Z-Selective Hydrostannylation of Terminal and Internal C-C Triple Bonds Initiated by the Trityl Cation

Article abstract of DOI:10.1021/acs.organomet.8b00409

A metal-free method for the anti-addition of n-Bu₃SnH across terminal and internal alkynes, including related α,β-unsaturated carboxyl compounds, is reported. The reaction is initiated by the trityl salt [Ph₃C]⁺[B(C₆F₅)₄]^- and proceeds through β-tin-stabilized vinyl cations. Their reduction by hydride transfer from n-Bu₃SnH explains the high Z-selectivity in the formation of the vinyl stannanes.

Full text of DOI:10.1021/acs.organomet.8b00409

Communication
Cite This: Organometallics XXXX, XXX, XXX−XXX
Z‑Selective Hydrostannylation of Terminal and Internal C−C Triple
Bonds Initiated by the Trityl Cation
́
́
Francis Forster, Victoria M. Rendon Lopez, and Martin Oestreich*
Institut fur Chemie, Technische Universitat Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A metal-free method for the anti-addition of n-
Bu3SnH across terminal and internal alkynes, including related
α,β-unsaturated carboxyl compounds, is reported. The
reaction is initiated by the trityl salt [Ph3C]+[B(C6F5)4]−
and proceeds through β-tin-stabilized vinyl cations. Their
reduction by hydride transfer from n-Bu3SnH explains the
high Z-selectivity in the formation of the vinyl stannanes.
n R3Sn group attached to an alkene is a versatile linchpin
Scheme 1. Competing Reaction Pathways: Dehydrogenative
Stannylation versus Hydrostannylation [Ar = 3,5-
(CF3)2C6H3 or C6F5]
A
in synthetic chemistry, e.g., in Stille-type cross-coupling
reactions.1 Methods for the stereoselective preparation of these
vinyl stannanes have therefore always been in demand. Known
procedures are diverse, and a direct way of their formation is
by the addition of hydrostannanes R3SnH across alkynes.2
These hydrostannylations can be either catalyzed by transition
metals3 or initiated by radical starters,4 often affording the vinyl
stannane with high E-selectivity despite their different
mechanisms. Conversely, hydrostannylations promoted by
Lewis acids arrive at Z-configuration of the vinyl stannane.5
We recently disclosed a dehydrogenative coupling of various
terminal alkynes I and n-Bu3SnH catalyzed by tethered Ru−S
complexes (I → III, Scheme 1, top).6 This transformation
proceeds through the intermediacy of bridged β-tin-stabilized
vinyl cations7 II (gray box, Scheme 1, middle). Intermediates
II emerge from I and the stannylium-ion-like tin electrophile
[n-Bu3Sn−S]+ that, in turn, is generated by cooperative Sn−H
bond activation at the Ru−S bond.8 The chemoselectivity of
the reaction is determined at the stage of II: deprotonation II
→ III (left) versus hydride reduction II → IV (right). The
neutral Ru−H formed is not sufficiently hydridic,9 but the
sulfur atom is able to abstract the proton from II, resulting in
the formation of alkynyl stannane III (Scheme 1, top). In the
light of this insight, we imagined that the chemoselectivity
could be reversed in the absence of this internal Brønsted base,
thereby forcing n-Bu3SnH into the role of the hydride source4c
and at the same time regenerating the tin electrophile for the
next formation of II from I. The trityl cation could fulfill this
purpose as it is typically employed for the generation of
stannylium ions by hydride abstraction,10 and the resulting
triphenylmethane would not participate further in reaction.
Consequently, the trityl cation could initiate the hydro-
stannylation of alkynes (I → IV, Scheme 1, bottom). Herein,
we report the Z-selective hydrostannylation of a broad range of
terminal and internal alkynes involving cationic intermedi-
ates.11
Initial experiments using the readily available trityl salt
[Ph3C]+[B(C6F5)4]− as initiator for the hydrostannylation of
phenyl acetylene (1a) furnished desired vinyl stannane Z-2a in
complex mixtures. A systematic screening of reaction
conditions revealed a dramatic influence of the solvent and
reaction time on the outcome (Table 1). Reactions performed
in CH2Cl2 largely yielded E-2a and substantial amounts n-
Bu4Sn; however, Z-to-E isomerization and decomposition
1
occurred at prolonged reaction times (entries 1−3). H and
Received: June 13, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00409
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Table 1. Reaction Optimization: The Crucial Role of
Solvent and Reaction Time
Table 2. Hydrostannylation of Phenyl Acetylene Derivatives
Initiated by the Trityl Cation
a
a
b
b
c
entry
solvent
T (°C)
t
Z-2/E-2/n-Bu4Sn
entry
alkyne 1
T (°C) t (h)
Z/E
yield of 2 (%)
d
1
2
3
4
5
6
7
8
9
10
CH2Cl2
CH2Cl2
CH2Cl2
PhH
PhF
PhCl
n-pentane
n-pentane
n-pentane
n-pentane
RT
RT
RT
RT
RT
RT
RT
RT
RT
0
5 min
1 h
20 h
1 h
1 h
1 h
5 min
1 h
48 h
24 h
15:51:34
0:54:46
0:52:47
0:1:99
0:42:58
6:38:56
95:0:5
89:0:11
0:47:53
>95:0:<5
1
2
3
4
5
6
7
8
1a (X = H)
1b (X = 4-F)
0
0
0
0
0
RT
60
0
24
2
2
2
2
1
24
2
2
1
>95:5
92:8
88:12
89:11
87:13
>95:5
96 (2a)
e
79 (2b)
74 (2c)
1c (X = 4-CF3)
1d (X = 4-Cl)
1e (X = 4-Br)
1f (X = 4-OMe)
1g (X = 4-NMe2)
1h (X = 4-Ph)
1i (X = 4-Me)
1j (X = 3-Me)
1k (X = 2-Me)
d
77 (2d)
d
74 (2e)
86 (2f)
f
g
(2g)
>95:5
>95:5
91:9
78 (2h)
78 (2i)
9
10
11
0
RT
0
d
95 (2j)
h
a
2
>95:5
82 (2k)
All reactions were performed according to general procedure 1 or 2
b
a
(see the Supporting Information for details). Ratios were estimated
by 119Sn{1H} NMR analysis. Full conversion of n-Bu3SnH was
observed in all reactions.
All reactions were performed according to general procedure 1 or 2
b
(see the Supporting Information for details). Ratios were determined
c
by 1H NMR analysis. Isolated yield after filtration over a plug of basic
d
e
Al2O3. Along with trace amounts of n-Bu4Sn. E/Z = 75:25 at RT for
1 h. Reaction performed in n-hexane instead of n-pentane. Only low
conversion of n-Bu3SnH was observed. E/Z = 80:20 at RT for 1 h.
f
g
119Sn NMR analysis showed full conversion of n-Bu3SnH after
5 min, and the 15% of Z-2a present after this short reaction
time then quickly disappeared. The same trend was seen in
aromatic solvents such as benzene, fluorobenzene, and
chlorobenzene (entries 4−6). This situation changed drasti-
cally with the use of n-pentane as solvent: Almost exclusive
formation of Z-2a was observed at short reaction times, and
isomerization to E-2a along with generation of n-Bu4Sn was
slowed down (entries 7−9). Lowering the reaction temper-
ature to 0 °C gave Z-2a as the only product (entry 10).
Having identified the optimized setup, we explored the
scope for phenyl acetylene derivatives (1a−k → Z-2a−k, Table
2). Regardless of the electronic property of the aryl group in
1a−i, almost all vinyl stannanes Z-2a−i were formed with high
control of the double bond geometry and in good to excellent
yields (entries 1−9). To secure high diastereoselectivities, the
hydrostannylation of electron-deficient 1b−e had to be run at
0 °C rather than RT (entries 2−5; see Table 2, footnote e).
Importantly, the formation of n-Bu4Sn was suppressed at that
temperature. Electron-rich 1f with an MeO group converted
smoothly into Z-2f, whereas Me2N-substituted 1g did not
afford Z-2g even at 60 °C (entries 6 and 7). Steric hindrance
was tolerated as verified for the three isomeric tolyl-substituted
acetylenes (1i−k → Z-2i−k, entries 9−11).
Next, we turned our attention to alkyl-substituted terminal
alkynes (1l−p → Z-2l−p, Table 3). Depending on the R
group, we observed the formation of alkynyl stannane 3 as the
product of dehydrogenative coupling. Linear hex-1-yne (1l)
gave only low conversion at RT but a higher reaction
temperature only led to a complex mixture (entry 1). In
turn, benzyl- and cyclopropyl-substituted 1m and 1n both
formed vinyl stannanes Z-2m and Z-2n chemoselectively
(entries 2 and 3). In contrast, cyclopentyl-substituted 1o
reacted to an almost equimolar mixture of Z-2o and 3o, and
that ratio could not be improved at lower temperatures due to
insufficient conversion (entry 4). The same applied to another
branched substrate, cyclohexyl-substituted 1p, although the
effect was less pronounced (entry 5). It seems that branching
h
in the propargylic position hampers the hydride transfer from
n-Bu3SnH to the β-tin-stabilized vinyl cation, turning proton
release or even protonation of n-Bu3SnH into a viable
alternative. Triisopropylsilyl-protected propargylic alcohol 1q
was afflicted with the same problem (entry 6). In addition to
that, there was no regiocontrol, and Z-2q and α-2q (not
shown) formed in almost equimolar ratio. Propiolic acid
methyl ester reacted cleanly but with opposite regioselectivity
(1r → α-2r):
Application of the standard protocol to internal alkynes and
related α,β-unsaturated carboxyl compounds12 was largely
satisfactory (Figure 1). No reaction was found for tolan despite
full conversion of n-Bu3SnH but this merely led to the
formation of n-Bu4Sn (not shown). However, the hydro-
stannylation of 1-phenylprop-1-yne (1s) yielded corresponding
vinyl stannane Z-2s diastereoselectively and with excellent
regioselectivity. An internal aliphatic alkyne was far less
reactive; 1t transformed slowly into Z-2t, barely reaching
50% conversion after 24 h at 60 °C. Silylated alkynes such as
1u generally resulted in complex reaction mixtures. Alkynones
1v and 1w afforded Z-2v and Z-2w with superb control of the
alkene geometry.
To conclude, we have introduced here a straightforward
method for the Z-selective preparation of vinyl stannanes by
alkyne hydrostannylation. The reaction is simply initiated by
catalytic amounts of the trityl cation, that is by its hydride
abstraction from the hydrostannane to formally generate a
stannylium ion. The hydrostannane adds anti across the C−C
B
DOI: 10.1021/acs.organomet.8b00409
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
a
Table 3. Hydrostannylation of Alkyl-Substituted Terminal Alkynes Initiated by the Trityl Cation
b
c
entry
alkyne 1
T (°C)
t (h)
Z-2/3
yield of 2 (%)
de
f
,
1
2
3
4
5
6
1l (R = n-Bu)
1m (R = Bn)
1n (R = cProp)
1o (R = cPent)
1p (R = Cy)
60
0
24
1/2
1
18
24
24
(2l)
>95:5
>95:5
55:45
91:9
76 (2m)
96 (2n)
83 (2o+3o)
96 (2p+3p)
51 (2q+3q)
RT
RT
RT
RT
d
g
1q (R = CH2OTIPS)
97:3
a
b
All reactions were performed according to general procedure 1 or 2 (see the Supporting Information for details). Ratios were estimated by
c
d
119Sn{1H} NMR analysis. Isolated yields after filtration over a plug of basic Al2O3. Only low conversion of n-Bu3SnH was observed at 0 °C and
RT, respectively. Reaction performed in n-hexane instead of n-pentane. Complex mixture was observed. Combined yield of Z-2q (24%) and α-
isomer α-2q (27%); the ratio of these regioisomers was nearly 1:1 prior to their separation by flash chromatography on silica gel.
e
f
g
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Cluster of Excellence
Unifying Concepts in Catalysis of the Deutsche Forschungsge-
meinschaft (EXC 314/2). V.M.R.L. (on leave from the
University of Guanajuato, Mexico) thanks the Consejo
́
Nacional de Ciencia y Tecnologia for a predoctoral fellowship
(No. 295157). M.O. is indebted to the Einstein Foundation
Berlin for an endowed professorship.
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* Supporting Information
■
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
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ORCID
Francis Forster: 0000-0002-0364-5491
́
́
Victoria M. Rendon Lopez: 0000-0002-0056-5787
Martin Oestreich: 0000-0002-1487-9218
C
DOI: 10.1021/acs.organomet.8b00409
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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DOI: 10.1021/acs.organomet.8b00409
Organometallics XXXX, XXX, XXX−XXX