Free radical hydrostannylation of unactivated alkenes with chiral trialkylstannanes

Article abstract of DOI:10.1021/om200473c

Free radical hydrostannylation of olefins with differing steric and electronic demands have been carried out using the chiral, nonracemic stannanes (1R,2S,5R)-menthyldiphenyltin hydride (7), bis[(1R,2S,5R)-menthyl]phenyltin hydride (8), and tris[(1R,2S,5R

Full text of DOI:10.1021/om200473c

ARTICLE
pubs.acs.org/Organometallics
Free Radical Hydrostannylation of Unactivated Alkenes with Chiral
Trialkylstannanes
Yvonne Kavanagh,†,‡ Leigh Ford,†,‡,§ and Carl H. Schiesser*,†,‡
†ARC Centre for Excellence for Free Radical Chemistry and Biotechnology and ‡School of Chemistry and Bio21 Molecular Science and
Biotechnology Institute, The University of Melbourne, Victoria 3010, Australia
ABSTRACT: Free radical hydrostannylation of olefins with differ-
ing steric and electronic demands have been carried out using the
chiral, nonracemic stannanes (1R,2S,5R)-menthyldiphenyltin
hydride (7), bis[(1R,2S,5R)-menthyl]phenyltin hydride (8), and
tris[(1R,2S,5R)-menthyl]tin hydride (9). These reactions resulted
in adducts 16ꢀ18 with yields that were found to depend on the
nature of the substituents on both alkene and stannane and could be carried out at low temperature initiated by triethylborane and
oxygen; MenPh2SnH (7) reacted with tert-butyldimethyl(1-phenylvinyloxy)silane (1) at ꢀ78 °C to afford adduct 19 in near-
quantitative yield. Somewhat surprisingly, addition of small quantities of diphenyl diselenide to these reactions failed to improve the
outcome under any circumstances.
’ INTRODUCTION
Roberts demonstrated that trialkylsilanes, in the presence of a
catalytic amount of a thiol, are capable of reducing alkyl halides
and other precursors.13 Now known as “polarity reversal cata-
lysis”, the success of this chemistry has been attributed to
favorable polar effects in the transition state for hydrogen atom
transfer from a sulfur- to a carbon-centered radical (e.g., 5) over
the less favorable transition state (e.g., 6), and this hypothesis has
been supported by computational chemistry;14 this technique
has been applied to the free radical hydrosilylation of alkenes.15,16
A similar catalytic phenomenon has been described by Crich for
reactions involving stannanes and benzeneselenol, a technique
that effectively extends the kinetic range of stannane-mediated
reactions.4 Importantly, benzeneselenol can be conveniently
prepared in situ by the reaction of diphenyl diselenide with
trialkyltin hydrides (Scheme 4).4
Computational chemistry suggests that polarity reversal cata-
lysis involving stannanes should be possible using a thiol as
catalyst.17 We were surprised that, given this history, we were
only able to locate one report that utililized Bu3SnH/ArSH to
hydrostannylate, and this chemistry involved terminal alkynes
such as phenylacetylene (Scheme 4).18
As part of an ongoing program of work aimed at developing
free radical methods for the preparation of chiral inter-
mediates,5,19ꢀ25 we have explored the use of chiral free radical
hydrostannylation chemistry to the synthesis of enantiopure
intermediates for use in the preparation of pharmaceutical
products. We now report that hydrostannylation of substituted
alkenes with (1R,2S,5R)-menthyldiphenyltin hydride25 (7), bis-
[(1R,2S,5R)-menthyl]phenyltin hydride25 (8), and tris[(1R,2S,
5R)-menthyl]tin hydride26 (9) proceeds in yields that depend on
the substituent on the alkene. Somewhat surprisingly, addition of
Single-enantiomer drug candidates are important to many
pharmaceutical companies, and as a consequence, new methods
for the preparation of enantiopure compounds are critical for the
development of next-generation pharmaceutical products.1
While motifs may vary, many of these candidates require chiral
benzylic alcohols as synthetic starting materials or intermediates;
a recent preparation of the antidepressant drug fluoxetine
(Prozac) is an example of this chemistry (Scheme 1).2 The
critical step in the example provided in Scheme 1 involves access
to ethyl (R)-3-hydroxy-3-phenylpropanoate, and this has tradi-
tionally been achieved through chiral resolution techniques2 or
through chiral reduction.3
We considered the possibility of carrying out asymmetric free
radical hydrostannylation chemistry on an appropriately con-
structed alkene to give a chiral building block suitable for further
elaboration (Scheme 2). To our disappointment, initial attempts
to hydrostannylate the enol ether 1 with tris[(1R,2S,5R)-
menthyl]tin hydride (tris[(1R,2S,5R)-2-(1-methylethyl)-5-
methylcyclohexyl]tin hydride) under radical conditions resulted
in returned starting material, even in the presence of benzene-
selenol as a polarity reversal catalyst.4,5 In contrast, triphenyl-
stannane afforded the product 2 in 20% yield and this could be
improved to 95% in the presence of benzeneselenol.5
Chiral free radical hydrostannylation reactions have been
reported on a number of occasions; these reactions proceed
most readily with electron-deficient alkenes, which is largely a
consequence of the nucleophilicty of the stannyl radical.6ꢀ12 For
example, Podestꢀa reported that reactions of (1R,2S,5R)-menthyl-
dimethylstannane with diphenylpropenoates and propeneni-
triles (e.g., 3) take place with high levels of diastereoselectivity
(Scheme 3),10 while the addition of achiral trialkylstannanes to
(1R,2S,5R)-menthyl crotonate (4) affords chiral tetraorganotin
compounds with moderate diastereoselectivity (Scheme 3).11,12
Received: June 1, 2011
Published: July 29, 2011
r
2011 American Chemical Society
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Scheme 1
Scheme 5
Scheme 2
Table 1. Reactions of Stannanes 7ꢀ 9 (1.0 equiv) with
Substrates 10ꢀ15 at 80 °C (AIBN) in the Absence of Solvent
or in 1.85 M Benzene Solutiona
entry
substrate
stannane
amt of 16ꢀ18, %b
119Sn NMR,c δ
1
10
7
8
9
7
8
9
7
8
9
7
8
9
7
8
9
7
8
9
95 (52)
93 (67)
99 (62)
94d (65)
n.r.e
n.r.e
73d (35)
n.r.e
ꢀ89.9
ꢀ79.3
ꢀ70.3
ꢀ92.5
2
Scheme 3
3
4
11
12
13
14
15
5
6
7
ꢀ97.0
8
9
n.r.e
10
11
12
13
14
15
15
17
18
99 (84)
99 (70)
97 (22)
93 (79)
88 (33)
n.r.e
n.r.e
n.r.e
n.r.e
ꢀ91.8
ꢀ79.3
ꢀ72.6
ꢀ94.5/ꢀ94.7f
ꢀ98.7/ꢀ98.8f
Scheme 4
a See text. All reaction mixtures were stirred at 80 ( 3 °C for 2.5 h. b 1
H
c
NMR estimated conversions (isolated yields in parentheses). CDCl3.
d Conversion based on unreacted stannane. e No reaction. f 1:1 mixture
of diastereoisomers.
treated with 1.0 equiv of 7 at 80 °C in the absence of solvent
(AIBN initiation), for 2.5 h. Due to viscosity issues, reactions
involving 8 or 9 were carried out in benzene at 1.85 M
concentration. All reactions were carried out under an atmo-
sphere of argon. Percentage conversions were estimated from the
1H NMR spectrum of each crude reaction mixture before
chromatographic separation yielded the tetraorganotin products
(16ꢀ18). The results of these studies are summarized in Table 1.
Inspection of Table 1 reveals that the substrates are divided
into two classes: those that add normally to substrates 10ꢀ15
and those that will not add. In each reacting substrate the
conversions are observed to be high, with isolated yields often
somewhat lower, reflecting difficulties in product isolation. The
unreacting substrates typically contain electron-donating groups
that are not well matched with the nucleophilic properties of the
incipient stannyl radical.5 It is particularly noteworthy that the
more sterically incumbered stannanes (8, 9) are more sensitive to
catalytic amounts of diphenyl diselenide appears to have little
effect on the outcomes of these reactions.
’ RESULTS AND DISCUSSION
Reactions of Chiral Stannanes with a Variety of Sub-
strates. Despite the results depicted in Scheme 3, Podestꢀa
reported that some menthyl-substituted stannanes fail to react
with methyl 1,2-diphenylacrylate under radical conditions,7 and
this was of some concern to us. We therefore chose to begin this
study by exploring the hydrostannylation chemistry of chiral
stannanes (7ꢀ9) with a variety of substrates (10ꢀ15) under the
“standard” radical hydrostannylation conditions reported pre-
viously by us (Scheme 5).5 To that end, each substrate was
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electronic demand and steric environment of the alkene. For
example, while the monomenthyl-substituted stannane 7
reacts well with all substrates with the exception of vinyl
acetate (15; entry 16), the dimenthylstannane 8 begins to
exhibit substrate selectivity in failing to react with the mildly
electron-rich alkene 1-heptene (11; entry 5) and the remain-
ing electron-rich systems (12 and 15, entries 8 and 17). On the
other hand, trimenthylstannane (9) reacts only with the
sterically unencumbered alkenes that are also electron-deficient
(10 and 13, entries 3 and 12). This phenomenon of bulky
stanannes performing either well or not at all has been reported
previously by Podestꢀa.7,27
It it interesting to note (and somewhat disappointing) that the
prochiral alkene methyl methacrylate failed to show any stereo-
selectivity in its reaction with either 7 or 8; products 16 and 17
(R = CO2Me; R0 = Me) were isolated as a 1:1 mixture of
diastereoisomers, as evidenced by NMR spectroscopy (entries 13
and 14).
In order to improve on these reaction conditions, and in an
attempt to “encourage” the unreactive systems to undergo
hydrostannylation, we repeated each of the reactions listed in
Table 1 in the presence of catalytic amounts of diphenyl
diselenide as described in our earlier publication.5 Unfortunately,
under no circumstance did we observe an improved outcome
and, more importantly, the unreactive alkenes still failed to react
under these conditions.
Scheme 6
Scheme 7
Scheme 8
Reactions of Chiral Stannanes with tert-Butyldimethylsi-
lyloxystyrene (1). We next turned our attention to the reactions
of the silyl enol ether 1 with chiral stannanes 7ꢀ9. We had
previously shown that 1 fails to react with tris[(1R,2S,5R)-
menthyl]tin hydride (9) under “standard” or “polarity reversal
catalyzed” conditions.5 It came as no surprise, therefore, that
bis[(1R,2S,5R)-menthyl]phenyltin hydride (8) also failed to
react under these conditions (Scheme 6).
To our (unexpected) delight, (1R,2S,5R)-menthyldiphenyltin
hydride (7) did react with 1 under the conditions detailed
in Table 1 (80°, neat, AIBN) to afford the adduct 19 in low
yield; however, when the amount of stannane was increased to
3 equiv, 19 was isolated in 93% yield. Interestingly, as was
observed for 14 (Table 1, entry 13), this reaction proceeded
without stereoselectivity.
The reactivities of stannanes 7ꢀ9 toward the enol ether 1 can
be attributed to steric congestion resulting from the interactions
of (multiple) menthyl substituents on the reagent with the bulky
tert-butyldimethylsilyl group on the oxygen atom in 1. This
congestion disfavors the equilibrium to form adduct radical 21
(Scheme 7), and as a consequence, high concentrations and
multiple equivalents of 7 are required for a successful outcome.
For the remaining chiral reagents, these unfavorable interactions
would appear to be too strong to be overcome through adjust-
ment of these parameters.
Table 2. Reactions of Stannanes Ph3SnH and 7 (2.0 equiv)
with Substrates 1 and 10 — 15 at Room Temperature Initiated
by Et3B/O2 (See Text)a
entry substrate stannane amt of 16, 19, or 20, %b 119Sn NMRc5
1
1
Ph3SnH
>99e (99) [20]
>99e(99)
>99e [97]
>99e
ꢀ124.8
ꢀ94.5/ꢀ94.6d
ꢀ109.4
ꢀ89.9
2
7
3
10
11
12
13
14
15
Ph3SnH
4
7
5
Ph3SnH
77 [93]
ꢀ110.3
ꢀ92.5
6
7
83
7
Ph3SnH
70 {77}f
98
>99e [63]
>99e
>99e {56}f
>99e
>99e [44]
n.r.g
ꢀ112.3
ꢀ97.0
ꢀ111.1
ꢀ91.8
ꢀ117.2
ꢀ94.5/ꢀ94.7d
ꢀ119.8
8
7
9
Ph3SnH
10
11
12
13
14
7
Ph3SnH
7
Ph3SnH
7
a See text. All reaction mixtures were kept at 23 °C for 3 h. b 1H NMR
estimated conversions: isolated yields are given in parentheses and
isolated yields from ref 5 in brackets (see text). c CDCl3. d 1:1 mixture of
diastereoisomers. e Quantitative. f Reference 31. g No reaction.
Low-Temperature Hydrostannylations. In an attempt to
induce stereoselectivity in this reaction, we considered carrying
out the radical hydrostannylation at reduced temperatures. While
there are a number of methods for initiating radical reactions at
low temperature,28 the method of choice from our perspective
involves the use of triethylborane and oxygen because of its ease
of application.29 To the best of our knowledge, low-temperature
radical hydrostannylation of alkenes employing this method of
initiation has not been reported previously; however, Oshima
reported the use of triethylborane as an initiator for the hydro-
stannylation of alkynes.30 As a consequence, we chose to explore
the feasibility of this chemistry using a model stannane: namely,
triphenyltin hydride.
To our delight, reaction of all substrates 1 and 10ꢀ15 with 2
equiv of triphenyltin hydride in cyclohexane (1 M), initiated by
Et3B/O2, proceeded with excellent conversion at room tempera-
ture (23 °C) to afford stannanes 20 (Scheme 8, Table 2). It
should be noted that even isoprenyl acetate (15) reacts with
Ph3SnH in quantitative yield under these conditions (entry 13).
These data are to be compared with the yields reported pre-
viously for the analogous reaction carried out under “standard”
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conditions (AIBN/80 °C);5,31 this comparison reveals that the
Et3B/O2 initiated reactions appear, on face value, to provide
superior outcomes (Table 2).
When these reactions were repeated with (1R,2S,5R)-
menthyldiphenyltin hydride (7), with the exception of 15, which
failed to react, high conversions to stannanes 16 and 19 were
observed by 1H NMR spectroscopy of the crude reaction
mixtures. In the case of the silylated substrate 1, the required
product 19 was isolated in 99% yield; unfortunately, no diaster-
eoselectivity was observed.
In a further attempt to induce diastereoselectivity, the reaction
of 1 with 7 was repeated at ꢀ78 °C; in this case a reaction time of
5 h was required. To our delight, high conversion to the required
product was observed under these conditions, with 19 isolated in
99% yield. Unfortunately, only moderate (1.3:1) diastereoselec-
tivity was observed and the diastereoisomers of 19 could not be
separated by chromatography.
General Procedure A for Hydrostannylation Reactions at 80 °C. A
few crystals of AIBN were added to a homogeneous mixture of stannane
(1.0 equiv) and olefin (1.0 equiv) in the absence of solvent (7 or
Ph3SnH) or in benzene (1.85 M, 8 or 9) under an inert atmosphere. The
mixture was heated at 80 °C for 2.5 h and cooled, the solvent removed in
vacuo (where appropriate), and the residue separated by flash column
chromatography (20/1 hexanes/EtOAc, 1% Et3N) to afford the hydro-
stannylated product 16ꢀ20 as detailed below.
General Procedure B for Hydrostannylation Reactions at 23 °C.
To the required olefin (1.0 equiv) was added stannane (Ph3SnH or 7,
2.0 equiv) followed by Et3B in cyclohexane (1 M, 1.0 equiv). The
mixture was stirred open to the atmosphere for 3 h and concentrated in
vacuo and the residue separated by flash chromatography (20/1
hexanes/EtOAc, 1% Et3N) to afford the hydrostannylated product
16ꢀ20 as detailed below.
Methyl 3-[((1R,2S,5R)-Menthyl)diphenylstannyl]propanoate (16; R =
CO2Me, R0 = H). Following general procedure A. Isolated as a colorless
oil (52%). [R]D27 = ꢀ18.15° (c = 5.35, CHCl3). 1H NMR (500 MHz,
CDCl3): δ 0.73 (3H, d, J = 7.0 Hz), 0.81 (3H, d, J = 7.0 Hz), 0.85 (3H, d,
J = 6.0 Hz), 0.95 (1H, qd, J = 3.0 Hz, 12.5 Hz), 1.05 (1H, qd, J = 3.0, 12.5
Hz), 1.28ꢀ1.36 (2H, m), 1.42ꢀ1.52 (3H, m), 1.63ꢀ1.70 (2H, m),
1.75ꢀ1.78 (1H, m), 1.99 (1H, td, J = 3.0, 12.5 Hz), 2.07 (3H, dt, J = 3.0,
8.5 Hz), 2.06ꢀ2.08 (1H, m), 2.46ꢀ2.61 (2H, m), 3.59 (3H, s), 7.33ꢀ
7.39 (6H, m), 7.45ꢀ7.57 (4H, m). 13C NMR (125 MHz): δ 5.1, 15.8,
21.8, 22.5, 26.7, 30.9, 34.1, 35.0, 35.2, 35.3, 41.2, 46.4, 51.5, 128.1, 128.2,
128.3, 137.0, 137.1, 139.7, 139.8, 175.6. 119Sn NMR (187 MHz,
CDCl3): δ ꢀ89.9. IR (neat) νmax 3064, 1729 cmꢀ1. HRMS: m/z calcd
for C26H36O2Sn 607.078 27 (M + Ag+), found 607.077 86.
Methyl 3-[Bis((1R,2S,5R)-menthyl)phenylstannyl]propanoate (17;
R = CO2Me, R0 = H). Following general procedure A. Isolated as a
colorless oil (67%). [R]D27 = ꢀ38.40° (c = 18.9, CHCl3). 1H NMR (500
MHz, CDCl3): δ 0.74 (3H, d, J = 7.0 Hz), 0.76 (3H, d, J = 7.0 Hz),
0.83ꢀ1.05 (16H, m), 1.20ꢀ1.46 (8H, m), 1.54ꢀ1.61 (2H, m), 1.67
(2H, td, J = 3.0, 12.0 Hz), 1.73ꢀ1.80 (4H, m), 1.96ꢀ2.02 (2H, m),
2.42ꢀ2.52 (2H, m), 3.63 (3H, s), 7.25ꢀ7.34 (3H, m), 7.38ꢀ7.45 (3H,
m). 13C NMR (125 MHz, CDCl3): δ 51.2, 16.1, 16.2, 21.8, 21.9, 22.4,
22.5, 26.7, 26.8, 31.2, 34.0, 35.0, 35.1, 35.26, 35.28, 35.33, 35.34, 41.4,
41.5, 46.5, 46.6, 51.3, 127.6, 127.7, 136.7, 143.3, 175.6. 119Sn NMR (187
MHz, CDCl3): δ ꢀ79.3. IR (neat): νmax 2952, 1734 cmꢀ1. HRMS: m/z
calcd for C30H50O2Sn 669.187 82 (M + Ag+), found 669.187 35.
Methyl 3-[Tris((1R,2S,5R)-menthyl)stannyl]propanoate (18; R =
CO2Me, R0 = H). Following general procedure A. Isolated as a white
solid (62%). [R]D27 = +57.7° (c = 17.4, CHCl3). 1H NMR (500 MHz,
CDCl3): δ 0.79 (9H, dd, J = 1.5, 6.5 Hz), 0.86 (9H, d, J = 4.5 Hz),
0.89ꢀ1.03 (6H, m), 0.97 (9H, dd, J = 1.5, 6.5 Hz), 1.07ꢀ1.13 (2H, m),
1.19ꢀ1.27 (6H, m), 1.39ꢀ1.48 (3H, m), 1.52ꢀ1.63 (3H, m), 1.70 (3H,
d, J = 12.0 Hz), 1.76 (3H, d, J = 12.5 Hz), 1.88ꢀ1.94 (3H, m), 2.46ꢀ2.58
(2H, m), 3.70 (3H, d, J = 2.0 Hz). 13C NMR (125 MHz, CDCl3): δ 7.33,
16.7, 22.2, 22.7, 27.1, 31.9, 33.9, 35.4, 35.5, 35.6, 41.8, 46.7, 51.5, 176.1.
119Sn NMR (187 MHz, CDCl3): δ ꢀ70.3. IR (neat): νmax 2947,
1743 cmꢀ1. HRMS: m/z calcd for C34H64O2Sn 715.302 46 (M+Ag+);
found 715.299 57.
’ CONCLUSIONS
Free radical hydrostannylations of olefins of differing steric
and electronic demand have been carried out using the chiral,
nonracemic stannanes (1R,2S,5R)-menthyldiphenyltin hydride
(7), bis[(1R,2S,5R)-menthyl]phenyltin hydride (8), and tris-
[(1R,2S,5R)-menthyl]tin hydride (9). These reactions resulted
in adduct yields (16ꢀ18) that were found to depend on the
natureofthe substituentsonbothalkeneandstannane. For alkenes
bearing electron-withdrawing groups, excellent yields of hydro-
stannylated products (16ꢀ18) were obtained irrespective of
stannane. For alkenes bearing electon-donating groups, the bulkier
stannanes (8 and 9) failed to react, while MenPh2SnH (7) reacted
with all substrates with good to excellent conversions, except for
vinyl acetate, which failed to react under all circumstances.
This work also demonstrated the feasibility of low-tempera-
ture hydrostannylation initiated by triethylborane and oxygen;
MenPh2SnH (7) reacted with tert-butyldimethyl(1-phenylviny-
loxy)silane (1) at ꢀ78 °C to afford adduct 19 in near-quantita-
tive yield. Despite being carried out at low temperature, little
diastereoselectivity was observed in this reaction.
Somewhat surprisingly, addition of small quantities of diphenyl
diselenide to these reactions failed to improve the outcome in
each case.
’ EXPERIMENTAL SECTION
(1R,2S,5R)-Menthyldiphenyltin hydride (7), bis[(1R,2S,5R)-men-
thyl]phenyltin hydride (8), and tris[(1R,2S,5R)-menthyl]tin hydride
(9) were prepared following literature procedures.25,26
General Methods. tert-Butyldimethyl(1-phenylvinyloxy)silane.32
To a solution of acetophenone (2.16 g, 18 mmol) in CH2Cl2 was added
triethylamine (2.5 mL, 20 mmol) at room temperature. After 1 h tert-
butyldimethylsilyl trifluoromethanesulfonate (5 g, 19 mmol) was added
and the resulting mixture was stirred at this temperature for a further 2 h.
To this solution was added cold phosphate buffer (pH 7, 30 mL), with
stirring, and the mixture was washed with CH2Cl2. The organic phases
were combined, dried (MgSO4), and concentrated in vacuo to yield the
title compound as a clear oil (4.2 g, 99%), which could be used directly or
Synthesis of Heptyl((1S,2S,5R)-menthyl)diphenylstannane (16; R =
(CH2)4CH3, R0 = H). Following general procedure A. Isolated as a
25
1
colorless oil (65%). [R]D = ꢀ18.73° (c = 4.15, CHCl3). H NMR
(500 MHz, CDCl3): δ 0.68 (3H, d, J = 6.5 Hz), 0.80 (3H, d, J = 6.5 Hz),
0.82 (3H, d, J = 6.5 Hz), 0.82 (3H, t, J = 7. Hz), 0.89ꢀ1.06 (4H, m),
1.18ꢀ1.35 (13H, m), 1.44 (1H, tt, J = 2.5, 12.0 Hz), 1.55ꢀ1.61 (2H, m),
1.87 (1H, td, J = 3.0, 12.0 Hz), 2.01 (1H, dt, 2.5, 9.5 Hz), 7.31ꢀ7.36 (6H,
m) and 7.46ꢀ7.51 (4H, m). 13C NMR (125 MHz, CDCl3): δ 10.9, 14.1,
15.8, 21.7, 22.5, 22.6, 26.7, 26.8, 28.7, 31.8, 33.9, 34.4, 34.5, 35.3, 35.4,
41.4, 46.6, 128.0, 128.1, 137.0, 140.7, 140.8. 119Sn NMR δ (187 MHz,
CDCl3): ꢀ92.5. IR (neat): νmax 2954 cmꢀ1. HRMS: m/z calcd for
C29H44OSn 619.151 04 (M + Ag+), found 619.150 52.
1
purified by column chromatography (petroleum spirits, 1% Et3N). H
NMR (500 MHz, CDCl3): δ 0.23 (6H, s), 1.06 (s, 9H), 4.43 (1H, d, J =
1.6Hz), 4.85 (1H, d, J = 1.6 Hz), 7.29ꢀ7.40 (3H, m), and 7.60ꢀ7.63 (2H,
m). 13C NMR δ (125 MHz, CDCl3): ꢀ4.6, 18.3, 25.8, 90.9, 125.3 128.0
128.1, 137.8, and 156.0.
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(2-Ethoxyethyl)((1S,2S,5R)-menthyl)diphenylstannane (16; R =
OEt, R0 = H). Following general procedure A. Isolated as a colorless oil
A. Isolated as a clear oil and as a 1:1 mixture of diastereoisomers (33%).
1H NMR (500 MHz, CDCl3): δ 0.74 (3H, d ꢁ 2, J = 7.0 Hz), 0.79 (3H,
dd, J = 2.5, 7.0 Hz), 0.83ꢀ0.89 (14H, m), 0.90ꢀ1.10 (3H, m), 1.17 (3H,
d ꢁ 2, J = 7.0 Hz), 1.19ꢀ1.31 (4H, m), 1.37ꢀ1.47 (3H, m), 1.55ꢀ1.63
(2H, m), 1.65ꢀ1.71 (2H, m), 1.75ꢀ1.82 (4H, m), 1.97ꢀ2.08 (2H, m),
2.54ꢀ2.65 (1H, m), 3.51 (3H, s ꢁ 2), 7.13ꢀ7.24 (3H, m), 7.44ꢀ7.46
(2H, m). 13C NMR (125 MHz, CDCl3): δ 16.20, 16.23, 16.28, 16.3,
16.34, 16.5, 21.0, 21.5, 21.91, 21.93, 21.97, 22.0, 22.59, 22.62, 22.64, 26.9,
33.84, 33.85, 33.9, 34.1, 35.32, 35.35, 35.38, 35.4, 35.5, 37.3, 37.4, 41.4,
41.5, 46.5, 46.7, 46.8, 51.4, 127.6, 127.62, 127.69, 127.7, 136.87, 136.88,
142.0, 142.1, 178.1, 178.3. 119Sn NMR δ (187 MHz, CDCl3): δ ꢀ98.7,
ꢀ98.8. IR (neat): νmax 2952, 1734 cmꢀ1. HRMS: m/z calcd for C31H52-
O2Sn 615.262 09 (M + K+), found 615.262 02.
27
(35%). [R]D = ꢀ19.4° (c = 6.5, CHCl3). 1H NMR (500 MHz,
CDCl3): δ 0.67 (3H, d, J = 6.5 Hz), 0.79 (3H, d, J = 7.0 Hz), 0.83 (3H, d,
J = 6.5 Hz), 0.91ꢀ1.06 (2H, m), 1.18 (3H, t, J = 7.0 Hz), 1.26ꢀ1.35 (2H,
m), 1.46 (1H, t, J = 3.0, 12.0 Hz), 1.58ꢀ1.68 (4H, m), 1.73ꢀ1.76 (1H,
m), 1.92 (1H, td, J = 3.0, 12.0 Hz), 2.01ꢀ2.04 (1H, m), 3.42 (2H, q, J =
7.02 Hz), 3.58ꢀ3.71 (2H, m), 7.32ꢀ7.36 (6H, m), 7.47ꢀ7.57 (4H, m).
13C NMR (125 MHz, CDCl3): δ 12.7, 15.3, 15.8, 21.8, 22.5, 26.7, 33.9,
35.0, 35.3, 35.4, 41.1, 46.4, 65.5, 66.7, 128.0, 128.1, 128.2, 137.0, 137.6,
140.3, 140.4. 119Sn NMR (187 MHz, CDCl3): δ ꢀ97.0. IR (neat): νmax
3063 cmꢀ1. HRMS: m/z calcd for C26H38OSn 593.098 76 (M + Ag+,
100%), found 593.099 01.
((1R,2S,5R)-Menthyl)(phenethyl)diphenylstannane (16; R = Ph,
R0 = H). Following general procedure A. Isolated as a colorless oil (84%).
[R]D27 = ꢀ10.2° (c = 43.25, CHCl3). 1H NMR (500 MHz, CDCl3): δ
0.67 (3H, d, J = 6.5 Hz), 0.77ꢀ0.80 (6H, m), 0.82ꢀ1.04 (2H, m),
1.20ꢀ1.29 (2H, m), 1.41 (1H, tt, J = 2.5, 11.5 Hz), 1.55 (2H, t, J = 8.5
Hz), 1.59ꢀ1.73 (3H, m), 1.85 (1H, td, J = 3.0, 12.5 Hz), 1.92ꢀ1.99 (1H,
m), 2.77ꢀ2.87 (2H, m), 7.14ꢀ7.16 (3H, m), 7.23ꢀ7.26 (2H, m),
7.32ꢀ7.35 (6H, m), 7.43ꢀ7.54 (4H, m). 13C NMR (125 MHz, CDCl3):
δ 12.8, 15.9, 21.9, 22.5, 26.7, 32.8, 34.0, 34.7, 35.3, 41.3, 46.5, 125.7,
127.7, 128.2, 128.3, 128.4, 137.0, 140.1, 140.2, 145.5. 119Sn NMR (187
MHz, CDCl3): δ ꢀ91.8. IR (neat): νmax 3063 cmꢀ1. HRMS: m/z calcd
for C30H38Sn 625.104 09 (M + Ag+, 38%), found 625.104 023.
Bis((1R,2S,5R)-menthyl)(phenethyl)phenylstannane (17; R = Ph,
R0 = H). Following general procedure A. Isolated as a colorless oil (70%).
[R]D27 = ꢀ39.6° (c = 15.75, CHCl3). 1H NMR (500 MHz, CDCl3): δ
0.97 (6H, d, J = 7.0 Hz), 1.03 (6H, d, J = 5.0 Hz), 1.05ꢀ1.24 (3H, m),
1.06 (3H, d, J = 7.5 Hz), 1.10 (3H, d, J = 7.0 Hz), 1.42ꢀ1.67 (9H, m),
1.83ꢀ1.98 (8H, m), 2.16ꢀ2.22 (2H, m), 2.98ꢀ3.09 (2H, m), 7.32ꢀ
7.35 (1H, m), 7.37ꢀ7.39 (2H, m), 7.45ꢀ7.53 (5H, m), 7.64ꢀ7.72 (2H,
m). 13C NMR (125 MHz, CDCl3): δ 13.3, 16.3, 16.4, 22.0, 22.1, 22.6, 33.3,
34.0, 34.1, 35.0, 35.1, 35.3, 35.4, 41.5, 41.7, 46.7, 127.7, 127.9, 128.3, 136.9,
142.1, 146.3. 119Sn NMR δ (187 MHz, CDCl3): δ ꢀ79.3 . IR (neat): νmax
3062 cmꢀ1. HRMS: m/z calcd for C34H52Sn 687.213 64 (M + Ag+, 100%),
found 687.212 95.
Methyl 3-(Triphenylstannyl)propanoate33 (20; R = CO2Me, R0 = H).
Following general procedure A. Isolated as a white solid (97%). Mp:
46ꢀ47 °C. 1H NMR (500 MHz, CDCl3): δ 1.73 (2H, t, J = 7.5 Hz), 2.74
(2H, t, J = 7.5 Hz), 3.52 (3H, s), 7.39ꢀ7.44 (9H, m), 7.55ꢀ7.67 (6H,
m). 13C NMR (125 MHz, CDCl3): δ 5.5, 30.7, 51.5, 128.5, 128.8, 137.0,
138.5, 175.1. 119Sn NMR (187 MHz, CDCl3): δ ꢀ109.4. IR (neat) νmax
3063, 1729 cmꢀ1. HRMS: m/z calcd for C22H22O2Sn 544.968 72 (M +
Ag+), found 544.968 51.
Heptyltriphenylstannane34 (20; R = (CH2)4CH3, R0 = H). Following
general procedure A. Isolated as a colorless oil that crystallized on
standing (93%). Mp: 55ꢀ56 °C. 1H NMR (500 MHz, CDCl3): δ 0.87
(3H, t, J = 7.0 Hz), 1.18ꢀ1.31 (6H, m), 1.37 (2H, quin, J = 7.5, 15.0 Hz),
1.53ꢀ1.54 (2H, m), 1.75 (2H, quin, 7.5, 15.0), 7.37ꢀ7.41 (9H, m) and
7.52ꢀ7.62 (6H, m). 13C NMR (125 MHz, CDCl3): δ 11.1, 14.0, 22.5,
26.6, 28.7, 31.7, 34.2, 128.4, 128.7, 137.0, 139.2 ppm. 119Sn NMR (187
MHz, CDCl3): δ ꢀ110.3. IR (neat): νmax 3063 cmꢀ1. HRMS: m/z calcd
for C25H30Sn 557.041 49 (M + Ag+), found 557.041 07.
(2-Ethoxyethyl)triphenylstannane (20; R = OEt, R0 = H). Following
1
general procedure A. Isolated as a colorless oil (77%). H NMR (500
MHz, CDCl3): δ 1.11 (3H, t, J = 7.0 Hz), 1.89 (2H, t, J = 7.5 Hz), 3.39
(2H, q, J = 7.0 Hz), 3.79 (2H, t, J = 7.5 Hz), 7.30ꢀ7.43 (9H, m),
7.54ꢀ7.67 (6H, m). 13C NMR (125 MHz, CDCl3): δ 13.6, 15.1, 65.6,
67.9, 128.3, 128.7, 137.1, 139.1. 119Sn NMR (187 MHz, CDCl3): δ
ꢀ112.3. IR (neat): νmax 3063 cmꢀ1. HRMS: m/z calcd for C22H24OSn
530.989 46 (M + Ag+), found 530.989 01.
Tris((1R,2S,5R)-menthyl)(phenethyl)stannane (18; R = Ph, R0 = H).
27
(Phenethyl)triphenylstannane33 (20; R = Ph, R0 = H). Following
general procedure A. Isolated as a while solid (63%). Mp: 127ꢀ128 °C.
1H NMR (500 MHz, CDCl3): δ 1.83 (2H, td, J = 1.0, 8.5 Hz), 2.99 (2H,
t, J = 8.5 Hz), 7.13ꢀ7.17 (3H, m), 7.22ꢀ7.25 (2H, m), 7.33ꢀ7.37 (6H,
m), 7.45ꢀ7.55 (9H, m). 13C NMR δ (125 MHz, CDCl3): 12.9, 32.5,
125.8, 127.8, 128.4, 128.5, 128.8, 138.6, 144.7. 119Sn NMR δ (187 MHz,
CDCl3): ꢀ111.1. IR (neat): νmax 3060 cmꢀ1. HRMS: m/z calcd for
C26H24Sn 562.994 54 (M + Ag+), found 562.994 13.
Following general procedure A. Isolated as a white solid (22%). [R]D
=
+37.37° (c = 3.7, CHCl3). 1H NMR (500 MHz, CDCl3): δ 0.82 (9H, d,
J = 6.5 Hz), 0.87 (9H, d, J = 6.0 Hz), 0.92ꢀ1.06 (5H, m), 0.99 (9H, d, J =
6.0 Hz), 1.31ꢀ1.81 (3H, m), 1.27ꢀ1.33 (6H, m), 1.47ꢀ1.54 (3H, m),
1.59ꢀ1.65 (6H, m), 1.72 (3H, dd, J = 2.5, 12.5 Hz), 1.77ꢀ1.79 (3H, m),
1.95ꢀ1.97 (3H, m), 2.84ꢀ2.92 (2H, m), 7.17ꢀ7.23 (3H, m), 7.29ꢀ
7.33 (2H, m). 13C NMR (125 MHz, CDCl3): δ 15.8, 16.8, 22.3, 22.7, 27.2,
33.8, 33.9, 35.1, 35.5, 35.7, 41.9, 46.9, 125.5, 127.6, 128.4, 146.7. 119Sn
NMR (187 MHz, CDCl3): δ ꢀ72.6. IR (neat): νmax 2945 cmꢀ1. HRMS:
m/z calcd for C38H66Sn 749.322 83 (M + Ag+, 100%), found 749.323 19.
Methyl (2R/2S)-3-[((1S,2S,5R)-Menthyl)diphenylstannyl]-2-methyl-
propanoate (16; R = CO2Me, R0 = Me). Following general procedure A.
(()-Methyl 2-Methyl-3-(triphenylstannyl)propanoate (20; R =
CO2Me, R0 = Me). Following general procedure A. Isolated as a colorless
oil (56%). 1H NMR (500 MHz, CDCl3): δ 1.27 (3H, dd, J = 2.0, 7.0 Hz),
1.66ꢀ1.71 (1H, m), 1.78ꢀ1.84 (1H, m), 2.86ꢀ2.94 (1H, m), 3.38 (1H, s),
7.38ꢀ7.39 (9H, m), 7.57ꢀ7.60 (6H, m). 13C NMR (125 MHz, CDCl3):
δ 16.5, 21.3, 37.1, 51.5, 128.4, 128.8, 137.0, 138.9, 177.5. 119Sn NMR
1
Isolated as a clear oil as a 1:1 mixture of diastereoisomers (79%). H
NMR (500 MHz, CDCl3): δ 0.69 (3H, d, J = 7.0 Hz), 0.71 (3H, d, J = 6.5
Hz), 0.76 (3H, d, J = 6.5 Hz), 0.83ꢀ0.85 (6H, m), 0.89ꢀ0.91 (3H, m),
0.98ꢀ1.06 (3H, m), 1.16ꢀ1.18 (3H, m), 1.23ꢀ1.44 (4H, m), 1.54ꢀ
1.66 (3H, m), 1.73ꢀ1.74 (1H, m), 1.92ꢀ1.98 (1H, m), 2.03ꢀ2.07 (1H,
m), 2.63ꢀ2.68 (1H, m), 3.45/3.46 (3H, s ꢁ 2), 7.33ꢀ7.36 (6H, m),
7.45ꢀ7.54 (4H, m). 13C NMR (125 MHz, CDCl3): δ 15.8, 16.0, 21.2,
21.3, 21.8, 25.5, 26.4, 33.6, 33.9, 34.8, 34.9, 35.2, 35.3, 37.2, 41.0, 41.1,
46.2, 46.3, 51.5, 128.0, 128.11, 128.12, 128.2, 128.3, 137.0, 137.10,
137.13, 139.8, 140.1, 140.2, 140.4. 119Sn NMR (187 MHz, CDCl3):
δ ꢀ94.5, ꢀ94.7. IR (neat): νmax 3063, 1733 cmꢀ1. HRMS: m/z calcd for
C27H38O2Sn 621.093 92 (M + Ag+), found 621.093 47.
(187 MHz, CDCl3): δ ꢀ117.2. IR (neat): νmax 3063, 1725 cmꢀ1
.
HRMS: m/z calcd for C23H24O2Sn 558.984 37 (M + Ag+), found
558.984 01.
(()-1-(Triphenylstannyl)propan-2-yl acetate (20; R = OAc, R0 =
Me). Following general procedure A. Isolated as a colorless oil (44%). 1H
NMR (500 MHz, CDCl3): δ 1.32 (3H, d, J = 7.0, 13.0 Hz), 1.69 (3H, s),
1.90 (1H, dd, J = 7.0 Hz), 1.95 (1H, dd, J = 7.0 Hz, CH2), 5.33 (1H, q, J =
7.0 Hz), 7.38ꢀ7.43 (9H, m), 7.53ꢀ7.64 (6H, m). 13C NMR (125 MHz,
CDCl3): δ 19.9, 20.9, 23.6, 70.6, 128.5, 128.9, 136.9, 138.5, 170.4. 119Sn
NMR (187 MHz, CDCl3): δ ꢀ119.8 ppm. IR (neat): νmax 3015,
1731 cmꢀ1. HRMS: m/z calcd for C23H24O2Sn 558.984 37 (M +
Ag+), found 558.983 95.
Methyl (2R/2S)-3-[Bis((1S,2S,5R)-menthyl)phenylstannyl]-2-meth-
ylpropanoate (17; R = CO2Me, R0 = Me). Following general procedure
4391
dx.doi.org/10.1021/om200473c |Organometallics 2011, 30, 4387–4392

Organometallics
ARTICLE
[2-(tert-Butyldimethylsilyloxy)-2-phenylethyl]triphenylstannane (20;
R = OTBDMS, R0 = Ph). Following general procedure B. Isolated as a
(12) Rahm, A.; Pereyre, M. J. Organomet. Chem. 1975, 88, 79.
(13) Roberts, B. P. Chem. Soc. Rev. 1999, 28, 25 and references cited
therein.
(14) Schiesser, C. H.; Skidmore, M. A. J. Chem. Soc. Perkin Trans. 2
1998, 2329.
(15) Haque, M. B.; Roberts, B. P.; Tocher, D. A. J. Chem. Soc., Perkin
Trans. 1 1998, 2881.
(16) Dang, H. S.; Roberts, B. P. Tetrahedron Lett. 1995, 36, 2875.
(17) Skidmore, M. A. Ph.D. Thesis, The University of Melbourne,
1999.
(18) Meurice, J.-C.; Vallier, M.; Ratier, M.; Duboudin, J.-G.; Pꢀetraud
J. Organomet. Chem. 1997, 542, 67.
(19) Dakternieks, D.; Dunn, K.; Perchyonok, V. T.; Schiesser, C. H.
Chem. Commun. 1999, 1665.
(20) Dakternieks, D.; Dunn, K.; Schiesser, C. H.; Tiekink, E. R. T.
J. Organomet. Chem. 2000, 605, 209.
(21) Dakternieks, D.; Dunn, K.; Schiesser, C. H.; Tiekink, E. R. T.
Dalton Trans. 2000, 3693.
1
colorless oil (99%). H NMR (400 MHz, CDCl3): δ ꢀ0.77 (3H, s),
ꢀ0.81 (3H, s), 0.74 (9H, s), 2.10 (1H, dd, J = 12.8, 7.6 Hz), 2.19 (1H, dd,
J = 13.2, 4.8 Hz), 5.21 (1H, dd, J = 7.2, 5.2 Hz), 7.17ꢀ7.20 (4H, m),
7.27ꢀ7.20 (15H, m), 7.60ꢀ7.63 (5H, m). 13C NMR (125 MHz, CDCl3):
δ ꢀ4.9, ꢀ4.7, 18.3, 25.9, 26.6, 73.8, 126.0, 127.0, 128.2, 128.3, 128.5,
137.0, 139.4, 146.6. 119SnNMR (187 MHz, CDCl3): δ ꢀ124.8. IR(neat):
νmax 3064 cmꢀ1. Anal. Calcd for C32H38OSiSn: C, 65.65; H, 6.54; Sn,
20.28. Found: C, 65.68; H, 6.53; Sn, 20.35.
[(2R/2S)-(tert-Butyldimethylsilyloxy)-2-phenylethyl]((1R,2S,5R)-
menthyl)diphenylstannane (19). To silyl enol ether 1 (0.23 g, 1 mmol)
was added MenPh2SnH (7; 1 g, 1.9 mmol), followed by Et3B (1 mL,
1 mmol). The mixture was stirred open to the atmosphere at ꢀ78 °C for
5 h, concentrated in vacuo, and purified by column chromatography
(petroleum spirits, 1% Et3N) to give the title compound as a clear oil and
1
as a 1.3:1 mixture of diastereoisomers (99%). H NMR (400 MHz,
(22) Dakternieks, D.; Perchyonok, V. T.; Schiesser, C. H. Tetrahe-
dron: Asymmetry 2003, 3057.
CDCl3): δ ꢀ0.27 (3H, s), ꢀ0.24 (3H, s), ꢀ0.19 (3H, s), ꢀ0.13 (3H, s),
0.59ꢀ1.31 (42H, m), 1.55ꢀ1.69 (12H, m), 1.80ꢀ1.97 (6H, m), 4.96
(1H, dd, J = 5.5, 9.2 Hz), 5.06 (1H, dd, J = 5.5, 7.6 Hz), 7.16ꢀ7.42 (30H,
m). 13C NMR (125 MHz, CDCl3): δ ꢀ4.8, ꢀ4.7, 15.9, 16.0, 18.2, 21.7,
22.4, 22.5, 25.4, 25.5, 25.9, 26.6, 33.6, 33.7, 34.4, 34.8, 35.1, 35.2, 35.3,
40.8, 41.0, 46.0, 46.1, 74.4, 74.5, 125.8, 125.9, 126.9, 127.0, 127.8, 127.95,
127.97, 128.00, 128.01, 128.09, 128.1, 137.0, 137.1, 140.1, 140.5, 146.9,
147.2. 119Sn NMR (187 MHz, CDCl3): δ ꢀ94.5, ꢀ94.6. IR (neat): νmax
3063 cmꢀ1. HRMS: m/z calcd for C36H52OSiSn 755.185 48 (M + Ag+),
found 755.185 12.
(23) Dakternieks, D; Duthie, A.; Zeng, L.; Perchyonok, V. T.;
Schiesser, C. H. Tetrahedron: Asymmetry 2004, 15, 2547.
(24) Beckman, J.; Dakternieks, D.; Duthie, A.; Floate, S. L.; Foitzik,
R. C.; Schiesser, C. H. J. Organomet. Chem. 2004, 689, 909.
(25) Dakternieks, D.; Dunn, K.; Henry, D. J.; Schiesser, C. H.;
Tiekink, E. R. T. Organometallics 1999, 18, 3342.
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J.-M.; Connil, M.-F.; Jousseaume, B. J. Organomet. Chem. 1996, 520, 101.
(27) Podestꢀa, J.; Chopa, A.; Koll, L.; Vitale, C.; Zꢀu~niga, A. Main
Group Met. Chem. 1991, 14, 101.
’ AUTHOR INFORMATION
(28) Kita, Y.; Matsugi, M. In Radicals in Organic Synthesis; Renaud,
P., Sibi, M., Eds.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 1.
(29) Miura, K.; Ichinose, Y.; Nozaki, K.; Fugami, K.; Oshima, K.;
Utimoto, K. Bull. Chem. Soc. Jpn. 1989, 62, 143.
Corresponding Author
*E-mail: carlhs@unimelb.edu.au.
(30) Nozaki, K.; Oshima, K.; Utimoto, K. J. Am. Chem. Soc. 1987,
109, 2547.
(31) Ford, L. Ph.D. Thesis, The University of Melbourne, 2007.
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1956, 352.
Present Addresses
§Department of Medicinal Chemistry, Monash Institute of
Pharmaceutical Sciences, Monash University, 381 Royal Parade,
Parkville, Victoria 3052, Australia.
(34) Ruff, J. K. J. Am. Chem. Soc. 1961, 83, 1798.
’ ACKNOWLEDGMENT
Generous support from the Australian Research Council
through the Centres of Excellence Program is gratefully
acknowledged.
’ REFERENCES
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