Polarity-reversal-catalyzed hydrostannylation reactions: Benzeneselenol-mediated homolytic hydrostannylation of electron-rich olefins

Article abstract of DOI:10.1002/hlca.200690214

Addition of 10 mol-% of diphenyl diselenide to hydrostannylation reactions involving electron-rich olefins results in a dramatic improvement in yield. For example, reaction of α-([(tert-butyl)dimethylsilyl]-oxy}styrene (1) with triphenylstannane (2a; 1.1

Full text of DOI:10.1002/hlca.200690214

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Polarity-Reversal-Catalyzed Hydrostannylation Reactions: Benzeneselenol-
Mediated Homolytic Hydrostannylation of Electron-Rich Olefins
by Leigh Ford, Uta Wille, and Carl H. Schiesser*
School of Chemistry, The University of Melbourne, Victoria, 3010, Australia
and
Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Victoria, 3010,
Australia
(e-mail: carlhs@unimelb.edu.au)
Dedicated to the memory of Professor Hanns Fischer. We take for granted his many contributions to the
field of radical chemistry. I am particularly indebted to his careful and patient explaining to me of the
ꢀpersistent radical effectꢁ over a drink, on a balmy New Hampshire evening.
Addition of 10 mol-% of diphenyl diselenide to hydrostannylation reactions involving electron-rich
olefins results in a dramatic improvement in yield. For example, reaction of a-{[(tert-butyl)dimethylsilyl]-
oxy}styrene (1) with triphenylstannane (2a; 1.1 equiv.) in the presence of PhSeSePh and 2,2’-azobis[2-
methylpropanenitrile] (AIBN) affords {2-{[(tert-butyl)dimethylsilyl]oxy}-2-phenylethyl}triphenylstan-
nane (3a) in 95% yield after 2h. This reaction presumably benefits, by the increased rate of H-atom
transfer, from the in situ generated polarity-reversal catalyst, benzeneselenol.
Introduction. – Intermolecular free-radical additions of stannyl radicals to multiple
bonds have emerged as important methods for the preparation of tetraorganylstan-
nanes which can be reacted further to afford new CÀC bonds through a variety of tran-
sition-metal-mediated coupling processes [1]. As part of an ongoing research program
exploring new stereoselective free-radical processes [2], we were interested in the
hydrostannylation of silyl enol ethers with chiral trialkylstannanes.
Previous work suggested to us that free-radical hydrostannylation reactions involv-
ing trialkylstannanes proceeded most efficiently with electron-deficient olefins [3], with
few examples of olefins bearing electron-donating groups providing synthetically useful
outcomes [4]. This guiding principle was indeed found to operate for reactions involv-
ing a-{[(tert-butyl)dimethylsilyl]oxy}styrene (={1-{[(tert-butyl)dimethylsilyl]oxy}ethe-
nyl}benzene; 1). Overnight reaction of 1 with triphenylstannane (2a; 1.1 equiv.) in
the presence of 2,2’-azobis[2-methylpropanenitrile] (AIBN) at 808 and in the absence
of solvent afforded a 20% yield of the desired stannane, {2-{[(tert-butyl)dimethylsilyl]-
oxy}-2-phenylethyl}triphenylstannane (3a), while the sterically more demanding chiral
stannane tri[(1R,3R,4S)-menthyl]stannane (=tris[(1R,2S,5R)-2-(1-methylethyl)-5-
methylcyclohexyl]stannane; 2b) [5] returned only starting material after prolonged
ꢂ 2006 Verlag Helvetica Chimica Acta AG, Zürich

Helvetica Chimica Acta – Vol. 89 (2006)
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heating with 1 at 1408 in the absence of solvent, even when large excesses of stannane
2b (5 equiv.) were employed (Scheme 1)1).
Scheme 1
While steric factors are undoubtedly partly responsible for these outcomes, it is
quite likely that reversible fragmentation is also responsible for the problems that we
encountered in these systems. It is well established that hydrostannylation reactions
of medial alkenes can be problematic because the addition of the nucleophilic stannyl
radical to the alkene is a reversible process [4]. In other words, the rate of (reversible)
b-fragmentation of the adduct radical 4 is competitive with H-atom abstraction from
the stannane (Scheme 2).
Scheme 2
Roberts demonstrated that trialkylsilanes, in the presence of a catalytic amount of a
thiol, are capable of reducing alkyl halides and other precursors [6]. Dubbed ꢀpolarity-
reversal catalysisꢁ by Roberts, the success of this chemistry, has been attributed to favor-
able polar effects in the transition state for H-atom transfer from an S- to a C-centered
radical (e.g., 5) over the less favorable transition state (e.g., 6) and has been supported
by computational-chemistry techniques [7]. A similar catalytic phenomenon has been
described by Crich and co-workers for reactions involving stannanes catalyzed by ben-
zeneselenol, a technique that effectively extends the kinetic range of stannane-medi-
ated reactions [8]. It is interesting to note that Roberts showed that alkenes could be
easily hydrosilylated by using silane/thiol mixtures [9].
1
)
It is interesting to note that the analogous trimethylsilyl enol ether product 2-{[(trimethylsilyl)oxy]-2-
phenylethyl}triphenylstannane failed to survive either of these reaction conditions.

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Given this rich history, we were surprised to discover that, apart from one example
that utilized Bu3SnH/ArSH to double hydrostannylate terminal alkynes [10], polarity-
AHCTREUNG
reversal catalysis has, to the best of our knowledge, not been applied to hydrostanny-
lation chemistry of alkenes. We now report that hydrostannylation of olefins, especially
electron-rich alkenes can be significantly improved by the introduction of a polarity-
reversal catalyst (benzeneselenol) into the reaction mixture.
Results and Discussion. – We began this study by examining the reaction of a-
{[(tert-butyl)dimethylsilyl]oxy}styrene (1) with tributylstannane (2c) at 808 in the
absence of solvent (AIBN initiator). Benzeneselenol (PhSeH) was generated by the
in situ method described by Crich and co-workers [8], that is by the rapid ionic reaction
of diphenyl diselenide with 2c (Scheme 3). The outcomes of this initial investigation are
listed in Table 1.
Scheme 3
Table 1. Reaction of Tributylstannane (2c) with Enol Ether 1 in the Presence of Diphenyl Diselenide (see
text)
Entry
2ca)
PhSeSePha)
2c (effective)b)
Yield of 3cc) [%]
1
2
3
4
5
0.97
1.05
1.05
1.50
1.31
0.08
0.05
0.08
0.11
0.14
0.89
1.0
0.97
1.39
1.17
30
43
55
99
87
a) Equiv. relative to 1. b) Adjusted taking Scheme 3 into account. c) Isolated yield.
Inspection of Table 1 reveals that good yields are possible for the hydrostannylation
of 1 without the need for forcing conditions, with ca. 10% PhSeSePh and 1.2–1.4 effec-
tive equiv. of Bu3
N
N
phenylethyl}stannane (3c) approaching quantitative. It should be noted that in the
absence of added catalyst, no reaction was observed even when 1.5 equiv. of the Bu3-
A
fer from benzeneselenol to 4 [11], resulting in reduced competition from the reversible
fragmentation depicted in Scheme 2.
We next turned our attention to the reaction of substrates 1, and 7–14 with triphe-
nylstannane (2a). The results of experiments in which these substrates were treated
with 1.1 and 0.1 equiv. of 2a and PhSeSePh, respectively, in the presence of AIBN at
808 and in the absence of solvent are reported in Table 2.
The data presented in Table 2 clearly highlight the beneficial effect that addition of
diphenyl diselenide, through its action as the polarity-reversal catalyst, benzeneselenol,
has on the electron-rich olefins in this study. As expected, Entries 2–4 show that hydro-
stannylation reactions in which either electron-deficient (see 8 and 9) or unsubstituted
terminal (see 10) olefins are employed derive no benefit through the introduction of a
polarity-reversal catalyst. We were surprised to discover that the hydrostannylation of

Helvetica Chimica Acta – Vol. 89 (2006)
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Table 2. Reaction of Triphenylstannane (2a; 1.1 equiv.) with Substrates 1 and 7–14 at 808 (AIBN) in the
Absence of Solvent (see text)a)
Entry
Substrateb)
Yield of 3a or 15c)d) [%]
Yield of 3a or 15d)e) [%]
1
2
3
4
5
6
7
8
9
7
8
9
10
11
12
13
14
1
trace
95
94
96
9263
94
78
90
95
trace
98
97
93
77
44
71
20
a) All reactions were performed at 80Æ38 for 2.5 h under stirring. b) Equiv. of reagent given relative to
this substrate. c) 10 mol-% of PhSeSePh added. d) Isolated yield. e) No catalyst added.
styrene (11) afforded stannane 15 (R=H, Y=Ph) in moderate yield after 40 min,
together with an unidentified by-product that may well be oligomeric in nature. The
formation of this by-product is completely suppressed by the addition of 10 mol-%
of PhSeSePh to the reaction mixture, affording the desired hydrostannylated product
in 92% yield in under 10 min. It is interesting to note that attempts to hydrostannylate
styrene and diethenylbenzene can be inefficient [4], but can be assisted by the addition
of Lewis acid catalysts [12] or sonication [13].
Entries 6–10 demonstrate the beneficial effect that the addition of PhSeSePh has on
chemistry involving electron-rich olefins, with the most dramatic improvement
observed for reactions involving the (tert-butyl)dimethylsilyl ether 1, in which the
yield rose from 20% to 95% after 2.5 h when 10 mol-% of PhSeSePh was included.
It is interesting to note that the hydrostannylation of vinyl acetate with triisobutylstan-
nane has been reported to proceed in 92% yield after 22 h at 508 [14].
The typical procedure (see Exper. Part) involved addition of diphenyl diselenide
(0.1 equiv.) to a homogeneous mixture of triphenylstannane (2a; 1.1 equiv.), olefin 1
(1.0 equiv.), and a few crystals of AIBN under Ar, followed by heating to 808 for 2.5
h. The product 3a was isolated directly from the reaction mixture by flash chromatog-
raphy. This procedure deserves comment. In previous, uncatalyzed experiments in
which large excesses of stannane were employed, tedious workup conditions that
included several repeated chromatographic purification steps were necessary to obtain
a product of acceptable purity. The conditions described above are a significant

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Helvetica Chimica Acta – Vol. 89 (2006)
improvement in that high-purity product is obtained after a single purification step,
largely because the by-product PhSeSnPh3 behaves discreetly upon chromatography.
Finally, our inability to improve the outcome of the reaction involving cyclohexene
(Table 2, Entry 1) deserves comment. As previously discussed, medial alkenes are
hydrostannylated inefficiently, an outcome ascribed to competitive reversible fragmen-
tation of the adduct radical. We suggest that Ph3SnC undergoes competitive homolytic
G
substitution at the Se-atom in benzeneselenol with expulsion of phenyl radical, and it
is this chemistry that dominates to the exclusion of homolytic addition to cyclohexene.
While we were initially surprised that polarity-reversal catalysis was unable to dis-
rupt this competitive equilibrium, our hypothesis is supported by available rate-con-
stant data. While, to the best of our knowledge, no rate data exist for the addition of
triphenylstannyl radicals to cyclohexene, based on data available for related systems
[15], this radical is likely to add to cyclohexene with a rate constant several orders of
magnitude slower than those for the analogous reactions involving terminal olefins,
and with a rate constant about one order of magnitude slower than that for homolytic
substitution at the Br-atom in bromobenzene [16]. Crich and co-workers had noted that
ꢀthe catalytic species, PhSeH, itself is more rapidly cleaved by Bu3
midesꢁ [17].
In conclusion, we have demonstrated that homolytic hydrostannylation reactions of
A
electron-rich alkenes derive considerable benefit, through improved yield as well as
reaction and purification conditions, by inclusion of benzeneselenol as polarity-reversal
catalyst.
Generous support from the Australian Research Council through the Centres of Excellence Program
is gratefully acknowledged.
Experimental Part
{2-{[(tert-Butyl)dimethylsilyl]oxy}-2-phenylethyl}triphenylstannane (3a). Diphenyl diselenide (0.1
equiv.) was added to a homogeneous mixture of triphenylstannane (2a; 1.1 equiv.), 1 (1.0 equiv.), and
a few crystals of AIBN, under Ar. The resulting yellow soln. rapidly became colorless. The mixture
was heated for 2.5 h at 808 and then cooled. The resulting residue was purified by flash chromatography
(Scharlau silica gel 60 (230–400 mesh), hexanes/AcOEt (1% Et3N) 10 :1): 3a (95%). Colorless oil. IR
E
1
(neat): 3064, 2953, 1950, 1877, 1817, 1429, 1219, 1074, 1060. H-NMR (400 MHz, CDCl3): À0.77 (s, 3
H); À0.81 (s, 3 H); 0.74 (s, 9 H); 2.10 (dd, J=7.6, 12.8, 1 H); 2.19 (dd, J=13.2, 4.8, 1 H); 5.21 (dd,
J=7.2, 5.2, 1 H); 7.16–7.20 (m, 4 H); 7.27–7.41 (m, 15 H); 7.60–7.63 (m, 1 H). 13C-NMR (100 MHz,
CDCl3): À4.9; À4.7; 18.3; 25.9 (3 C); 26.6; 73.8; 126.0; 127.0; 128.2; 128.3 (6 C); 128.5; 137.0 (6 C);
139.4 (3 C); 146.6. 119Sn-NMR (150 MHz, CDCl3): 124.8 ((Bu3
N
N
C32H38OSiSn: C 65.65, H 6.54, Sn 20.28; found: C 65.68, H 6.53, Sn 20.35.
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Received April 27, 2006