Diastereoselective silylene transfer reactions to chiral enantiopure alkenes: Effects of ligand size and substrate bias

Article abstract of DOI:10.1039/c6dt04612f

Silylenes are useful reactive intermediates for the stereoselective construction of compounds containing carbon-silicon bonds. Despite their synthetic utility, the development of either an enantioselective or diastereoselective metal-catalyzed silylene tr

Full text of DOI:10.1039/c6dt04612f

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Diastereoselective silylene transfer reactions to
chiral enantiopure alkenes: effects of ligand size
and substrate bias†
Cite this: DOI: 10.1039/c6dt04612f
Christina Z. Rotsides and K. A. Woerpel*
Silylenes are useful reactive intermediates for the stereoselective construction of compounds containing
carbon–silicon bonds. Despite their synthetic utility, the development of either an enantioselective or
diastereoselective metal-catalyzed silylene transfer reaction, in which ligands on the metal catalyst
control stereoselectivity, has not been achieved. In this article, we report that the structure of the alkene is
the most important for controlling stereoselectivity in these reactions. The stereochemical course of kine-
tically controlled silacyclopropanation reactions was not affected by the nature or chirality of the ligands
on the metal. When silylene transfer reactions were reversible, however, products can be formed with a
high degree of diastereoselectivity (90 : 10 d.r.).
Received 6th December 2016,
Accepted 25th January 2017
DOI: 10.1039/c6dt04612f
rsc.li/dalton
Introduction
The reactions of metal silylene complexes with various functio-
nalized organic compounds provide routes to synthesize pro-
ducts with diverse structures.1 Methods to control the
diastereoselectivity of silylene transfer reactions, however, are
limited.2 In contrast to the well-established diastereoselective
and enantioselective syntheses of cyclopropanes from metal
carbenes,3 stereoselective silylene transfer to alkenes has been
much more difficult to achieve.4 In the few reported examples,
a sterically bulky substituent on the alkene prevented the
addition of the silylene to the more sterically encumbered
face, resulting in diastereoselectivity.4 The only examples of
obtaining single enantiomers from reactions using silylenes as
reactive intermediates rely on the use of chiral enantiopure
substrates5 or chiral auxiliaries.6 Although diastereoselective
additions of nucleophiles to chiral silylenes have been
reported,7 there is no known asymmetric or diastereoselective
silylene transfer reaction using a chiral ligand as the source of
stereodifferentiation.
Fig. 1 Systematic study of diastereoselective silylene transfer reactions.
Silacyclopropanation reactions were examined in the presence
of chiral ligands that differed in size, electron-donating ability,
and absolute configuration. These studies provide evidence that
the metal-catalyzed silacyclopropanation of alkenes involves a
ligand-bound metal–silylene complex in which the reactivity is
determined by steric interactions between the ligand and the
substituents on the alkene. Electronic factors are less significant
in controlling both reactivity and stereoselectivity.
Results and discussion
The effects of the structure of the catalyst on silylene transfer
reactions were first evaluated using chiral non-racemic bis
(homoallylic) ether 4 (eqn (1)) and diene 6 (eqn (2)). These sub-
strates were designed to place the stereocenter at some dis-
tance from the reactive alkene functional group, but close
enough to enable the determination of facial selectivity on the
alkene by measuring the ratio of diastereomers by nuclear
magnetic resonance (NMR) spectroscopy. Initial experiments
confirmed that the remote stereocenters of alkene 4 and diene
6 exerted no stereoselectivity on the silver salt-catalyzed sila-
cyclopropanation reaction in the absence of chiral ligands.
Control experiments established that the silylene transfer reac-
To understand the role that ligands play in controlling
stereoselectivity in silylene transfer reactions, we performed
a systematic study of both silver- and copper-catalyzed silylene
transfer reactions with chiral enantiopure alkenes to
provide diastereomeric silylene transfer products (Fig. 1).
Department of Chemistry, New York University, New York, NY, 10003, USA.
E-mail: kwoerpel@nyu.edu
†Electronic supplementary information (ESI) available: Additional experimental
details, ligands used, details of computational studies, and 1H and 13C NMR
spectra of new compounds. See DOI: 10.1039/c6dt04612f
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tion was irreversible, so this selectivity is not the result of
The effects of the structure of the catalyst on silylene trans-
thermodynamic control. Consequently, these substrates could fer reactions were then evaluated using chiral, non-racemic
be used to evaluate the influence of chiral ligands on stereo- homoallylic ether 18.10 Both silver11 and copper12 catalysts
selectivity in the absence of steric effects from the substrate.
were evaluated because of their propensity to form metal
silylenes with silacyclopropane 2 and their ability to coordinate
to various ligands.13
The silacyclopropanation of methyl ether 18 occurred with
modest stereoselectivity with both copper and silver catalysts
in the absence of a chiral ligand. Under either silver- (Table 1,
entries 1–4) or copper- (Table 1, entries 5–7) catalyzed
conditions, the reaction was irreversible, and the product ratio
was consistently about 70 : 30. This selectivity was relatively
unaffected by the nature of the metal salt or its counterion.
The high yields observed in the absence of a chiral ligand indi-
cate that neither the steric hindrance of 18 nor the presence of
an electron-withdrawing alkoxy group affected silylene trans-
fer. The steric hindrance of this substrate also likely prevented
rearrangement reactions from occurring.14
The effects of chiral ligands on the diastereoselectivity of
the silacyclopropanation of alkene 18 were examined (Fig. 3).
In the presence of either a bidentate bis(phosphine)9 (8) or a
tridentate bis(oxazoline)15 (20) ligand, no reaction was
observed. A bidentate phosphine ligand could be too strongly
electron-donating to allow the metal-catalyzed silylene transfer
ð1Þ
ð2Þ
The silacyclopropanation reaction was insensitive to the
presence of the chiral ligands, however (Fig. 2).8 Ligands had
no effect on the ratio of isomers or the rate of silylene transfer.
The presence of the enantiomeric ligands (R)- and (S)-BINAP9
and their quantity seemed to exert no influence on the extent
of conversion and selectivity as a function of time, as deter-
mined by 1H NMR spectroscopy. Only strongly donating
ligands such as pyrrolidine 12 inhibited the reaction, indicat-
ing that the steric size of the ligands and the alkene did not
affect the rate of silylene transfer. The fact that selectivity was
independent of the presence of the ligand suggests that these
ligands were likely not the optimal structures for achieving an
enantioselective reaction.
Table 1 Silylene transfer to alkene 18
The outcomes of reactions of unsaturated substrates 4 and
6 did not change with the reaction conditions.8 Changing the
amount of the silver catalyst from 1 mol% to 15 mol%,
decreasing the temperature to −20 °C or −78 °C, and increas-
ing the reaction time to 22 hours did not affect diastereo-
selectivity. Using polar and non-polar solvents such as toluene,
trifluorotoluene, diethyl ether, chloroform, and 1,2-dichlor-
oethene did not seem to affect the rate of the reaction, except
for when tetrahydrofuran was used a solvent, in which case the
reaction was completely inhibited.
Entry
MX
Yielda
d.r.a
1
2
3
4
5
6
7
AgO2CCF3
AgOTf
AgOTs
Ag3PO4
[Cu(OTf)]2·PhH
CuCl
77
73
96
87
87
79
69
71 : 29
71 : 29
71 : 29
67 : 33
70 : 30
71 : 29
57 : 43
CuI
a Determined by 1H NMR spectroscopy.
Fig. 2 Effect of ligand on silver-catalyzed silylene transfer to 4.
Fig. 3 Effect of the ligand on copper-catalyzed silylene transfer to 18.
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to occur; similar observations have been made in other of the copper-catalyzed reaction, the selectivity with the chiral
systems.16 In the case of the tridentate ligand 7, there might non-racemic alkene (60 : 40 d.r.) was higher than for the
not be an open coordination site for interaction with silacyclo- racemic alkene (50 : 50 d.r.). Although this difference is small,
propane 2 to initiate catalysis, considering the small number it is beyond experimental error, and it was reproducible. This
of ligands that can coordinate to copper or silver.13 In contrast, observation might result if complexes containing two alkenes
silylene transfer proceeded smoothly over 30 minutes when were present in the transition state for silacyclopropanation.22
bidentate bis(oxazolines)17 (21) or mono(oxazoline) ligands18
(22) were used. The diastereoselectivity of the reaction,
however, was not affected by the chiral ligand. Similar results
were obtained with bis(homoallylic)ether 4 and a larger set of
ligands (Fig. 2).
ð3Þ
The lack of influence of the ligand on stereoselectivity
could be explained in different ways. It could result because,
in the transition state for silylene transfer, the substituents on
The use of chiral ligands for copper in the silacyclopropana-
the chiral ligand are too far from the developing stereocenter
tion of homoallylic ether 23 did not alter the diastereo-
to control the facial selectivity. Alternatively, the homoallylic
selectivity to any appreciable degree (Fig. 4). The presence of
stereocenter could be large enough that it did not permit the
these ligands also had little effect on the rates of conversion of
development of an optimal geometry to observe ligand-
the reaction. This observation contrasts with the silylene trans-
controlled selectivity. Because the reactivity of 4 was also
fer to homoallylic ether 18, where the presence of ligands 8
unaffected by most ligands used, this possibility is less likely,
and 20 (Fig. 3, above) and bis(homoallylic) ether 4, where
however.
ligand 12 (Fig. 2, above) blocked silacyclopropanation. These
In contrast, when less sterically congested secondary homo-
results suggest that the lack of reactivity with homoallylic ether
allylic ethers were used, the reaction efficiency was sensitive to
18 was not caused by ineffective catalysis by a complex
both the metal–ligand complex and the size of the substituent
between ligand 20 and the copper salt. Instead, it suggests that
on the alkene. Racemic homoallylic ethers 23 and 24, which
the resulting metal–ligand–silylene complex was too sterically
differ only in the size of the silyloxy group on the oxygen atom,
hindered to transfer the silylene unit to the hindered alkene,
were subjected to silylene transfer conditions (Table 2).
therefore terminating the catalytic cycle for silylene transfer.20
Substrates protected as the larger tert-butyldiphenylsilyl ether
Silylene transfer to the enantiomeric homoallylic ether (R)-
(24) reacted more slowly than the substrates protected as the
23 and enantiomeric ligands (R)- and (S)-BINAP were evaluated
smaller tert-butyldimethylsilyl ether 23.19 This effect was con-
in this series to discern how diastereomeric pairs of com-
sistent for both silver and copper catalysts. This sensitivity to
ponents might affect the efficiency of the silylene transfer reac-
the steric size of the substituent suggests some interaction of
tion (eqn (4)). These experiments did not result in any vari-
the alkene substituents with the approaching metal–silylene
ation in diastereoselectivity or conversion, again indicating
complex in the transition state for silacyclopropanation, con-
that bis(phosphines) are not the optimal ligands for develop-
sidering that the generation of the metal–silylene complex
ing enantioselective processes involving metal silylenes. The
likely does not involve the alkenes 23 or 24.20
results do indicate, however, that mismatched stereochemistry
To determine the feasibility of the development of an asym-
was not the source of diminished yields and poor selectivities
metric silylene transfer reaction, the silacyclopropanation of
when phosphines were used. They also indicate that the ligand
enantiomerically enriched alkene (S)-23, prepared using an
is located far from the reactive alkene during the silylene trans-
asymmetric
allylation
reaction,21
was
evaluated.
fer step. Similar results were obtained with ether 4 and the
enantiomeric BINAP ligands.
Silacyclopropanation with both silver and copper catalysts
occurred with modest diastereoselectivity (eqn (3)). In the case
Table 2 Steric effects of homoallylic ethers 23 and 24
Entry
MX
R
Conversiona (%)
d.r.a
1
2
3
4
AgOTs
[Cu(OTf)]2·PhH
AgOTs
Me
Me
Ph
Ph
55
42
26
24
62 : 48
50 : 50
50 : 50
50 : 50
[Cu(OTf)]2·PhH
a Determined by 1H NMR spectroscopy.
Fig. 4 Ligand screening for alkene 23.
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calculations (B3LYP/6-31G*) on a model system with a SiMe3
i
group in place of the more computationally demanding Pr3Si
ð4Þ
group.25 Low-energy conformers of both isomers would likely
place the large silyloxy group anti to the di-tert-butylsilyl
group. The syn isomer 29a places the methyl group gauche to
the methylene unit of the silacyclopropane (and therefore also
near a tert-butyl group), an interaction that is absent in the
lowest energy conformers of the anti isomer 29b.
Computationally, this interaction leads to an approximately
1 kcal mol−1 difference in energy between the isomers.
Experiments with allylic silyl ethers, such as the chiral,
non-racemic alkene 28, provided additional evidence that the
silylene transfer reaction is sensitive to the size of the metal–
ligand complex (eqn (5)). When alkene 28 was subjected to the
silylene transfer conditions in either the absence or presence
of chiral ligands, diastereoselectivity was not affected.8 Trends
in reactivity, however, were evident. When ligands were
employed, reaction times were extended and conversion was
diminished. In particular, the use of bulky phosphine ligands
such as (S,S)-NORPHOS (30) and (S,S)-DIOP (31) completely
inhibited the reaction (eqn (6)), in contrast to their minimal
effects with homoallylic silyl ethers (eqn (4)).23
i
Considering that the experiments involved the larger Pr3Si
ether,19 these interactions should be even more destabilizing,
leading to the observed selectivity (eqn (7)).
A variety of mechanisms for metal-catalyzed silylene trans-
fer can be envisioned (Fig. 6). The simplest mechanism (a) is
one in which the metal facilitates the liberation of a free
silylene, which then undergoes concerted silacyclopropanation
with an alkene. This mechanism is unlikely, however. If this
mechanism were operating, then the influence of the ligand
on the reaction rate would have been the same for all sub-
strates, which was not the case.
A second mechanism, which involves the dissociation of
the ligand from the metal silylene prior to transfer to the
alkene (b), can also be discounted. In such a mechanism, the
ligand could not completely inhibit the reactivity of the silyl-
ene unless the silver catalyst was completely sequestered by
the ligand. This response would be consistent for all alkenes
examined, which is not the case: some ligands slowed reac-
tions with some alkenes but not with others.
ð5Þ
ð6Þ
Despite the difficulty associated with coaxing sterically
encumbered alkenes to react, stereoselectivity could be influ-
enced if the silylene transfer reactions were reversible.
Although the silver-catalyzed reaction of the hindered triiso-
propylsilyl ether 28 was irreversible, the copper-catalyzed reac-
tion was reversible (eqn (7)). When the copper-catalyzed reac-
tion was allowed to equilibrate overnight, a 90 : 10 ratio of
isomers was obtained. Even after this long reaction time,
however, rearrangement to allylic silanes was not observed.24
The most reasonable explanation for these results involves
a mechanism in which both the ligand and alkene are involved
in the stereochemistry-determining step (c). The rate of any
particular reaction would depend upon the combination of the
two factors, the substitution of the alkene and the nature of
the ligand on the metal–silylene complex. This trend was
ð7Þ
Although the stereochemical configuration of the major
diastereomer of silacyclopropane 29 could not be assigned
unambiguously, the thermodynamic product is likely to be the
anti isomer 29b (Fig. 5). This assignment was made by consid-
ering the conformational preferences of these sterically con-
gested products. Conformational analysis of each isomer of
the silacyclopropane was aided by density-functional theory
Fig. 5 Low energy conformers of syn and anti 29.
Fig. 6 Possible mechanisms for silylene transfer to an alkene.
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observed: the lowest levels of reactivity were observed when a (125 MHz, C6D6, diagnostic peaks) δ 132.5, 132.4, 130.5, 130.2,
strongly donating ligand was combined with a sterically 71.0, 70.7, 30.3, 30.2, 19.9, 19.8, 18.88, 18.86, 17.7, 17.4, 3.3,
encumbered alkene. The fact that the ligand could not control 2.9; 29Si NMR (99 MHz, C6D6, diagnostic peaks) δ −48.1, −48.2.
the facial selectivity only indicates that the interaction between
Silacyclopropanes 19a and 19b
the ligand and the substituents on the alkene did not differen-
tiate between diastereomeric transition states.
The representative procedure for the synthesis of silacyclo-
propanes was followed using alkene 18 (0.100 mL, 0.505 M in
C6D6, 0.0505 mmol), silacyclopropane 2 (0.100 mL, 0.548 M in
C6D6, 0.0548 mmol), mesitylene (0.0040 mL, 0.028 mmol,
internal standard), and AgOTs (0.100 mL, 0.025 M in C6D6,
0.0025 mmol) in C6D6 (0.200 mL). Silacyclopropanes 19a and
19b were formed in 96% combined yield based on the com-
parison of the area of the standard peak (δ 6.72) and the
methoxy protons (71 : 29 d.r.): 1H NMR (600 MHz, C6D6,
diagnostic peaks) δ 3.12 (s, 1.2H), 3.05 (s, 3H), 2.51 (dd, J =
15.7, 2.2, 1H), 1.99 (dd, J = 15.5, 6.2, 0.4H), 1.18 (s, 3.6H), 1.14
(s, 9H), 1.07 (s, 3.6H), 1.05 (s, 9H), 0.40–0.36 (m, 1H), 0.30 (dd,
J = 10.6, 9.5, 0.4H); 13C NMR (125 MHz, C6D6, diagnostic
peaks) δ 87.3, 87.2, 53.5, 53.1, 51.0, 50.9, 48.4, 47.9, 45.8, 45.5,
42.4, 42.3, 38.8, 38.7, 31.2, 31.0, 30.4, 30.3, 22.2, 22.0,
21.8, 21.7, 13.5, 12.6, 7.8, 7.2; 29Si NMR (99 MHz, C6D6)
δ −49.2, −49.8.
Conclusion
In conclusion, the stereoselective formation of sila-
cyclopropanes is possible if the reaction were reversible and
the alkene were sufficiently bulky. Through a systematic study
of different alkenes, we determined that the steric factors out-
weigh the electronic ones in the metal-catalyzed transfer of
silylenes to alkenes. These observations are consistent with a
mechanism involving the silylene, metal atom, ligand, and
alkene in the stereochemistry-determining step.
Experimental details
General procedures
General procedures are provided in the ESI.†
Silacyclopropanes 25a and 25b
The representative procedure for the synthesis of silacyclo-
propanes was followed using alkene (S)-23 (0.100 mL, 0.503 M
in C6D6, 0.0503 mmol), silacyclopropane 2 (0.100 mL, 0.550 M
in C6D6, 0.0550 mmol), mesitylene (0.0020 mL, 0.014 mmol,
internal standard), and [CuOTf]2·PhH (0.060 mL, 0.013 mM in
C6D6, 0.0016 mmol Cu) in C6D6 (0.240 mL). Silacyclopropanes
25a and 25b were formed in 82% yield based on the compari-
son of the area of the standard peak (δ 6.72) and the methine
protons (52 : 48 d.r.): 1H NMR (500 MHz, C6D6, diagnostic
peaks) δ 7.44 (d, J = 7.5, 2H), 7.41 (d, J = 7.5, 2H), 7.19 (t, J =
7.6, 4H), 7.10–7.07 (m, 2H), 4.86–4.80 (m, 2H), 2.43–2.28 (m,
2H), 2.09–1.98 (m, 2H), 1.18–1.13 (m, 1H), 1.08 (s, 9H), 1.06 (s,
9H), 1.04 (s, 9H and m, 1H), 1.04 (s, 9H), 1.03 (s, 9H), 1.02 (s,
18H), 0.91–0.85 (m, 2H), 0.31–0.23 (m, 2H), 0.17 (s, 3H), 0.16
(s, 3H), −0.02 (s, 3H), −0.04 (s, 3H); 13C NMR (125 MHz, C6D6,
diagnostic peaks) δ 147.1, 146.8, 127.73, 127.69, 126.83,
126.76, 78.5, 77.1, 44.8, 44.7, 31.2, 31.1, 30.18, 30.15, 26.6,
26.5, 11.5, 10.5, 3.9, 3.7, −3.87, −3.91, −4.1, −4.3; 29Si NMR
(99 MHz, C6D6, diagnostic peaks) δ −48.7, −49.6.
Representative procedure for silylene transfer without a ligand
(silacyclopropanes 5a and 5b)
To a solution of alkene 4 (0.100 mL, 0.496 M in C6D6,
0.0496 mmol) was added silacyclopropane 2 (0.110 mL, 0.750
M in C6D6, 0.0830 mmol), mesitylene (0.0020 mL, 0.014 mmol,
internal standard), and AgO2CCF3 (0.110 mL, 0.014 M in C6D6,
0.0015 mmol) in C6D6 (0.185 mL). Silacyclopropanes 5a and 5b
were formed in 88% combined yield based on the comparison
of the area of the standard peak (δ 6.72) and the silacyclo-
1
propane protons (54 : 46 d.r.): H NMR (600 MHz, C6D6, diagnos-
tic peaks) δ 4.08–3.99 (m, 2H), 1.69–1.59 (m, 2H), 1.24 (d, J =
6.1, 3H), 1.23 (d, J = 6.1, 3H), 1.17–1.15 (m, 42H), 1.15 (s, 9H),
1.039 (s, 9H), 1.037 (s, 9H), 0.94–0.87 (m, 4H), 0.32–0.24 (m,
2H); 13C NMR (150 MHz, C6D6, diagnostic peaks) δ 69.37,
69.36, 43.63, 43.58, 24.40, 24.36, 19.43, 19.38, 18.89, 18.86,
18.81, 18.78, 14.9, 14.8, 13.33, 13.26, 4.2, 4.1; 29Si NMR
(99 MHz, C6D6, diagnostic peaks) δ −49.0, −49.1.
Vinylsilacyclopropanes 7a and 7b
Silacyclopropanes 26a and 26b
The representative procedure for the synthesis of silacyclo-
propanes was followed using diene 6 (0.100 mL, 0.510 M in The representative procedure for the synthesis of silacyclo-
C6D6, 0.0510 mmol), silacyclopropane 2 (0.110 mL, 0.750 M in propanes was followed using alkene 24 (0.019 g, 0.051 mmol),
C6D6, 0.0830 mmol), mesitylene (0.0020 mL, 0.014 mmol, silacyclopropane 2 (0.100 mL, 0.550 M in C6D6, 0.0550 mmol),
internal standard), and AgO2CCF3 (0.085 mL, 0.018 M in C6D6, mesitylene (0.0020 mL, 0.014 mmol, internal standard), and
0.0015 mmol) in C6D6 (0.205 mL). Vinylsilacyclopropanes 7a AgO2CCF3 (0.0004 g, 0.0022 mmol) in C6D6 (0.400 mL).
and 7b were formed in 93% combined yield based on the com- Silacyclopropanes 26a and 26b were formed in 26% combined
parison of the area of the standard peak (δ 6.72) and the yield based on the comparison of the area of the standard
1
alkene protons (51 : 49 d.r.): H NMR (500 MHz, C6D6, diagno- peak (δ 6.72) and the methine protons (50 : 50 d.r.): 1H NMR
stic peaks) δ 5.95 (dd, J = 15.1, 7.6, 1H), 5.89 (dd, J = 15.2, 7.5, (500 MHz, C6D6, diagnostic peaks) δ 7.91–7.88 (m, 4H),
1H), 5.64–5.55 (m, 2H), 1.36–1.34 (m, 6H), 1.13 (s, 9H), 1.12 (s, 7.67–7.65 (m, 2H), 7.63–7.62 (m, 1H), 7.46–7.44 (m, 2H),
9H), 0.98 (s, 9H), 0.97 (s, 9H), 0.77–0.71 (s, 2H); 13C NMR 7.37–7.36 (m, 1H), 4.99 (dd, J = 8.5, 4.9, 1H), 4.90–4.80 (m,
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1H), 2.32 (dd, J = 8.3, 6.7, 2H), 2.29–2.24 (m, 2H), 1.20 (s, 9H),
1.18 (s, 9H), 0.97 (s, 9H), 0.91 (s, 18H), 0.85 (s, 9H), 0.56 (dd,
J = 12.2, 10.8, 1H), 0.08 (dd, J = 10.6, 9.2, 1H), −0.02 to −0.06
(m, 1H).
M. Tanaka, H. Fujiyama and H. Sakurai, Organometallics,
2006, 25, 6159–6161.
8 See the ESI† for more details.
9 For a review discussing this family of phosphine ligands,
see: H. Shimizu, I. Nagasaki and T. Saito, Tetrahedron,
2005, 61, 5405–5432.
Silacyclopropanes 29a and 29b
10 Homoallylic ether 18 was prepared from (R)-camphor and
the literature precedent suggests that the alkoxy substituent
occupies the exo position. See: V. Dimitrov, S. Simova and
K. Kostova, Tetrahedron, 1996, 52, 1699–1706.
11 J. Ćiraković, T. G. Driver and K. A. Woerpel, J. Am. Chem.
Soc., 2002, 124, 9370–9371.
12 A. K. Franz and K. A. Woerpel, J. Am. Chem. Soc., 1999, 121,
949–957.
13 For a discussion of copper and silver coordination chem-
istry, see: L.-J. Baker, G. A. Bowmaker, R. D. Hart,
P. J. Harvey, P. C. Healy and A. J. White, Inorg. Chem., 1994,
33, 3925–3931.
14 P. A. Cleary and K. A. Woerpel, Org. Lett., 2005, 7, 5531–
5533.
15 For structural studies of tridentate bis(oxazoline) ligands to
silver and copper salts, see: H.-W. Kuai, X.-C. Cheng,
D.-H. Li, T. Hu and X.-H. Zhu, J. Solid State Chem., 2015,
228, 65–75.
Note: Enantiomeric substrate ent-28 was used. The representa-
tive procedure for the syntheses of silacyclopropanes was fol-
lowed using alkene ent-28 (0.100 mL, 0.485 M in C6D6,
0.0485 mmol), silacyclopropane 2 (0.100 mL, 0.550 M in C6D6,
0.0550 mmol), mesitylene (0.0020 mL, 0.014 mmol, internal
standard) and AgO2CCF3 (0.100 mL, 0.0145 M in C6D6,
0.00145 mmol) in C6D6 (0.200 mL). Silacyclopropanes ent-29a
and ent-29b were formed in 77% yield based on the compari-
son of the area of the standard peak (δ 6.72) and the methine
proton (54 : 46 d.r.): 1H NMR (600 MHz, C6D6, diagnostic
peaks) δ 4.22–4.12 (m, 2H), 1.48 (d, J = 6.0, 3H), 1.46 (d, J = 6.0,
3H), 1.18 (s, 9H), 1.08 (s, 9H), 1.06 (s, 9H), 0.97 (s, 9H), 0.77
(dd, J = 12.7, 10.9, 1H), 0.58 (dd, J = 11.1, 9.3, 1H), 0.19 (t, J =
10.4, 1H); 13C NMR (125 MHz, C6D6, diagnostic peaks) δ 72.8,
72.1, 31.00, 30.96, 30.22, 30.18, 29.6, 29.5, 19.2, 19.1, 19.0,
13.9, 13.7, 4.1, 1.0; 29Si NMR (99 MHz, C6D6, diagnostic peaks)
δ −47.4, −48.9.
16 For a review covering silver-catalyzed atom-transfer reac-
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19 Replacing a methyl group on the silicon atom with a
phenyl group increases the volume of a silyl group, see:
J. S. Panek, A. Prock, K. Eriks and W. P. Giering,
Organometallics, 1990, 9, 2175–2176.
20 T. G. Driver and K. A. Woerpel, J. Am. Chem. Soc., 2004, 126,
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21 G. E. Keck, K. H. Tarbet and L. S. Geraci, J. Am. Chem. Soc.,
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22 For a discussion of the origin of non-linear effects in asym-
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23 For asymmetric transformations using DIOP and
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Acknowledgements
This research was supported by the National Science
Foundation (CHE-1362709). C. Z. R. thanks the Margaret
Strauss Kramer Graduate Fellowship (NYU) for fellowship
support. K. A. W. thanks the Global Research Initiative, NYU
and NYU Florence, for a fellowship. The NMR spectra collected
were supported by an S10 grant from the National Institutes of
Health (OD016343). We thank Dr Chin Lin (NYU) for his assist-
ance with NMR spectroscopy, mass spectrometry, and optical
rotation.
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