Stereoselective Base-Catalyzed 1,1-Silaboration of Terminal Alkynes

Article abstract of DOI:10.1002/anie.201913544

A base-catalyzed reaction that enables stereoselective 1,1-silaboration of terminal alkynes is described. This method not only offers a new strategy to functionalize simple and readily accessible alkynes beyond 1,2-difunctionalization, but also provides a

Full text of DOI:10.1002/anie.201913544

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Accepted Article
Title: Stereoselective Base-Catalyzed 1,1-Silaboration of Terminal
Alkynes
Authors: Ruben Martin, Yiting Gu, Yaya Duan, and Yangyang Shen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913544
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10.1002/anie.201913544
Angewandte Chemie International Edition
COMMUNICATION
Stereoselective Base-Catalyzed 1,1-Silaboration of Terminal Alkynes
Yiting Gu, Yaya Duan, Yangyang Shen and Ruben Martin*
Abstract: A base-catalyzed protocol that enables a stereoselective
1,1-silaboration of terminal alkynes is described. This method does
not only offer a new strategy to functionalize simple and readily
accessible alkynes beyond 1,2-difunctionalization events, but also
provides an unconventional atom- and step-economical approach to
rapidly and reliably access versatile geminal silylboranes in the
Driven by the versatility and inherent modularity of organosilicon
and organoboron reagents as synthons in organic synthesis,[9] we
recently wondered whether it would be possible to design an
atom-economical stereoselective 1,1-silaboration of alkynes in
the absence of transition metals or organometallic species. To this
end, we hypothesized that a base-catalyzed protocol in the
absence of transition metals with an exquisite stereoselectivity pattern. presence of readily available silylborane might be suited for our
purposes (Scheme 1, bottom). Specifically, we anticipated that in
situ generated ate-complexes of type
I might enable a
In recent years, the catalytic functionalization of p-systems has
gained momentum as a powerful, yet practical, alternative to the
use of organic (pseudo)halides for building up molecular
complexity.[1] Among these, particular attention has been devoted
to designing de novo catalytic routes en route to densely
functionalized alkenes via site-selective difunctionalization of
alkyne congeners.[2] Although 1,2-difunctionalizations of alkynes
via one or two-electron manifolds have become routine (Scheme
1, top right),[3-5] a related 1,1-difunctionalization event is not as
commonly practiced as one might initially anticipate. At present,
these techniques remain primarily confined to the use of transition
metal catalysts – in most instances requiring sophisticated ligand
sets –,[6] or stoichiometric organometallic reagents.[7] Despite the
stereoselective [1,2]-silyl shift[10] with concomitant deprotonation
of the acetylenic sp C–H bond, thus setting the basis for the base
catalyst turnover. If successful, such a route would represent,
conceptuality and practicality aside, a new platform for preparing
geminal organometallic linkages bearing two chemically distinct,
yet modular, C–B and C–Si bonds. In our continuing interest in
silylation and borylation reactions,[11] we report herein the
successful realization of this goal. Unlike related 1,1-diborations
that typically require either transition metal catalysts or
organometallic reagents,[12] our catalytic 1,1-silaboration event
offers a strategic gateway to rapidly and reliably access densely
functionalized alkenes in a highly stereocontrolled manner via
orthogonal C–Si and C–B cleavage, thus leading to new
knowledge in retrosynthetic design.
advances realized,[8]
a
stereoselective catalytic 1,1-
heterodifunctionalization blueprint that obviates the need for
transition metals while forging two different C–heteroatom bonds
still constitutes an elusive cartography in catalytic endeavors
(Scheme 1, top left).
Ha
Ph
BPin
SiEt3
KHMDS (20 mol%)
DME, 70 ºC
Ph
Ha
+
Et3SiBpin
2a
1a
Entry
3a
3a (%)[a]
97 (91)[b]
heterodifunctionalization of terminal alkynes (Y, Z = heteroatom)
Deviation from standard conditions
none
1
2
1,1-selectivity
1,2-selectivity
H
no transition
metal
KHMDS (10 mol%) was used
LiHMDS instead of KHMDS
NaHMDS instead of KHMDS
Mg(HMDS)2 instead of KHMDS
+ 18-crown-6 (20 mol%)
catalyst
34
15
36
–
H
Y
Z
3
Y & Z
4
path b
path a
H
5
unknown
ample
precedents
6
–
base-catalyzed 1,1-silaboration of terminal alkynes (this work)
t-BuOK instead of KHMDS
toluene instead of DME
7
32
95
32
41
R3Si
8
H
R3SiBPin
BPin
R3Si
BPin
dioxane instead of DME
9
base
X
reaction conducted at 25 °C
Et3SiSiEt3 (Et3SiH) instead of Et3SiBPin
B2Pin2 instead of Et3SiBPin
10
11
12
H
H
[c]
“H”
I
–
no transition metal
mild & atom-economical
exquisite stereoselectivity
[d]
–
Scheme 2. Optimization of the reaction conditions. 1a (0.40 mmol), 2a (0.4
mmol), KHMDS (0.08 mmol, 20 mol%) in DME (2 mL) at 70 ºC for 12 h. [a] Yields
determined by GC using decane as internal standard. [b] Isolated yield, average
Scheme 1. Base-catalyzed heterodifunctionalization of terminal alkynes.
[c]
[d]
of two independent runs.
no traces of hydro(di)silylation were found.
no
traces of hydro(di)boration were found. DME = 1,2-dimethoxyethane; BPin =
4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl. HMDS = hexamethyldisilazide.
[a]
Y. Gu, Y. Duan, Y. Shen, Prof. R. Martin
Institute of Chemical Research of Catalonia (ICIQ)
The Barcelona Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: rmartinromo@iciq.es
We began our investigations by studying the 1,1-silaboration of
phenylacetylene (1a) with Et3SiBPin (2a) en route to 3a (Scheme
2). The choice of 2a instead of commonly employed PhMe2SiBPin
(2b) was not arbitrary.[13] From a synthetic standpoint, the use of
2a offers the advantage of selectively functionalizing the vinyl–Si
bond in 3a at later stages, thus avoiding unnecessary site-
selectivity issues arising from the presence of multiple sp2 C–Si
Prof. R. Martin
Catalan Institution for Research and Advanced Studies (ICREA)
Passeig Lluïs Companys, 23, 08010 Barcelona Spain
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201913544
Angewandte Chemie International Edition
COMMUNICATION
bonds in 3b.[14] After systematic optimization,[15] a protocol based
on KHMDS (20 mol%) in DME at 70 ºC provided the best results,
(entry 1).[16] This transformation merits some further discussion:
1) no transition metal complex is required; 2) the reaction is
conducted with a 1a:2a ratio of 1:1 and catalytic amounts of
KHMDS, thus offering an atom- and step-economical route to 3a;
3) two different, yet chemically distinguishable, organometallic
bonds can be forged from available alkynes with exquisite control
of the stereoselectivity. Intriguingly, the escorting cation exerted a
profound influence on the reaction outcome. Indeed, the
corresponding Li, Na or Mg analogues provided lower yields of 3a,
if any (entries 2-5), possibly due to the different size and
coordination ability of the cation. The key role of potassium
counterions was further corroborated by observing no conversion
of 1a to 3a with 18-crown-6 (entry 6) whereas lower yields were
found when KOtBu was used (entry 7). Likewise, the nature of the
counteranion and the solvent employed was equally important
(entry 8 and 9). The importance of boron–interelement reagents
was illustrated by the lack of reactivity found when employing
either Et3SiH,[17] (Et3Si)2 or B2Pin2, respectively (entry 11 and 12).
presence of nitriles (3l), carbazoles (3n) or esters (3u) could all
be well-accommodated. Interestingly, aryl halides (3c, 3d, 3g, 3h
and 3m),[18] sulfonates 3j), ethers (3f, 3i, 3k and 3p) or sulfides
(3o) – all common counterparts in the cross-coupling arena –[19]
did not interfere with our targeted 1,1-silaboration, hence opening
up an orthogonal gateway for further derivatization via
conventional transition metal-catalyzed reactions. Particularly
noteworthy was the observation that the 1,1-silaboration of 1a
could be executed on a gram scale, delivering 3a in 81% yield
without noticeable erosion in yield or diastereoselectivity. The
ability to obtain 3q and 3r as single products is particularly
noteworthy if one takes into consideration the known proclivity of
silylboranes to enable C2– or C4–silylation of electron-poor
azines under basic conditions via nucleophilic aromatic
substitution (SNAr) pathways.[11a] Substrates bearing more than
one terminal alkyne can either be selectively (3s) or exhaustively
coupled with silylborane (3t) by carefully adjusting the
stoichiometry of the reaction. As shown for 3u, the reaction can
be applied to non-aromatic acetylenes, yet with significant lower
selectivity, suggesting that allene-type intermediates might come
into play in certain cases (see below).[20]
Ha
BPin
SiEt3
KHMDS (20 mol%)
DME, 70 ºC
Ha
+
R3SiBPin
2a-b
Ar1
Ar2
Me4NF
Ar2CHO
as Scheme 2
(entry 1)
1a-u
3a-u
Ph
Ph
THF, 60 ºC
Ar2=p-CF3C6H4
92% yield
1a
O
R
6, 74%
BPin
[Si]
BPin
SiEt3
R
BPin
SiEt3
Ph
Ar1
Pd(Ph3)4 (5 mol%)
KOH(aq)
Ar1
SiEt3
TBAF, THF
or
3a, [Si] = SiEt3, 92%, 81%[a] 3c, R = F, 80%,
3e, R = Me, 84%
3f, R = OCF2H, 89%
Ph
Ph
3d, R = Cl, 83%
3a
X
3b, [Si] = SiMe2Ph, 87%
Ar1I, dioxane
NBS, MeCN
5
7 (X=H), 79%
8 (X=Br), 80%
BPin
Ar1=p-OMeC6H4
88% yield
SiEt3
R
O
3k, R =
, 65%
Rh(acac)(CO)2
(6 mol%)
dppb (6 mol%)
Ph
O
SiEt3
PdCl2(MeCN)2
(5 mol%)
CuCl2, DCE, 80 ºC
3g, R = F, 70%
Ar1
O
3h, R = Br, 89%[b]
3i, R = OCF3, 79%
3j, R = OTs, 92%
S
3l, R = CN, 85%
3m, R = Cl, 93%[b]
3n, R = N-carbazole, 80%[b]
BPin
3m
Me
O
Ph
9, 62%
4, 77%
Z:E = 5:1
Me
Me
S
BPin
Me
BPin
SiEt3
Me
Me
SiEt3
Scheme 4. Synthetic Applicability.
Z
SiEt3
Y
3q (Y = H; Z = N), 89%
3r (Y = N; Z =H), 85%
S
MeO
3o, 84%
3p, 85%
The stereoselective synthesis of densely functionalized olefins –
fundamental motifs in pharmaceuticals, material sciences and
liquid crystals –,[21] continues to pose a challenge in preparative
organic chemistry.[22] Driven by this observation, we wondered
whether our 1,1-silaboration platform could be used for such
purpose by selectively functionalizing two distinguishable
organometallic C–Si and C–B bonds at later stages.[23] As shown
in Scheme 4, this was indeed the case. Specifically, 4 and 5 could
easily be obtained from 3a via Pd-catalyzed Suzuki-Miyaura
coupling with an aryl iodide[24] or Rh-catalyzed 1,4-addition with
a,b-unsaturated ketones.[25] The versatility of the triethylsilyl group
as a masked nucleophile is evident by the results compiled in
Scheme 4 (right pathways). Indeed, proto– or bromodesilylation
could easily be accomplished from 5 with TBAF (7) or NBS (8)
whereas sp3–sp3 bonds can easily be forged with Me4NF and an
appropriate aldehyde counterpart (6).[7a] The conversion of 1a into
9 via 5 is particularly noteworthy, allowing to incorporate three
different aryl groups via b-selective C–H functionalization with Pd
catalysts and Cu(II) as oxidant.[26] Taken together, the results
Et3Si
BPin
BPin
SiEt3
Bpin
EtO
BPin
SiEt3
SiEt3
O
SiEt3
Bpin
3s, 82%
3t, 86%[c]
3u, 81% (E:Z/3:1)
H
Scheme 3. Scope of terminal alkyne. Reaction conditions: As scheme 2, entry
1; Yield of Isolated product, average of at least two independent runs. [a] 10
mmol scale. [b] T = rt. [c] 2a (3.5 equiv).
With a reliable set of conditions in hand, we focused our attention
on studying the generality of our method (Scheme 3). As evident
from the results compiled in Scheme 3, the 1,1-silaboration turned
out to be widely applicable for a wide range of arene substituents
at the alkyne terminus. Moreover, 3b was within reach in an
otherwise identical yield to that shown for 3a. Notably, the
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10.1002/anie.201913544
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COMMUNICATION
shown in Scheme 4 stand as a testament to the prospective
impact of our base-catalyzed 1,1-silaboration of terminal alkynes.
attenuating the reactivity of I while forming the targeted 3v-3w in
a highly stereoselective fashion.
deuterium labelling experiments
solvent-dependent stereoselectivity pattern
D
H
as Scheme 2
(entry 1)
as Scheme 2
(entry 1)
X
single isomers
E/Z mixtures
BPin
SiEt3
BPin
SiEt3
KHMDS
(20 mol%)
KHMDS
(20 mol%)
H
H
Ph
Ph
H
Ar
X = D
(96%D)
X = H
in d8-Tol
BPin
BPin
+
Ar
Ar
Ph
3a-D1, 89%
93%D
3a, 92%
<5% D
DME, 70 ºC
PhMe, 70 ºC
2a
SiEt3
SiEt3
1v (Ar =p-COMeC6H4)
1w (Ar =p-CONMe2C6H4)
3v:3v’, 79% (2:1)
3w:3w’, 83% (2:1)
3v, 80%
3w, 85%
1a-D1
Et3SiBpin (1 equiv)
KHMDS (20 mol%)
D(H)
D(H)
+
BPin
BPin
involvement of allene-type intermediates
+
Ph
Ar
Ar
H
DME, 70 ºC
via
H
SiEt3
SiEt3
3a-D1
24% (50% D)
favoring a concerted [1,2]-shift/deprotonation pathway
SiEt3
3j-D1
40% (36% D)
as Scheme 2
(entry 1)
1j
(1a-D1:1j=1:1)
Ar = p-TsO-C6H4
F
F
SiEt3
Ar1 = p-CF3C6H4
Ar2 = p-CF2HC6H4
BPin
Ar1
Ar2
F
“H”
II
1x
11, 61%
BPin
PhMe2Si
H
BPin
PhSiMe2Li
BPin
X
Scheme 6. Solvent-dependent selectivity and intermediacy of allene species.
Ar
DME/THF
-78 ºC to 70 ºC
no reaction
SiMe2Ph
Ph
via
I
“H”
Ph
3b
10
In summary, we have documented an unconventional atom-
economical 1,1-silaboration of terminal alkynes enabled by
catalytic amounts of KHMDS. This protocol is characterized by its
excellent stereoselectivity pattern, constituting a complementary
approach to existing methods en route to geminal organometallic
reagents that typically require transition metal complexes and a
new gateway to rapidly and reliably convert simple alkynes into
stereodefined trisubstituted alkenes via orthogonal C–Si and C–
B bond-cleavage. Further extensions to other related processes
are currently underway in our laboratories.
Scheme 5. Deuterium labelling experiments.
Next, we turned our attention to unravelling the mechanistic
intricacies of our 1,1-silaboration event via deuterium-labelling
experiments (Scheme 5, top). As shown, reliable deuterium
transfer en route to 3a-D1 was only observed if 1a-D1 was used
as substrate (top left), arguing against radical-type scenarios via
hydrogen atom transfer (HAT) with solvents possessing weak C–
H bonds such as toluene (top right).[27] Notably, a deuterium-
labelling crossover experiment with 1a-D1 and 1j revealed a non-
negligible H/D scrambling in the 1,1-silaboration products 3a and
3j. These experiments suggested that our 1,1-difunctionalization
might operate via initial deprotonation of the acetylenic sp C–H
bond (pKa (PhCºC–H) = 23) by KHMDS (pKa = 27) followed by
addition to Et3SiBPin. The resulting silylboronate species (I,
Scheme 1) might evolve via a concerted pathway consisting of a
[1,2]-shift[28] followed by a downhill protonolysis of an in situ
generated vinyl organometallic reagent (pKa » 40-45) with the
acetylenic sp C–H bond, thus turning over the key potassium
acetylide species.[29-31] This notion could be corroborated by the
lack of reactivity found when exposing 10 to PhSiMe2Li (bottom).
While our available data suggested a stereoselective [1,2]-shift of
in situ generated I en route to Z-configured 1,1-silylborylated
alkenes (3a-3u, Scheme 3), care should be taken when
generalizing this. Indeed, we found an intriguing dichotomy
exerted by both the substituents on the arene and the solvent
Acknowledgements
We thank ICIQ and FEDER/MICIU – AEI/PGC2018-096839-B-
100 for financial support. Y. G. and Y. S. thank Severo and China
Scholarship Council (CSC) for predoctoral fellowships.
Keywords: alkyne • 1,1-difunctionalization • silaboration • atom-
economy
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This article is protected by copyright. All rights reserved.

10.1002/anie.201913544
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Yiting Gu, Yaya Duan, Yangyang Shen,
Ruben Martin*
R3Si
stereoselective
[1,2]-shift
H
R3SiBPin
BPin
R3Si
BPin
H
base
X
&
protonation
H
“H”
Page No. – Page No.
no transition metal
concerted C–Si &
C–H bond-formation
mild & atom-economical
exquisite stereoselectivity
Stereoselective Base-Catalyzed 1,1-
Silaboration of Terminal Alkynes
A base-catalyzed protocol that enables an atom-economical 1,1-silaboration of
terminal alkyne is described. This protocol is distinguished by its mild conditions
and exquisite stereoselectivity pattern, offering a complementary approach to
conventional 1,2- or 1,1-difunctionalization techniques that typically require
transition metal complexes or stoichiometric organometallics, and an opportunity to
build up molecular complexity by subsequent orthogonal C–Si and C–B cleavage.
This article is protected by copyright. All rights reserved.