An analysis of the influences dictating regioselectivity in platinum-catalyzed hydrosilylations of internal alkynes

Article abstract of DOI:10.1016/j.tet.2014.03.012

A full account of our studies on internal alkyne hydrosilylations using platinum catalysis is described. We demonstrate that these transformations are highly governed by the electronic characteristics of the alkyne substituents, wherein the hydride will add preferentially to the more electron-deficient alkyne carbon. The steric and coordinative capabilities of the substituents influence the selectivity to a much lesser extent, with propargylic alcohols a lone exception. The choice of silane is relevant in some cases; specific silanes will afford high regioselectivities while others are much less selective. Ultimately, the regioselectivity of addition can be quite predictable using ¹³C NMR chemical shift data, allowing this reactivity to be incorporated into purposeful reaction design.

Full text of DOI:10.1016/j.tet.2014.03.012

Tetrahedron 70 (2014) 4232e4244
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An analysis of the influences dictating regioselectivity in platinum-
catalyzed hydrosilylations of internal alkynes
*
Douglas A. Rooke, Zachary A. Menard, Eric M. Ferreira
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A full account of our studies on internal alkyne hydrosilylations using platinum catalysis is described. We
demonstrate that these transformations are highly governed by the electronic characteristics of the al-
kyne substituents, wherein the hydride will add preferentially to the more electron-deficient alkyne
carbon. The steric and coordinative capabilities of the substituents influence the selectivity to a much
lesser extent, with propargylic alcohols a lone exception. The choice of silane is relevant in some cases;
specific silanes will afford high regioselectivities while others are much less selective. Ultimately, the
regioselectivity of addition can be quite predictable using 13C NMR chemical shift data, allowing this
reactivity to be incorporated into purposeful reaction design.
Received 16 January 2014
Received in revised form 1 March 2014
Accepted 4 March 2014
Available online 13 March 2014
Keywords:
Hydrosilylation
Alkyne
Ó 2014 Elsevier Ltd. All rights reserved.
Catalysis
Platinum
Regioselectivity
1. Introduction
The metal-catalyzed hydrosilylation reaction of alkenes and al-
kynes is a well-studied transformation.1 Vinylsilanes, the products
of alkyne hydrosilylation, can serve as useful precursors for Tam-
aoeFleming oxidations,2 nucleophilic additions,3 Hiyama cou-
plings,4 and halodesilylation reactions.5 The widespread synthetic
use of vinylsilanes rests upon the ability for their syntheses to be
facile with high regio- and stereocontrol. An overwhelming ma-
jority of research on alkyne hydrosilylation has focused on the anti-
Markovnikov addition of a silane across a terminal alkyne, a trans-
formation that has reliably provided the terminal silane. Con-
versely, internal alkynes have received considerably less attention,
which can be at least partially attributed to the difficulty of
achieving regioselective transformations with this substrate class
(Fig. 1).
Fig. 1. Contrasting hydrosilylations of terminal and internal alkynes.
aspects our findings.7 Herein, we present a comprehensive account
of our study, illustrating how regioselective hydrosilylations of in-
ternal alkynes can be accomplished depending on the nature of the
two alkyne substituents.
Our entry into the field of metal-catalyzed hydrosilylations be-
gan with our desire to obtain (E)-
mers of the products of our platinum catalyzed 1,2-silyl migration
reaction of
-hydroxypropargylsilanes.6 To our delight, this class of
a-silylenones, the geometric iso-
1.1. Background
a
internal alkyne substrates afforded excellent regio- and stereo-
selectivity in catalytic hydrosilylation. We sought to fully un-
derstand the effects that influenced this process and its
characteristic selectivity, and we previously reported several
There are many different methods reported to afford hydro-
silylation products. However, the selectivity of SieH addition can be
a limitation of further synthetic utility for these methods. Radically
induced hydrosilylation requires a silane that can easily undergo
homolytic cleavage of the SieH bond (Eq. 1).8 The intermediate
vinyl radical of this silane addition can be configurationally labile.
High stereoselectivities have been achieved in certain cases, but for
others the process is less effective. Lewis acids like AlCl3 can
* Corresponding author. Tel.: þ1 970 491 6379; fax: þ1 970 491 1801; e-mail
addresses: emferr@mail.colostate.edu, eric.ferreira@colostate.edu (E.M. Ferreira).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.03.012

D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
4233
catalyze the trans addition of a hydrosilane across an alkyne (e.g.,
Eq. 2).9
resulting Pt(IV) complex undergoes migratory insertion across an
olefin or alkyne, delivering a hydride to one of the two carbons. The
resulting CePteSi complex can then undergo reductive elimination
to afford the cis hydrosilylation products.
The ChalkeHarrod mechanism has been widely accepted and
can generally be applied to platinum-based hydrosilylation re-
actions. However, a number of phenomena are still unexplained by
this mechanism. First, the other widely used catalyst in hydro-
silylation is Karstedt’s catalyst, a Pt(0)edivinyldisiloxane complex,
which may implicate a similar catalytic mechanism invoking Pt(0)
and Pt(II) oxidation states.16 Second, it has been shown that alter-
native operational platinum catalysts, such as Pt(cod)Cl2, are not
active but instead are precatalysts that must undergo an induction
period. Roy and Taylor have studied this induction period in depth
and have concluded that the classical ChalkeHarrod mechanism
can proceed through both Pt(II)ePt(IV) and Pt(0)ePt(II) catalytic
cycles.17 As long as the ChalkeHarrod mechanism holds, we can
presume that the hydride is delivered during the migratory in-
sertion step, and this step is thus playing a significant role in de-
termining the regioselectivity and stereoselectivity of the net
hydrosilylation.
ð1Þ
ð2Þ
The strong Lewis acids that promote this transformation, how-
ever, can also catalyze the isomerization of the alkene. Early tran-
sition metal complexes, such as titanocene10 and organoyttrium
compounds11 have been shown to catalyze the cis-addition of a si-
lane across an alkyne. Due to issues primarily related to regiose-
lectivity and functional group tolerance, the abovementioned
methods of hydrosilylation have received little attention in the
context of synthetic utility. Conversely, late transition metals have
been the primary focus of research in regard to hydrosilylation.
The hydrosilylation chemistry catalyzed by late transition
metals is both diverse and well-documented. Two separate mech-
anisms are cited for the observed regioselectivity of the hydro-
silylation. One mechanism (the CrabtreeeOjima mechanism) has
been observed in several cases using late transition metal catalysts
based on Rh, Ir, and Ru.12 This mechanism proceeds via a metal-
locarbene species, which leads to possible E/Z isomerizations. An
exceptional catalytic system based on this mechanism, Trost and
Ball have shown that CpRu(MeCN)3PF6 catalyzes an exclusively
trans addition of silane with excellent selectivity and for a wide
variety of internal alkynes (Eq. 3).13
Today, regioselective hydrosilylation on internal alkynes re-
mains difficult, and research efforts have often focused on
exploiting sterics dictated by both the substrate and catalyst sys-
ꢀ
tem. Marko and co-workers have performed extensive work on the
hydrosilylation of alkynes using bulky Pt/NHC catalysts developed
in their lab.18 Although chiefly focused on terminal alkynes,
a number of internal alkynes were also investigated with some
providing high regioselectivities. In 2011, Cook developed an active
catalytic system for the hydrosilylation of propargylic alcohols.19
Predictably, terminal alkynes showed excellent selectivity, while
internal alkynes were much less selective unless significant steric
bulk was employed to influence the addition. A recent report from
Hosoya and co-workers illustrates Pd-catalyzed hydrosilylations of
electron-deficient alkynes, where high regioselectivities have been
achieved.20 There have also been a few isolated examples where
the alkyne electronic influence has been observed, often in sub-
strate syntheses or specialized cases.21
The notion that electronic effects can dictate alkyne hydro-
silylation was introduced by Tsipis in 1980. He hypothesized that
the polarization of an alkyne would direct the hydride addition to
the more electropositive carbon.22 Up until this point, since most
hydrosilylation centered around terminal alkynes and alkenes,
sterics were believed to be the single most contributing factor af-
fecting regioselectivity. Tsipis noted that the magnitude of differ-
ence in 13C chemical shifts for terminal alkynes is rather large
(w15e20 ppm).23 However, for internal alkynes, the difference in
alkyne chemical shifts is markedly less (approximately 3e5 ppm).
These data could serve as predictors for which alkyne carbon has
the lowest lying LUMO. He therefore posited that the hydro-
silylation regioselectivity was impacted by this polarization in ad-
dition to the steric effects (Fig. 3).
ð3Þ
Pt-catalyzed hydrosilylations, alternatively, generally proceed
though the standard ChalkeHarrod mechanism,14 which provides
complementary syn-addition products. Platinum and palladium
catalysts are widely used for these hydrosilylation reactions and
generally give products arising from the cis-addition of silane
across an alkyne. In 1957, Speier was the first to report platinum-
catalyzed hydrosilylations using H2PtCl6 (now termed Speier’s
Catalyst).15 Today this catalyst is one of the most commonly used
hydrosilylation catalysts in an industrial setting. The cis selectivity
of addition is reflective of the ChalkeHarrod mechanism for Pt
hydrosilylation (Fig. 2), wherein Pt(II) coordinates to an olefin (later
applied to alkynes), and oxidative addition of the silane occurs. The
Alami and co-workers, in their investigations of ortho-
substituted arylacetylenes, showed one of the most salient first
examples of highly selective internal alkyne hydrosilylation using
PtO2 (Fig. 4).24 Interestingly, it was found that almost any sub-
stituent, from eNO2 to ei-Pr, induced selectivity for the
a-silyl
isomer. This occurrence was observed even for cases involving
diarylacetylenesdthe silicon species will preferentially add to the
alkyne carbon bearing the ortho-substituted aryl group.
More telling of the absolute electronic effects of this silane ad-
dition were their analyses of para-substituted diarylacetylenes
(Fig. 5). Both alkyne carbons of these diarylacetylenes should have
similar steric environments, and the para-substituent should exert
a negligible coordinative effect. Therefore, any addition selectivity
would likely be a consequence of alkyne polarization. In comparing
Fig. 2. The ChalkeHarrod mechanism for metal-catalyzed hydrosilylation.

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D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
Fig. 6. Potential generalization of internal alkyne hydrosilylation based on electronic
induction.
regioselectivity is difficult to control in many cases; given the
synthetic utility of vinylsilanes, we sought to comprehensively
evaluate this transformation to develop a full understanding of how
this method can be properly exploited in synthetic contexts.
2. Results and discussion
2.1. Alkyne hydrosilylationdconditions and scope of elec-
tronic effects
Fig. 3. Tsipis’s proposal for electronic effects in alkyne hydrosilylation.
As previously mentioned, we entered the field of hydrosilylation
to selectively make (E)-silylenones as a complement to our 1,2-
silicon migration method that affords (Z)-silylenones.6 We envi-
sioned that the highly polarized ynone substrates would induce high
selectivity for the desired
a-silyl regioisomer. To our delight, treat-
ment of various ynones and trialkylsilanes with catalytic PtCl2 pro-
ceeded under mild conditions to afford a-silylenones with generally
excellent yields and selectivities (Table 1).25 Even the hydrosilylation
Table 1
Pt-catalyzed hydrosilylations of ynones
Fig. 4. Alami’s hydrosilylations of aryl and diarylacetylenes.
Entry
1
Substrate
Major product
Time (h) Yield (%)
ba
a
/
1.5
98
>19:1
2
3
1.5
99
>19:1
9
65
>19:1
Fig. 5. Alami’s hydrosilylations of para-substituted diarylacetylenes.
the depicted hydrosilylations, it is apparent that the electron-
withdrawing para-nitrophenyl group induces -silyl selectivity,
whereas the electron-donating para-methoxyphenyl reverses se-
lectivity to give the -silyl isomer predominantly.
These observations integrate well into Tsipis’s original hypoth-
esis that the silylplatinum hydride will undergo migratory insertion
across an alkyne, preferentially delivering the hydride to the more
electropositive carbon. The ChalkeHarrod mechanism for Pt-cata-
lyzed hydrosilylations can be readily applied to this concept, and
a
4
5
2
2
92
>19:1
b
87
2:1
selectivity distributions for the
a and b isomers of a vinylsilane can
ultimately be governed by the polarization of the alkyne (Fig. 6). In
spite of these findings, it has still been generally regarded that
a
Isolated yield; a/b
ratio determined by 1H NMR.

D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
4235
of highly hindered pivaloyl alkyne 4 afforded >19:1 regioselectivity
for the -silylenone, albeit in a more moderate yield.
One of the abovementioned substrates, the hydrosilylation of
propargyl silyl ether 8 showed only slight preference for the -silyl
difference being that CH2Cl2 was found to be a generally better
solvent than PhCH3.26 All substrates, with the exception of alde-
hyde 24, reacted efficiently, and the resulting products were iso-
lated in high yields. Also notably, in all cases the hydrosilylations
a
a
regioisomer (entry 5). We originally attributed this to the ethereal
group coordinating to the silylplatinum intermediate, the inductive
effect of the oxygenation, or some combination of these effects.
Nonetheless, our interest was piqued, and we sought to elucidate
this method as a useful, facile means to access stereodefined tri-
substituted vinylsilanes.
proceeded with high selectivity favoring the
esters 10 and 12 provided >19:1 and 16:1
spectively, showing excellent differentiation over the n-butyl
group. Branched esters 14 and 16 were unsurprisingly less regio-
selective, likely due to the increased steric bulk, affording still re-
a
-silyl isomer. Linear
selectivity, re-
a/b
spectable 9.3:1 and 7.5:1 a/b selectivities. Weinreb (18) and phenyl
Based on the high levels of regioselectivity observed in the
hydrosilylations of ynones, we anticipated that other carbonyl-
based electron-withdrawing groups, such as esters and amides
would induce comparable reactivities and selectivities. To probe
this hypothesis, a number of alkynes were subjected to conditions
similar to our hydrosilylations of ynones (Table 2), with the salient
(20) amides also demonstrated high selectivity. Even a carboxylic
acid (22) was tolerated under the reaction conditions. The hydro-
silylation of aldehyde 24 was the only observed outlier, affording
a 58% yield in the best case and with varying regioselectivity
(3.1e5.8:1). We attribute this outcome to differential carbonyl
hydrosilylations of the product vinylsilanes. Competitive aldehyde
hydrosilylation was consuming the product enals; differential re-
Table 2
activities of the a and b-isomers led to these variable results.
Pt-catalyzed hydrosilylations of alkynesdvarious electron-withdrawing groups
We also evaluated a number of methyl ynoate substrates by
varying the other alkyne substituent and observing differences in
regioselectivity of addition as well as in reactivity (Table 3). In-
creasing the size of this substituent (i.e., alkyne 26) severely re-
tarded the rate of reaction, yet still resulted in a 96% yield and
>19:1 a/b selectivity (entry 2). More interestingly, substituents that
Entry
1
Substrate
Major product
Time (h)
Yield (%)
ba
a
/
Table 3
3
95
>19:1
Pt-catalyzed hydrosilylations of ynoatesdvarying the
b
-substituent
2
3
3
3
93
16:1
Entry
1
Substrate
Major product
Time (h)
Yield (%)
ba
a
/
3
95
>19:1
93
9.3:1
2
150
96
>19:1
4
5
6
7
5
95
7.5:1
3
4
11
90
1:3.5
89
>19:1
7
97
1:1.1
24
90
12:1
5
6
12
3
92
3.5:1
7
8
5.5
3
95
13:1
88
5.2:1
58
3.1e5.8:1
a
Isolated yield; a/b
ratio determined by 1H NMR.
a
Isolated yield; a/b
ratio determined by 1H NMR.

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D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
affected the polarization of the alkyne showed dramatically re-
duced hydrosilylation selectivity. Upon treatment of phenyl-
substituted methyl ynoate 28 with PtCl2 and HSiEt3, the major
utilization of the CeSi bond;29 this transformation generally re-
quires an activated silicon intermediate (i.e., not a simple tri-
alkylsilane). For these reactions, the vinylsilane must again contain
oxygenation, halogenation, or a surrogate carbon-based species
that can generate this oxygenation or halogenation in situ. Conse-
quently, methods for the selective installation of these aforemen-
tioned silicon groups would ideally lead to more general
application in synthesis.
product was the b-silyl regioisomer (1:3.5 a/b). Introducing oxy-
genation at the propargylic position similarly eroded selectivity,
which had also been observed with ynone 8 (Table 1, entry 5). The
reaction with propargylic acetate 30 also resulted in slight in-
version of selectivity, affording a 1:1.1 a/b mixture of isomers. Di-
minished inductive effects can be observed in the hydrosilylation of
homopropargylic acetate 34. We anticipated that an additional
methylene unit between the alkyne and the acetate group would
decrease the inductive electronic effect of the acetate as well as
reduce the possibility of coordination to the silylplatinum in-
termediate. Indeed, this substrate gave a higher regioselectivity,
Thus, a number of different commercially available silanes,
possessing various silane substituents, were tested with methyl
heptynoate. As can be seen in Table 4, the yields for the trans-
formations were consistently good. Even sterically large silanes
(e.g., HSi(t-Bu)Me2, entry 2) were competent reactants, albeit ap-
preciably slower. For the silanes with carbon-based substituents
(entries 1e6), there appeared to be a slight but observable elec-
tronic trend. Silanes bearing substituents capable of inductively
withdrawing electron density (e.g., allyl, phenyl) appeared to erode
the regioselectivity of the addition. In particular, HSiPh3 provided
5.2:1 a/b.
A number of commercially available Pt catalysts can be used for
these hydrosilylation reactions with comparable outcomes. These
catalysts were tested on substrate 10 in a series of similar condition
sets with reasonable amounts of catalyst loadings (Scheme 1).
a much lower regioselectivity (entry 6, 6.0:1 a/b). One potential
rationalization is that the HePteSi intermediate is less hydridic
than ones with standard trialkylsilanes, possessing more anionic
character on the silicon atom. The lessened hydridic nature of this
species would consequently be less susceptible to the electronic
influences dictating the alkyne insertion step.
For the alkoxysilanes evaluated (entries 7e10), there was no
discernibly clear trend in regioselection. The regioselectivities were
generally lower than the examples using trialkylsilanes, which may
be reflective of a similarly diminished hydridic nature as in the
HSiPh3 case. The difference between triethoxysilane and trime-
thoxysilane, however, cannot be explained solely by this effect, and
more subtle influences are clearly dictating this reactivity. Grati-
fyingly, 1,1,1,3,5,5,5-heptamethyltrisiloxane (HSi(OTMS)2Me) was
quite effective in providing similar regioselectivities (entry 10).30
Vinylsilanes based on these species were subsequently found to
be efficient participants in both halodesilylations and Hiya-
maeDenmark cross couplings.7,31
To further evaluate the influence of various silane reagents on
regioselectivity, we tested six different silanes with a significantly
less electronically-differentiated system (Table 5). The hydro-
silylation of homopropargylic acetate 45 with triethylsilane gives
a 3.8:1 selectivity for vinylsilane 46 (entry 1). In noticeable contrast
to the hydrosilylations of methyl heptynoate (10), where regiose-
lectivity was impacted by silane substitution, all six different si-
lanes effected a relatively similar regioselective outcome in this
case.
Scheme 1. Pt catalyst evaluation in hydrosilylation of ynoate 10.
Reactions with both PtCl2 (5 mol %) and Zeise’s dimer [{(C2H4)
PtCl2}2] (5 mol % Pt) were essentially identical and showed exqui-
site selectivity for the
a-isomer. Karstedt’s catalyst [Pt(dvds)]
(1 mol %)27 was competent in the hydrosilylation, showing similarly
high yields for enoate 11 but with somewhat diminished selectivity
(9.8:1 a/b).
The hydrosilylation of internal alkynes can also be performed
with remarkably low catalyst loadings (Scheme 2). The high solu-
bility and activity of [{(C2H4)PtCl2}2] in CH2Cl2 made it an ideal
choice for a low catalyst loading reaction. For example, the hydro-
silylation of alkyne 10 using 0.01 mol % catalyst effected the
transformation to vinylsilane 11 in high yield and regioselectivity.28
In the cases of ynone internal alkyne hydrosilylation, it was
observed that propargylic oxygenation had a pronounced effect on
the reaction regioselectivity (vide supra, 8/9). More noticeably,
the hydrosilylation of propargylic acetate 30 afforded a reversal of
regioselectivity, favoring the b-silyl product relative to the methyl
ester. Because of the apparent influence of propargylic oxygenation,
we sought to investigate whether these functionalities could steer
regioselectivity relative to other moieties. As an initial set of ex-
periments, secondary propargylic alcohol 52 was protected as
a trifluoroacetate (53), an acetate (55), and a triethylsilyl ether (57),
and each alkyne was subjected to identical hydrosilylation condi-
tions (Table 6). As illustrated, when the electron-withdrawing na-
ture of the propargylic substituent is decreased, the regioselectivity
of addition is lessened.
Scheme 2. Hydrosilylation of ynoate 10dlow catalyst loading.
After evaluating the effects of carbonyl-based electron-with-
drawing groups as well as the different alkyne substituents, we felt
it prudent to investigate different silanes as a third possible arm of
substrate control. Simple, vinyltrialkylsilanes are inherently useful
for due to their high stability; however, often a more activated
silicon species is generally necessary for more general synthetic
utility. Reactions, such as the TamaoeFleming oxidation allow for
vinylsilanes to serve as stable, masked carbonyls that can be un-
veiled later in a synthesis.2 These CeSi bond oxidations primarily
require halogen-, oxygen-, or aryl-substituted silicon species. The
HiyamaeDenmark coupling represents another important
This trend was consistent with primary esters as well (Table 7).
We subjected acetate protected 2-butyn-1-ol (60) to our hydro-
silylation conditions, which only afforded modest selectivity (3.7:1)
for the
fluoroacetate group (62), selectivity was greatly increased to a more
synthetically useful 17:1 ratio of isomers. To directly compare
the influence of the two different propargyl acetates, protected diol
a-silyl isomer. Using the more electron withdrawing tri-
a/b

D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
4237
Table 4
Table 5
Pt-catalyzed hydrosilylations of ynoate 10dvarying the silane
Pt-catalyzed hydrosilylations of alkyne 45dvarying the silane
Entry
1
Silane
HSiEt3
Major product
Time (h)
Yield (%)
ba
Entry
1
Silane
HSiEt3
Major product
Time (h)
5.5
Yield (%)
ba
a
/
a
/
3
94
90
>19:1
3.8:1
2
3
4
HSiMe2Ph
1.5
93
3.1:1
2
3
4
HSiMe2t-Bu
HSiMe2Bn
45
16
12
74
10:1
HSiPh3
16
98
4.5:1
98
17.8:1
HSiMe2(OEt)
2.5
94
HSiMe2(allyl)
90
3.9:1
10.4:1
5
6
HSi(OEt)3
4
87
4.2:1
5
6
7
HSiMe2Ph
2
23
2
95
10.2:1
HSi(OMe)3
4.5
88
3.2:1
HSiPh3
94
6.0:1
a
Isolated yield; a/b
ratio determined by 1H NMR.
HSiMe2(OEt)
81
10.3:1
trifluoroacetate. This result also serves as evidence that co-
ordination is not a significant contributor to regioselection, as the
acetate should exert stronger coordinative influence on the catalyst
than the trifluoroacetate, and it would therefore likely favor the
observed minor product more appreciably.
8
HSi(OEt)3
8
86
6.4:1
2.2. Steric and coordination analysis
9
HSi(OMe)3
6.5
5
81
1.6:1
Taking into account all of these observations, it appeared clear
that alkyne polarization plays the major role in dictating the
hydrosilylation regioselectivity. Still, we wanted to evaluate if ste-
rics and/or coordination were also influential.
10b
HSi(OTMS)2Me
98
>19:1
To evaluate the influence of steric hindrance on hydrosilylation
regioselectivity, we turned to all aliphatic alkyne substituents to
eliminate any significant electronic or coordinative effects. First we
subjected 5-decyne (66) to our hydrosilylation conditions (Scheme
3). The reaction proceeded in 3 h, a reaction time comparable to the
aforementioned methyl heptynoate substrate (10). The reaction
time for tert-butyl alkyne 68 was substantially longer (120 h),
presumably reflecting the difficulty of coordination of the silyle-
platinumehydride complex to the alkyne as well as the steric en-
cumbrance during migratory insertion step.
a
Isolated yield;
a
/b
ratio determined by 1H NMR.
Using Pt(dvds) as catalyst.
b
64 was evaluated (entry 3) and the catalyst system was able to
effectively differentiate both alkyne carbons (3.8:1 ), favoring
silyl addition on the carbon proximal to the more electron deficient
a/b

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D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
Table 6
Pt-catalyzed hydrosilylations of substituted secondary propargylic alcohols
Entry
1
Substrate
Major product
Time (h)
12
Yield (%)
ba
Scheme 3. Steric effects on Pt-catalyzed hydrosilylations.
a
/
95
>19:1
differentially substituted ynoates in Table 2 (entries 1e4). As the
ester substituent increased in size, regioselectivity slightly but
measurably decreased.
The potential for catalyst coordination and its impact on regio-
selectivity also needed to be established. A recent example,
Tomooka and co-workers employed a tethered, cleavable dime-
thylvinyl silyl directing group for the platinum-catalyzed hydro-
silylation of alkynes (Fig. 7).32 When the directing group was
proximal, exquisite levels of regioselectivity were observed; as the
distance between the alkyne and the directing group increased,
however, the overall selectivity decreased accordingly.
2
3
16
16
97
7.1:1
98
1.0:1
a
Isolated yield; a/b
ratio determined by 1H NMR.
Table 7
Pt-catalyzed hydrosilylations of primary propargylic esters
Fig. 7. Tomooka’s internal alkyne hydrosilylationdalkene directing capability.
Entry
1
Substrate
Major product
Time (h) Yield (%)
ba
a
/
The hydrosilylations of a number of previously described sub-
strates with propargylic oxygenation (e.g., 8, 30, 32) displayed
significantly diminished regioselectivity. Although these outcomes
were consistent with solely electronic governance, we postulated
that the proximal oxygen could also be influential via coordination
of either the platinum or silicon atoms of intermediate 71, and
consequently direct silicon addition toward the side of said group
(Fig. 8).
10
90
3.7:1
2
3
8
89
17:1
60
69
3.8:1
a
Fig. 8. The potential for regioselectivity via catalyst coordination.
Isolated yield; a/b
ratio determined by 1H NMR.
To more definitively ascertain any potential effects of co-
ordination on hydrosilylation regioselectivity, a primary prop-
argylic alcohol was protected with groups of varying coordinative
properties but similar inductive properties (Table 8). For example,
we anticipated the methoxymethyl group (entry 1) would be more
capable of catalyst coordination than a silyl ether (entry 3). These
Interestingly, even though one side of alkyne 68 is significantly
more hindered than the other, only a 1:2.2 ratio of regioisomers
is observed. This suggests that sterics may not be nearly as in-
fluential on the regioselectivity as the overall polarization of the
alkyne. This premise is consistent with the hydrosilylations of the
a/b

D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
4239
Table 8
Alcohols themselves, however, showed a substantial capability
for impacting regioselectivity, implicating a potential coordinative
effect. Examples are illustrated in Table 10. Excellent selectivity was
observed for the hydrosilylation of primary and secondary prop-
argyl alcohols 52 and 85; however, 2-butyn-1-ol (87) was curiously
less selective. Sterics could potentially play a role with the relatively
small methyl alkyne substituent.21 Although highly hindered al-
kyne 89 showed moderate regioselectivity, similarly hindered, yet
more significantly polarized alkyne 91 showed a much higher
Pt-catalyzed hydrosilylations of protected propargylic alcohols
Entry
1
Substrate
Major product
Time (h)
16
Yield (%)
ba
a
/
isomeric ratio (9.2:1 a/b). The effect of the alcohol functional group
did not extend to homopropargylic systems; much lower regiose-
lectivities were observed (entry 6).
73
2.8:1
2
3
16
16
96
3.2:1
Table 10
Pt-catalyzed alkyne hydrosilylationsdthe effect of alcohols
78
2.7:1
Entry
1
Substrate
Major product
Time (h)
Yield (%)
ba
a
/
a
Isolated yield; a/b
ratio determined by 1H NMR.
3
86
>19:1
substrates were then subjected to hydrosilylation conditions using
Pt(dvds). As can be seen, very similar
served across all three substrates.
To further study the possibility of oxygen coordination influence
of regioselectivity, an identical study was carried out with homo-
propargylic alcohol derivatives (Table 9).
a/b product ratios were ob-
2
3
4
5
24
24
92
>19:1
97
3.3:1
Table 9
Pt-catalyzed hydrosilylations of protected homopropargylic alcohols
88
3.2:1
Entry
1
Substrate
Major product
Time (h)
16
Yield (%)
ba
a
/
5
6
24
6
81
9.2:1
93
2.1:1
97
3.0:1
2
3
16
16
98
2.0:1
a
Isolated yield; a/b
ratio determined by 1H NMR.
95
1.9:1
The high regioselectivities observed in these propargylic alcohol
hydrosilylations were intriguing. One could surmise that the alco-
hol may participate in a favorable hydrogen bond interaction with
the catalytic platinum intermediate, thus acting as a directing
group for the addition process. Our current hypothesis is that the d-
electrons on the Pt center are engaging in this hydrogen bond (95,
Fig. 9).33
a
Isolated yield; a/b
ratio determined by 1H NMR.
The above tables perhaps best illustrate the lack of influence
that coordinating groups have on hydrosilylation regioselectivity.
Both the propargylic and the homopropargylic systems have es-
sentially identical observed product ratios even though these
ranges of substrates are likely to have different coordinative
properties.
An additional important observation was made during the
studies of these propargylic alcohols and derivatives. In general, the

4240
D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
Fig. 9. Possible catalyst coordination mode for alcohol moieties.
three catalyst precursors we had been evaluating (PtCl2, Zeise’s
dimer, and Pt(dvds)) behaved quite similarly across most sub-
strates. The hydrosilylations of methyl heptynoate (Scheme 1) are
indicative of this similarity. The propargylic alcohol substrates,
however, afforded much lower yields than typically observed for
hydrosilylations when PtCl2 or Zeise’s dimer were employed (Table
11). HCl, a potential byproduct of the Pt(II) salt catalyst induction
period, may promote the decomposition of the product or starting
material alcohols through ionization pathways. Pt(dvds) was able
to circumvent the problem of acid generation affording excellent
Fig. 10. Alami’s catalytic hydrostannylation of diarylacetylenesdcorrelation with 13C
NMR data.
As illustrated in Table 12, the hydrosilylations of select methyl
ynoate substrates afforded sequentially decreasing regioselectiv-
ities even though the four respective substituents are of relatively
similar size. An evaluation of alkyne 13C chemical shifts for each
compound clearly shows that each group exerts a noticeably dif-
ferent inductive effect.
yield and selectivity (92% yield, >19:1 a/b) of the vinylsilane.
Table 11
Pt-catalyzed hydrosilylations of propargylic alcoholsdeffect of precatalyst
Table 12
Pt-catalyzed ynoate hydrosilylationsdcorrelation to 13C NMR data
Entry
Catalyst
Time (h)
Yield (%)
a/b
1
2
3
PtCl2 (5 mol %)
6
4
5
45
>19:1
47
>19:1
92
Entry
Substrate
13C chemical shifts
(C (C
Dd(C
bꢀCa
)
a/
ba
[{(C2H4)PtCl2}2] (2.5 mol %)
Pt(dvds) (1 mol %)
d
a
)
d b)
>19:1
1
2
73.0
74.0
90.1
85.0
17.1
11.0
>19:1
2.3. Predictability in regioselection
5.2:1
Toward overall synthetic utility, we believed that it was im-
portant for this transformation to be as predictable as possible.
Therefore, we were curious if the characteristic data of the alkyne
substrates could lead to better predictability. A few isolated ex-
amples have employed 13C NMR chemical shift data to differentiate
alkyne carbons and rationalize hydrosilylation regioselectivity.
Tsipis’s seminal report, which hypothesized an electronic effect
influencing selectivity, discussed such 13C NMR data in the evalu-
ation of a number of aliphatic alkynes.22 Others have invoked the
analysis of 13C NMR data to explain the high regioselectivity for the
hydrosilylation of terminal alkynes.34 More recently, Alami and co-
workers correlated 13C NMR data to the regioselectivity of
palladium-catalyzed hydrostannylations (Fig. 10), specifically fo-
cusing on the differences between ortho-substituted diary-
lacetylenes and their para-substituted counterparts.35
3
4
77.4
81.6
84.2
78.0
6.8
3.5:1
0.9:1
ꢀ3.6
a
Determined by 1H NMR.
The hydrosilylation of methyl heptynoate (10, entry 1) produces
the enoate with high regioselectivity; this corresponds with a high
difference in alkyne carbon chemical shifts (
propargylic acetate 34 shows markedly less
D
a
¼17.1 ppm). Homo-
In this work, it was shown that the para-substituted phenyl
-selectivity, and ac-
diarylacetylenes featured relatively small differences in the
b
a and
cordingly the alkyne carbons are less differentiated (11.0 ppm). On
the other end of the spectrum, a propargylic acetate moiety (i.e.,
alkyne 30) seemingly exerts a stronger electronic influence than
the methyl ester, showing a slight preference for the
regioisomer and a negative difference between (C ) and
Overall, the magnitude and direction of the difference between
alkyne carbon chemical shifts directly corresponds to observed
regioselectivity of hydrosilylation.
This correlation extended to the aforementioned alkynes fea-
turing propargylic or homopropargylic oxygenation (Fig. 11). As
13C chemical shifts. The corresponding hydrostannylation regio-
selectivities were consistent with these differences. In contrast, the
ortho-substituted systems possessed much greater chemical shift
differences, and correlating larger regioselectivities were observed.
A subsequent report by Gevorgyan and co-workers indicated that
13C chemical shifts should not be applied with these ortho-
substituted cases.36 With this backdrop, we were eager to probe
this concept to the internal alkyne hydrosilylations we had been
investigating.
b
-silyl
(C ).
d
b
d
a

D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
4241
into purposeful reaction design. Table 13 highlights an example.
Alcohol 96 can be protected with common groups that feature
different electronic properties. The correlating inductive impact on
the alkyne carbons can be readily measured by NMR. As evidenced,
synthetically viable selectivity can be achieved in this hydro-
silylation by the judicious choice of alcohol protecting group (i.e.,
the trifluoroacetate). This example illustrates how reasonable
predictability can be achieved in future alkyne hydrosilylations.
3. Conclusion
In summary, the hydrosilylation of internal alkynes, once con-
sidered to be generally unselective, can be harnessed quite effec-
tively using substrate control. We have demonstrated the
hydrosilylations of numerous internal alkynes using mild condi-
tions and simple, stable Pt catalysts. In many cases, these reactions
proceed in high yield and selectivity. To further elucidate selectivity
controls, the effects of electronics, steric hindrance, and co-
ordination were comprehensively evaluated. Notably, it was found
that alkyne polarization afforded the greatest influence over the
regioselectivity of silane addition. This electronic effect can be
reasonably quantified using 13C NMR data. A greater magnitude
between alkyne carbon chemical shifts correlates to greater ob-
served regioselectivity of silane addition.37 The effects of sterics are
measurable but significantly less influential. Although proximal
oxygen atoms could conceivably direct selectivity, after evaluation
of numerous substrates it can be determined that the inductive
effect of the oxygen group probably affects overall regioselectivity
more so than coordination, with alcohols themselves providing an
exception. This expansion of hydrosilylation methodology will
hopefully serve the synthetic community with its ability to gener-
ate synthetically useful stereodefined trisubstituted alkenes.
Fig. 11. 13C NMR analysis of propargylic and homopropargylic alcohols and derivatives.
discussed, similar hydrosilylation regioselectivities were observed
in both classes regardless of the potential coordinative abilities of
the respective functional groups. In evaluating the 13C NMR data, it
was clear the correlation of this data and their related electronic
implications were well aligned with the observed regioselection.
These data also corroborated the lack of a coordinative effect for the
compounds in Tables 8 and 9. Alternatively, this data also strongly
implicates the directing impact of a propargylic alcohol (i.e., alkyne
85), while indicating the absence of a similar effect for a homo-
propargylic alcohol.
Taking this information fully into account, the electronic effect
in internal alkyne hydrosilylation should be able to be incorporated
Table 13
Pt-catalyzed alkyne hydrosilylationsdprotecting group strategy by correlation with 13C NMR data
Entry
Substrate
13C chemical shifts
(C
Dd(C
bꢀCa
)
a/
ba
d
a
)
d(Cb)
1
2
77.8
76.5
81.3
81.8
3.5
5.3
1.4:1
2.5:1
2.7:1
3
4
76.5
74.7
82.0
83.0
5.5
8.3
4.3:1
(76% isol. yield)
a
Determined by 1H NMR.

4242
D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
4. Experimental section
90%, 10.4:1
a
/
b
. Rf¼0.41 in 19:1 hexanes/Et2O. IR (film) 2958, 1718,
1433, 1250, 1199 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
6.18 (t, J¼7.1 Hz,
4.1. Materials and methods
1H), 5.74 (ddt, J¼16.3, 10.5, 8.1 Hz, 1H), 4.86e4.82 (comp. m, 2H),
3.72 (s, 3H), 2.36 (q, J¼7.2 Hz, 2H), 1.65 (d, J¼8.0 Hz, 2H), 1.44e1.28
(comp. m, 4H), 0.90 (t, J¼7.1 Hz, 3H), 0.12 (s, 6H); 13C NMR
Methylene chloride, tetrahydrofuran, ether, and toluene were
purified by passing through activated alumina columns. All other
reagents were used as received unless otherwise noted. Commer-
cially available chemicals were purchased from Alfa Aesar (Ward
Hill, MA), SigmaeAldrich (St. Louis, MO), Gelest (Morrisville, PA),
Oakwood Products (West Columbia, SC), Strem (Newburport, MA),
and TCI America (Portland, OR). Qualitative TLC analysis was per-
(100 MHz, CDCl3)
d 171.2, 153.7, 134.9, 134.5, 113.9, 51.5, 31.9, 31.7,
23.6, 22.8, 14.4, ꢀ3.1; HRMS (DARTþ) m/z calcd for (MþNH4)þ
[C13H28NO2Si]þ: 258.1889, found 258.1884.
4.2.4. Methyl (E)-2-(dimethyl(phenyl)silyl)hept-2-enoate
(39). Hydrosilylation performed under Ar. Colorless oil. Yield 95%,
ꢁ
formed on 250 mm thick, 60 A, glass backed, F254 silica (Silicycle,
10.2:1
a
/b
. Rf¼0.29 in 19:1 hexanes/Et2O. IR (film) 2957, 1717, 1606,
Quebec City, Canada). Visualization was accomplished with UV
light and exposure to either p-anisaldehyde or KMnO4 stain solu-
tions followed by heating. Flash chromatography was performed
using Silicycle silica gel (230e400 mesh). 1H NMR spectra were
acquired on either a Varian Mercury 300 (at 300 MHz), a Varian
Inova 400 (at 400 MHz), or a Varian 400 MR (at 400 MHz) and are
1200, 835 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
7.54e7.52 (comp. m,
2H), 7.37e7.34 (comp. m, 3H), 6.20 (t, J¼7.2 Hz,1H), 3.65 (s, 3H), 2.39
(q, J¼7.3 Hz, 2H),1.43e1.27 (comp. m, 4H), 0.89 (t, J¼7.2 Hz, 3H), 0.42
(s, 6H); 13C NMR (100 MHz, CDCl3)
d 171.2, 155.4, 138.1, 134.60,
134.58, 129.7, 128.3, 51.6, 32.2, 31.7, 23.0, 14.5, ꢀ1.9; HRMS (APCIþ)
m/z calcd for (MþNH4)þ [C16H28NO2Si]þ: 294.1889, found 294.1888.
reported relative to SiMe4 (d
0.00). 13C NMR spectra were acquired
on either a Varian Inova 400 (at 100 MHz), a Varian Mercury 300 (at
75 MHz), or a Varian 400 MR (at 100 MHz) and are reported relative
4.2.5. Methyl (E)-2-(triphenylsilyl)hept-2-enoate (40) and methyl
(E)-3-(triphenylsilyl)hept-2-enoate. Hydrosilylation performed un-
to SiMe4
(
d
0.0). All IR spectra were obtained on NaCl plates (film)
der Ar. Colorless oil. Yield 94%, 6.0:1
Et2O.
-Isomer: IR (film) 2955, 1715, 1485, 1203, 1109 cmꢀ1; 1H NMR
(400 MHz, CDCl3)
a
/
b
. Rf¼0.26 in 19:1 hexanes/
with a Bruker Tensor 27. High resolution mass spectrometry data
were acquired by the Colorado State University Central Instrument
Facility on an Agilent 6210 TOF LC/MS.
a
d
7.58 (app. d, J¼8.0 Hz, 6H), 7.45e7.36 (comp. m,
9H), 6.32 (t, J¼7.3 Hz, 1H), 3.48 (s, 3H), 2.47 (q, J¼7.3 Hz, 2H),
1.44e1.28 (comp. m, 4H), 0.90 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz,
4.2. General procedure for Pt-catalyzed alkyne
hydrosilylations
CDCl3) d 171.0, 158.9, 136.6, 134.1, 131.7, 130.0, 128.2, 51.5, 32.3, 31.5,
22.9, 14.3; HRMS (APCIþ) m/z calcd for (MþNH4)þ [C26H32NO2Si]þ:
418.2202, found 418.2183.
b-Isomer: IR (film) 3070, 2956, 1721,
To a solution of the alkyne substrate and silane (1.1 equiv) in
CH2Cl2 (0.2 M in substrate) was added PtCl2 (5 mol %) under either
argon or an ambient atmosphere. The reaction mixture was
allowed to stir at room temperature until TLC indicated consump-
tion of the starting material, at which point the reaction mixture
was filtered through a small plug of silica gel, washing with Et2O
(w3ꢁ reaction volume). The solvent was removed by rotary evap-
oration, and the resulting residue was purified by flash chroma-
tography to afford the vinylsilane. The crude reaction mixture was
analyzed by 1H NMR to determine the isomeric ratio prior to pu-
1485, 1714, 700 cmꢀ1 1H NMR (400 MHz, CDCl3)
; d 7.58e7.56 (m,
6H), 7.48e7.38 (comp. m, 9H), 6.23 (s, 1H), 3.72 (s, 3H), 2.75 (app. t,
J¼7.5 Hz, 2H), 1.42e1.11 (comp. m, 4H), 0.67 (t, J¼7.0 Hz, 3H); 13C
NMR (100 MHz, CDCl3) d 165.9, 161.6, 136.7, 133.0, 130.2, 128.3, 51.3,
32.2, 32.0, 23.3, 13.9; LRMS (ESIþ) m/z calcd for (MþH)þ
[C26H29O2Si]þ: 401.2, found 401.2.
4.2.6. Methyl (E)-2-(ethoxydimethylsilyl)hept-2-enoate
(41). Hydrosilylation performed under Ar. Colorless oil. Yield 81%,
10.3:1
a
/
b
. Rf¼0.52 in 9:1 hexanes/Et2O. IR (film) 2960, 1719, 1252,
rification. Typically, the
inseparable by flash column chromatography, although larger col-
umns were sometimes effective at separation.
a
and
b
isomers of the vinylsilanes were
1199, 835 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
6.38 (t, J¼7.2 Hz, 1H),
3.73 (s, 3H), 3.69 (q, J¼7.0 Hz, 2H), 2.42 (q, J¼7.3 Hz, 2H),1.44 (quint,
J¼7.4 Hz, 2H), 1.34 (sextet, J¼7.4 Hz, 2H), 1.19 (t, J¼7.0 Hz, 3H), 0.91
(t, J¼7.2 Hz, 3H), 0.25 (s, 6H); 13C NMR (100 MHz, CDCl3)
155.1, 134.1, 59.0, 51.4, 31.6, 31.4, 22.7, 18.7, 14.2, ꢀ1.4.
d 170.7,
4.2.1. Methyl (E)-2-(tert-butyldimethylsilyl)hept-2-enoate
(36). Hydrosilylation performed under Ar. Colorless oil. Yield 74%,
10.0:1
a
/
b
. Rf¼0.56 in 19:1 hexanes/Et2O. IR (film) 2957, 1718, 1604,
4.2.7. Methyl (E)-2-(triethoxysilyl)hept-2-enoate (42) and methyl
(E)-3-(triethoxysilyl)hept-2-enoate. Hydrosilylation performed un-
1465, 1200, 835 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
6.07 (t, J¼7.2 Hz,
1H), 3.71 (s, 3H), 2.26 (q, J¼7.3 Hz, 2H), 1.45e1.28 (comp. m, 4H),
der Ar. Colorless oil. Yield 86%, 6.4:1
Et2O ( -isomer), 0.43 in 9:1 hexanes/Et2O (
(film) 2975, 1720, 1609, 1434, 1391, 1206, 788 cmꢀ1
(400 MHz, CDCl3)
a
/
b
. Rf¼0.35 in 9:1 hexanes/
-isomer). -Isomer: IR
1H NMR
0.94e0.89 (comp. m, 12H), 0.11 (s, 6H); 13C NMR (100 MHz, CDCl3)
a
b
a
d
172.3, 151.2, 134.8, 51.5, 32.2, 31.7, 27.1, 22.8, 17.8, 14.4, ꢀ5.3; HRMS
;
(APCIþ) m/z calcd for (MþH)þ [C14H29OSi]þ: 257.1937, found
d
6.68 (t, J¼7.2 Hz, 1H), 3.84 (q, J¼7.0 Hz, 6H), 3.73
257.1917.
(s, 3H), 2.46 (q, J¼7.2 Hz, 2H), 1.43 (quint, J¼7.4 Hz, 2H), 1.33 (sextet,
J¼7.4 Hz, 2H), 1.21 (t, J¼7.0 Hz, 9H), 0.90 (t, J¼6.9 Hz, 3H); 13C NMR
4.2.2. Methyl (E)-2-(benzyldimethylsilyl)hept-2-enoate
(37). Hydrosilylation performed under air. Colorless oil. Yield
(100 MHz, CDCl3) d 169.8, 160.5, 127.5, 59.3, 51.7, 31.8, 31.4, 22.9,
18.6, 14.3; LRMS (ESIþ) m/z calcd for (MþH)þ [C14H29O5Si]þ: 305.2,
98%, 17.8:1
a
/b
. Rf¼0.53 in 19:1 hexanes/Et2O. IR (film) 2958, 1717,
found 305.2. b-Isomer: IR (film) 2976, 1727, 1434, 1082, 963,
1601, 1493, 1199 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
7.19 (t, J¼7.5 Hz,
783 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
6.31 (s,1H), 3.84 (q, J¼7.1 Hz,
2H), 7.06 (t, J¼7.4 Hz, 1H), 6.99 (d, J¼6.9 Hz, 2H), 6.06 (t, J¼7.2 Hz,
1H), 3.73 (s, 3H), 2.34 (q, J¼7.3 Hz, 2H), 2.23 (s, 2H), 1.40e1.24
(comp. m, 4H), 0.89 (t, J¼7.2 Hz, 3H), 0.09 (s, 6H); 13C NMR
6H), 3.72 (s, 3H), 2.67 (t, J¼7.5 Hz, 2H), 1.50e1.33 (comp. m, 2H) 1.25
(t, J¼7.0 Hz, 9H), 0.93 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d
166.3,157.2,130.1, 59.2, 51.5, 31.9, 31.1, 23.6,18.6,14.4; LRMS (ESIþ)
(100 MHz, CDCl3)
d
170.8, 153.9, 139.7, 133.7, 128.5, 128.4, 128.3,
m/z calcd for (MþH)þ [C14H29O5Si]þ: 305.2, found 305.2.
128.2,124.2, 51.2, 31.6, 31.2, 25.7, 22.5,14.0, ꢀ3.4; HRMS (DARTþ) m/
z calcd for (MþNH4)þ [C17H30NO2Si]þ: 308.2046, found 308.2040.
4.2.8. Methyl (E)-2-(trimethoxysilyl)hept-2-enoate (43) and methyl
(E)-3-(trimethoxysilyl)hept-2-enoate. Hydrosilylation
under Ar. Colorless oil. Yield 81%, 1.6:1
. Rf¼0.30 in 9:1 hexanes/
EtOAc ( -isomer), 0.46 in 9:1 hexanes/Et2O ( -isomer). -Isomer: IR
performed
4. 2. 3. Methyl (E)-2-(allyldimethylsilyl)hept-2-enoate
(38). Hydrosilylation performed under air. Colorless oil. Yield
a/b
a
b
a

D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
4243
(film) 2954, 2844,1721,1608,1207 cmꢀ1; 1H NMR (400 MHz, CDCl3)
4.2:1
a
/
b
. Rf¼0.24 in 9:1 hexanes/Et2O. (Data for
a
-isomer only.) IR
d
6.69 (t, J¼7.2 Hz, 1H), 3.76 (s, 3H), 3.58 (s, 9H), 1.48e1.41 (m, 2H),
(film) 2974, 1744, 1618, 1388, 1242, 959, 781 cmꢀ1 1H NMR
;
1.34 (sextet, J¼7.3 Hz, 2H), 0.90 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz,
(400 MHz, CDCl3)
d
6.22 (t, J¼7.0 Hz, 1H), 4.06 (t, J¼7.6 Hz, 2H), 3.81
CDCl3)
d
169.0,161.2,125.5, 51.4, 50.9, 31.3, 22.4,13.8; LRMS (ESIþ) m/
(q, J¼7.6 Hz, 6H), 2.47 (t, J¼7.3 Hz, 2H), 2.17 (q, J¼7.2 Hz, 2H), 2.03 (s,
z calcd for (MþH)þ [C11H22O5Si]þ: 263.1, found 263.1.
b-Isomer: IR
3H), 1.43e1.29 (comp. m, 4H), 1.22 (t, J¼7.1 Hz, 9H), 0.91 (t, J¼7.3 Hz,
(film) 2954, 2844, 1727, 1194, 1089, 813 cmꢀ1
;
1H NMR (400 MHz,
3H); 13C NMR (100 MHz, CDCl3)
d 171.5, 149.4, 127.9, 64.1, 58.9, 31.8,
CDCl3)
d
6.28 (s, 1H), 3.73 (s, 3H), 3.59 (s, 9H), 2.66 (t, J¼7.8 Hz, 2H),
28.81, 28.77, 22.9, 21.5, 18.6, 14.4; HRMS (ESIþ) m/z calcd for
(MþNa)þ [C16H32NaO5Si]þ: 355.1917, found 355.1906.
1.49e1.34 (comp. m, 4H), 0.93 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz,
CDCl3)
d 165.9, 155.4, 130.4, 51.4, 51.1, 31.6, 30.8, 23.4, 14.2; LRMS
(ESIþ) m/z calcd for (MþH)þ [C11H22O5Si]þ: 263.1, found 263.1.
4.2.15. (E)-3-(Trimethoxysilyl)oct-3-en-1-yl
(51). Hydrosilylation performed under Ar. Colorless oil. Yield 88%,
3.2:1 -isomer only.) IR
. Rf¼0.31 in 9:1 hexanes/EtOAc. (Data for
(film) 2958, 2842, 1744, 1617, 1243, 811 cmꢀ1 1H NMR (400 MHz,
CDCl3)
6.21 (t, J¼7.1 Hz,1H), 4.06 (t, J¼7.5 Hz, 2H), 3.57 (s, 9H), 2.48
acetate
4.2.9. Methyl (E)-2-(1,1,1,3,5,5,5-heptamethyltrisiloxan-3-yl)hept-2-
enoate (44). Hydrosilylation performed under air using Pt(dvds).
a/
b
a
;
Pale yellow oil. Yield 98%, >19:1
IR (film) 2959, 1721, 1609, 1258, 1200, 1075 cmꢀ1
(400 MHz, CDCl3)
6.44 (t, J¼7.2 Hz, 1H), 3.72 (s, 3H), 2.43 (q,
J¼7.3 Hz, 2H),1.46e1.31 (comp. m, 2H), 0.92 (t, J¼7.2 Hz, 3H), 0.18 (s,
a/
b
. Rf¼0.52 in 19:1 hexanes/Et2O.
d
;
1H NMR
(t, J¼7.5 Hz, 2H), 2.29 (q, J¼7.1 Hz, 2H), 2.03 (s, 3H),1.42e1.30 (comp.
d
m, 4H), 0.92 (t, J¼7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 171.5,
150.1,126.8, 64.1, 51.1, 31.8, 28.8, 21.7, 22.9, 21.5,14.4; HRMS (ESIþ) m/
z calcd for (MþNa)þ [C13H26O5SiNa]þ: 313.1447, found 313.1446.
3H), 0.10 (s, 18H); 13C NMR (100 MHz, CDCl3)
d 170.1, 156.1, 133.5,
51.0, 31.2, 31.2, 22.5, 14.0, 1.9, 0.3; HRMS (ESIþ) m/z calcd for
(MþH)þ [C15H35O4Si3]þ: 363.1838, found 363.1846.
Acknowledgements
4.2.10. (E)-3-(Triethylsilyl)oct-3-en-1-yl
(46). Hydrosilylation performed under Ar. Colorless oil. Yield 90%,
3.8:1 -isomer only.) IR
. Rf¼0.38 in 19:1 hexanes/Et2O. (Data for
(film) 2956, 1745, 1464, 1239, 1031, 718 cmꢀ1 1H NMR (400 MHz,
CDCl3)
acetate
Colorado State University is acknowledged for support of our
research program. Eli Lilly and Company (_100004312) is gratefully
acknowledged for a graduate research fellowship to D.A.R.
a
/
b
a
;
d
5.81 (t, J¼7.0 Hz, 1H), 3.95 (app. t, J¼8.0 Hz, 2H), 2.44 (dd,
References and notes
J¼8.3, 7.6 Hz, 2H), 2.17 (q, J¼7.1 Hz, 2H), 2.05 (s, 3H), 1.39e1.33
(comp. m, 4H), 0.92 (t, J¼7.8 Hz, 9H), 0.60 (q, J¼7.8 Hz, 6H); 13C NMR
1. (a) Marciniec, B. Hydrosilylation In Advances in Silicon Science; Marciniec, B.,
Ed.; Springer: Dordrecht, Netherlands, 2009, Chapters 1e2; (b) Trost, B. M.; Ball,
Z. T. Synthesis 2005, 853e887.
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14. Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16e21.
15. (a) Speier, J. L.; Webster, J. A.; Barnes, G. A. J. Am. Chem. Soc. 1957, 79, 974e979;
(b) Their report was disclosed subsequent to a patent by Wagner using Pt/C
a few months earlier. Wagner, G. H.; Strother, C. O. U.S. Patent 2,632,013, Mar.
17, 1953.
(100 MHz, CDCl3)
d 171.5, 146.0, 132.1, 64.2, 32.3, 29.4, 28.9, 22.9,
21.5, 14.5, 7.8, 3.4; HRMS (DARTþ) m/z calcd for (MþNH4)þ
[C16H36NO2Si]þ: 302.2515, found 302.2518.
4.2.11. (E)-3-(Dimethyl(phenyl)silyl)oct-3-en-1-yl
(47). Hydrosilylation performed under Ar. Colorless oil. Yield 93%,
3.1:1 -isomer only.) IR
. Rf¼0.53 in 9:1 hexanes/EtOAc. (Data for
(film) 2958, 1743, 1241, 1110, 1033, 818 cmꢀ1 1H NMR (400 MHz,
CDCl3)
acetate
a/b
a
;
d
7.51e7.48 (m, 2H), 7.35e7.32 (comp. m, 3H), 5.96 (t, J¼7.0 Hz,
1H), 3.86 (app. t, J¼7.8 Hz, 2H), 2.45 (t, J¼7.6 Hz, 2H), 2.18 (q, J¼7.1 Hz,
2H), 1.97 (s, 3H), 1.39e1.31 (comp. m, 4H), 0.92 (t, J¼7.1 Hz, 3H), 0.36
(s, 6H); 13C NMR (100 MHz, CDCl3)
d 171.3, 146.4, 138.9, 134.2, 133.6,
129.2,128.0, 64.0, 32.0, 29.1, 29.0, 22.8, 21.3,14.3, ꢀ2.5; HRMS (APCIþ)
m/z calcd for (MþNH4)þ [C18H32NO2Si]þ: 322.2202, found 322.2191.
4.2.12. (E)-3-(Triphenylsilyl)oct-3-en-1-yl
(48). Hydrosilylation performed under Ar. White solid. Yield 98%,
4.5:1 -isomer only.) IR
. Rf¼0.47 in 9:1 hexanes/EtOAc. (Data for
(film) 3069, 2957, 1741, 1428, 1240, 702 cmꢀ1 1H NMR (400 MHz,
CDCl3) 7.56e7.54 (m, 6H), 7.43e7.35 (comp. m, 9H), 6.13 (t,
acetate
a
/b
a
16. Steffanut, P.; Osborn, J. A.; DeCian, A.; Fisher, J. Chem.dEur. J. 1998, 4,
2008e2017.
;
17. Roy, A. K.; Taylor, R. B. J. Am. Chem. Soc. 2002, 124, 9510e9524.
d
ꢀ
18. Berthon-Gelloz, G.; Schumers, J.-M.; De Bo, G.; Marko, I. E. J. Org. Chem. 2008,
J¼7.0 Hz, 1H), 3.75 (t, J¼7.6 Hz, 2H), 2.58 (t, J¼7.6 Hz, 2H), 2.29 (q,
73, 4190e4197.
J¼7.1 Hz, 2H),1.86 (s, 3H),1.40e1.32 (comp. m, 4H), 0.91 (t, J¼7.0 Hz,
19. (a) McLaughlin, M. G.; Cook, M. J. Chem. Commun. 2011, 11104e11106; (b)
McAdam, C. A.; McLaughlin, M. G.; Johnston, A. J. S.; Chen, J.; Walter, M. W.;
Cook, M. J. Org. Biomol. Chem. 2013, 11, 4488e4502.
20. Sumida, Y.; Kato, T.; Yoshida, S.; Hosoya, T. Org. Lett. 2012, 14, 1552e1555.
21. (a) Stork, G.; Jung, M. E.; Colvin, E.; Noel, Y. J. Am. Chem. Soc. 1974, 96,
3684e3686; (b) Thorimbert, S.; Malacria, M. Tetrahedron Lett. 1996, 37,
8483e8486; (c) Li, J.; Suh, J. M.; Chin, E. Org. Lett. 2010, 12, 4712e4715.
22. Tsipis, C. A. J. Organomet. Chem. 1980, 187, 427e446.
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24. (a) Hamze, A.; Provot, O.; Alami, M.; Brion, J.-D. Org. Lett. 2005, 7, 5625e5628;
(b) Hamze, A.; Provot, O.; Brion, J.-D.; Alami, M. Synthesis 2007, 2025e2036.
25. In this and all cases of the Pt-catalyzed hydrosilylations evaluated, we only
observed (E)-selectivity. No (Z)-isomers were observed.
26. We attribute this efficiency largely to the increased solubility of the Pt catalyst
in CH2Cl2.
27. Of note, 1 mol % Karstedt’s catalyst was employed due to the fact that it is
a 3 wt % solution in PDMS and removing large amounts of PDMS can be quite
cumbersome.
28. We did find that reactions would not proceed to completion with lower cat-
alyst loadings.
29. Chang, W. T.; Smith, R. C.; Regens, C. S.; Bailey, A. D.; Werner, N. S.; Denmark, S.
E. Org. React. 2011, 75, 213e745.
3H); 13C NMR (100 MHz, CDCl3)
d 171.4, 151.4, 136.8, 134.8, 130.0,
128.3, 64.3, 32.1, 29.7, 23.1, 21.4, 14.5; HRMS (APCIþ) m/z calcd for
(MþNH4)þ [C28H36NO2Si]þ: 446.2515, found 446.2508.
4.2.13. (E)-3-(Ethoxydimethylsilyl)oct-3-en-1-yl
(49). Hydrosilylation performed under Ar. Colorless oil. Yield 94%,
3.9:1
. Rf¼0.31 in 9:1 hexanes/EtOAc. (Data for
-isomer only.) 1H
NMR (400 MHz, CDCl3)
acetate
a/b
a
d
5.98 (t, J¼6.9 Hz, 1H), 4.01 (t, J¼7.8 Hz, 2H),
3.63 (q, J¼7.0 Hz, 2H), 2.50 (t, J¼7.8 Hz, 2H), 2.17 (q, J¼7.1 Hz, 2H), 2.05
(s, 3H),1.40e1.32 (comp. m, 4H),1.19 (t, J¼7.0Hz, 3H), 0.92(t, J¼7.1 Hz,
3H), 0.20 (s, 6H); 13C NMR (100 MHz, CDCl3)
d 171.5,146.3,134.3, 64.2,
58.8, 32.0, 28.8, 28.7, 22.9, 21.5, 18.9, 14.5, ꢀ1.6; HRMS (APCIþ) m/z
calcd for (MþH)þ [C14H28O3Si]þ: 273.1808, found 273.1800.
4.2.14. (E)-3-(Triethoxysilyl)oct-3-en-1-yl
acetate
(50). Hydrosilylation performed under Ar. Colorless oil. Yield 87%,

4244
D.A. Rooke et al. / Tetrahedron 70 (2014) 4232e4244
30. For examples of the use of this reagent in Pt catalyzed hydrosilylations of
terminal alkynes, see: (a) Hamze, A.; Provot, O.; Brion, J.-D.; Alami, M. J. Or-
ganomet. Chem. 2008, 693, 2789e2797; (b) Sonnek, G.; Drahs, E.; Jancke, H.;
Hamann, H. J. Organomet. Chem. 1990, 386, 29e35.
Although we do not eliminate that as a possibility, we do not prefer that in-
teraction in our case, as we observe hydride delivery to the alkyne b-carbon.
34. Matsumoto, H.; Hoshino, Y.; Nagai, Y. Chem. Lett. 1982, 1663e1666.
35. Alami, M.; Liron, F.; Gervais, M.; Peyrat, J.-F.; Brion, J.-D. Angew. Chem., Int. Ed.
2002, 41, 1578e1580.
31. Rooke, D. A.; Ferreira, E. M. Org. Lett. 2012, 14, 3328e3331.
32. Kawasaki, Y.; Ishikawa, Y.; Igawa, K.; Tomooka, K. J. Am. Chem. Soc. 2011, 133,
20712e20715.
36. Rubin, M.; Trofimov, A.; Gevorgyan, V. J. Am. Chem. Soc. 2005, 127,
10243e10249.
33. (a) The regioselectivities observed here are quite different than those in Cook’s
studies of propargylic alcohols. We attribute this difference to his use of a bulky
XPhos ligand in conjunction with a Pt catalyst. See Ref. 19. (b) Cook invoked
hydrogen bonding between the alcohol hydrogen and the platinum hydride.
37. For other elegant examples of the application of this correlation, see: (a) Hy-
droalumination: Igawa, K.; Tomooka, K. Angew. Chem., Int. Ed. 2006, 45,
232e234; (b) Au- and Zn-catalyzed hydroxyanilide addition: Wang, Y.; Liu, L.;
Zhang, L. Chem. Sci. 2013, 4, 739e746.