 (C(CH3)3), 33.93
(SiCHCH2), 43.76 (SiCHCH2), 126.39 (Ph o-C), 128.26 (Ph
p-C), 128.63 (Ph m-C), 129.21 (Mes m-C), 129.65 (Mes m-C),
130.56 (Mes i-C), 134.67 (Mes i-C), 135.68 (SiCHdCPh), 137.35
(Ph i-C), 138.56 (Mes p-C), 139.43 (Mes p-C), 142.92 (Mes o-C),
144.76 (Mes o-C), 160.70 (SiCHdCPh); 29Si NMR (C6D6) δ-13.0
(Mes2Si, JH-Si = 6).
1
ration, yielding a yellow oil (172 mg). H NMR spectroscopic
analysis revealed that the residue contained alkyne 13, cyclo-
propene 14, and alkene 15 in a ratio of 61:33:6, respectively.16
The mixture was separated by preparative thin-layer chroma-
tography (silica gel, 3:1 hexanes:CH2Cl2), yielding alkyne 13 and
alkene 15 as a colorless oil, in a ratio of 92:6, respectively, as
determined by GC analysis (87 mg, 0.43 mmol, 41%).
8: as a mixture with 6a and 7; 1H NMR (C6D6) δ 0.95 (s, 9 H,
C(CH3)3), 2.12 (s, 6 H, Mes p-CH3), 2.44 (s, 12 H, Mes o-CH3),
6.22 (d, 1 H, Si-CHdCH-t-Bu, J = 19 Hz), 6.32 (d, 1 H, Si-
CHdCH-t-Bu, J = 18 Hz), 6.77 (s, 4 H, Mes-H), 7.00-7.05 (m,
3 H, Ph-H), 7.05 (d, 1 H, Si-CHdCH-Ph, J = 18 Hz),33 7.16 (d,
1 H, Si-CHdCH-Ph, J = 18 Hz),33 7.26-7.27 (m, 2 H, Ph-H);
29Si NMR (C6D6) δ -21.7.
13: IR (cm-1) 700 (s), 996 (m), 1028 (s), 1080 (m), 1145 (s),
1205 (s), 1257 (w), 1450 (m), 1498 (m), 1603 (w), 2238 (w),
2827 (m), 2855 (m), 2938 (s), 3028 (w), 3059 (w); 1H NMR
(C6D6) δ 1.24 (s, 3 H, CtC-CCH3), 1.57 (s, 3 H, CH3-CtC),
2.38 (d, 1 H, PhCH, J = 7.2 Hz), 3.08 (s, 3 H, MeO), 3.53 (d, 1 H,
MeOCH, J = 7.2 Hz), 7.04 (t, 1 H, Ph p-H, J = 7.8 Hz), 7.14 (t,
2 H, Ph m-H, J = 7.8 Hz), 7.43 (d, 2 H, Ph o-H, J = 7.8 Hz);
13C NMR (C6D6) δ 3.48 (CH3-CtC), 13.36 (CtC-CCH3), 16.88
(CtC-CCH3), 33.81 (PhCH), 58.52 (MeO), 68.51 (MeOCH),
71.94 (CH3-CtC), 85.53 (CH3-CtC), 126.46 (Ph p-C), 128.21
(Ph m-C), 130.95 (Ph o-C), 135.43 (Ph i-C); high-resolution EI-MS
for C14H16O (Mþ) m/z calcd 200.1201, found 200.1192.
1
9: H NMR (C6D6) δ 0.97 (s, 9 H, C(CH3)3), 1.07 (t, 3 H,
OCH2CH3, J = 6.9 Hz), 1.29 (dd, 1 H, SiCHCH2, J = 14, 11
Hz), 1.68 (dd, 1 H, SiCHCH2, J = 15, 2.1 Hz), 2.06 (s, 3 H, Mes
p-CH3), 2.07 (s, 3 H, Mes p-CH3), 2.58 (s, 6 H, Mes o-CH3), 2.64
(s, 6 H, Mes o-CH3), 2.66 (dt, 1 H, SiCHCH2, J = 10, 1.8 Hz),
3.55 (AB, 1 H, OCH2CH3, JAB = 9.6, J = 7.0), 3.58 (AB, 1 H,
OCH2CH3, JAB = 9.6, J = 7.0 Hz), 5.90 (d, 1 H, SiCdCH,
J = 2.4, 3JH-Si = 21), 6.69 (s, 2 H, Mes-H), 6.73 (s, 2 H, Mes-H);
13C NMR (C6D6) δ 14.52 (OCH2CH3), 21.01 (Mes p-CH3),
23.70 (Mes o-CH3), 23.94 (Mes o-CH3), 25.17 (SiCHCH2), 30.03
(C(CH3)3), 32.28 (C(CH3)3), 47.32 (SiCHCH2), 63.94 (OCH2-
CH3), 119.55 (SiCdCH), 129.15 (Mes m-C), 129.27 (Mes m-C),
131.20 (Mes i-C), 132.36 (Mes i-C), 139.05 (Mes p-C), 139.20
(Mes p-C), 143.84 (Mes o-C), 144.11 (Mes o-C), 164.82
14: IR (cm-1) 802 (m), 1026 (m), 1075 (m), 1261 (m), 1451 (m),
1
1605 (m), 1651 (m), 2857 (m), 2923 (s), 2958 (s), 3025 (w); H
NMR (C6D6) δ 1.58 (s, 3 H, CH3-CtC), 1.64 (s, 3 H, CtC-
CCH3), 1.95 (s, 3 H, CdC-CH3), 7.01-7.03 (m, 1 H, Ph p-H),
7.12-7.15 (m, 2 H, Ph m-H), 7.50-7.51 (m, 2 H, Ph o-H); 13
C
3
(SiCdCH); 29Si NMR (C6D6) δ -3.5 (Mes2Si, JH-Si = 21);
high-resolution EI-MS for C28H40SiO (Mþ) m/z calcd 420.2849,
found 420.2853.
NMR (C6D6) δ 3.58 (CH3-CtC), 9.21 (CdC-CH3), 15.04
(CtC-CCH3), 24.65 (CtC-CCH3), 70.13 (CH3-CtC), 86.12
(CH3-CtC), 117.05 (PhCdCMe), 117.90 (PhCdCMe), 128.129
(Ph p-C), 128.84 (Ph i-C), 128.88 (Ph m-C), 129.11 (Ph o-C);
high-resolution EI-MS for C14H14 (Mþ) m/z calcd 182.1096,
found 182.1092.
Preparation of (trans,trans-2-Methoxy-1-methyl-3-phenyl-
cyclopropyl)[2H]ethyne, 11. A colorless solution of alkyne 10c
(175 mg, 0.94 mmol) in THF (3.0 mL) was cooled to -78 °C, and
BuLi (0.59 mL, 1.6 mmol) was added dropwise. Upon complete
addition of the BuLi, the solution appeared clear and light
yellow. Over the course of an hour, the color of the solution
changed to green. The reaction mixture was then allowed to
warm to RT, after which it appeared opaque and deep brown.
D2O (2 mL) was added to the reaction mixture; the color faded
instantly to pale yellow. The reaction mixture was then diluted
with Et2O, and the aqueous and organic layers were separated.
The aqueous layer was extracted with Et2O (3 ꢀ 3 mL); the
combined organic layers were dried over MgSO4, and the
solvent was removed by rotary evaporation, yielding a vibrant,
yellow oil (151 mg, 85% yield). Deuterated alkyne 11 was
contaminated with 5% alkene 15.16 11: 2H NMR (C6H6) δ
1.71 (s, CtCD).
Reaction of 1-(trans,trans-2-Methoxy-1-methyl-3-phenylcyclo-
propyl)-1-propyne, 13, and 1,1-Dimesitylneopentylsilene, 4. A
solution containing 1,1-dimesitylneopentylsilene, 4, and fluoro-
silane 5 in a ratio of 85:15, respectively, dissolved in C6D6 (0.5 mL)
was prepared from fluorosilane 5 (53 mg, 0.17 mmol). To this
solution, alkyne 13 (70 mg, 0.35 mmol) in C6D6 (0.5 mL) was
added. The progress of the reaction was monitored over 6 days,
after which time no silene remained in the reaction mixture. The
relative ratio of alkyne 13 to residual fluorosilane 5 remained
unchanged, as determined by 1H NMR spectroscopy. The solvent
was removed by rotary evaporation, yielding a pale yellow residue
(138 mg). The crude mixture was separated by preparative
chromatography (silica gel, 3:1 hexanes:CH2Cl2), yielding a mix-
ture of vinylsilane 163a and disiloxane 1715b (83:17, 15 mg),
fluorosilane 5, and a mixture of alkyne 13 and alkene 15.16
Reaction of Silacyclobutene 7 with Sodium Methoxide. Excess
sodium methoxide (30 mg) was added to a mixture of silyl-
acetylene 6a, silacyclobutene 7, vinylsilane 8, and fluorovinylsi-
lane 5 (38 mg) dissolved in THF (2 mL). The solution was
refluxed for 18 h. The reaction mixture was then cooled and
added to water (2 mL). The mixture was extracted with Et2O,
(30) Due to overlap with the m,p-PhH signals of 6a, the chemical shift
was estimated from the gCOSY spectrum.
(31) Due to overlap with the m,p-PhH signals of 6a, this J value was
estimated from the ROESY spectrum.
(32) Due to overlap with the m,p-PhH signals of 6a, this J value was
estimated from the 1H-29Si gHMBC spectrum.
(33) The chemical shift and J value were estimated from the 1H-29Si
gHMBC spectrum.

Article
Organometallics, Vol. 29, No. 22, 2010 5981
then brine, and the solvents were removed under vacuum to
yield a light yellow oil (35 mg). The oil was purified by
preparative thin-layer chromatography (1:1 hexanes:CH2Cl2)
to give ring-opened product 18 as a mixture with silylacetylene
6a and vinylsilane 8 in a ratio of 24:70:6, respectively (21 mg).34
18: 1H NMR (C6D6) δ 0.79 (s, 9 H, C(CH3)3), 1.99 (d, 2 H,
CH2t-Bu, J = 7.2 Hz), 2.10 (s, 6 H, Mes p-CH3), 2.40 (s, 12 H,
Mes o-CH3), 2.70 (s, 2 H, SiCH2), 3.04 (s, 3 H, MeO), 5.35 (t,
1 H, CHdCPh, J = 7.5 Hz), 6.67 (s, 4 H, Mes-H), 7.09 - 7.10
(Ph-H); 13C NMR (C6D6) δ 20.98 (Mes p-CH3), 23.90 (Mes
o-CH3), 29.38 (C(CH3)3), 31.16 (C(CH3)3), 31.46 (SiCH2), 43.41
(CH2t-Bu), 49.48 (MeO), 124.87 (Ph-CdC(H)CH2), 128.42-
128.53 (Ph o,m,p-C), 129.65 (Mes m-C), 132.18 (Mes i-C), 137.70
(Ph-CdC(H)CH2), 139.01 (Mes p-C), 143.34 (Ph i-C), 144.23
(Mes o-C); 29Si NMR (C6D6) δ -1.6; high-resolution EI-MS for
C33H44SiO (Mþ) m/z calcd 484.3161, found 484.3146.
Acknowledgment. We thank the NSERC (Canada),
the University of Western Ontario, and the Ontario
Photonics Consortium for financial support.
Supporting Information Available: Experimental details con-
cerning the addition of all alkynes to 4 and 1H NMR spectra for
all new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
(34) The mixture was also contaminated with the product formed
from the reaction of vinylsilane 5 and sodium methoxide.