Table 1. Cyclopropanation of vinyl silanes by diazomethane.[a]
particles generated in situ.[41] Importantly, the adjunction of
N or P donor ligands to the Pd catalyst completely inhibits
the cyclopropanation reaction,[33,41] that is, the coordinating
ability of the olefin is essential for effective Pd-catalyzed cy-
clopropanation.[20] Our previous success in the use of PdII
salts and diazo alkanes for the cyclopropanation of vinyl
boronates[39,46] and related tin analogues[47] suggested that al-
kenyl silanes would be ideal substrates, since they coordi-
nate rather strongly to late transition metals.[48,49] Moreover,
the paucity of reports on the use of this catalyst system on
alkenyl silanes and, to the best of our knowledge, the ab-
sence of data on its scope and limitations further spurred
our interest.[25,29,32]
Entry
1
Vinyl silane
Product
Yield [%][b]
83 (100)
2[c]
47 (100)[d]
3
4
5
6
86 (100)
90 (100)
87 (100)
91 (100)
Herein we report on the use of alkenyl silanes as sub-
strates for Pd-catalyzed cyclopropanation. These olefins can
be efficiently transformed in the presence of diazomethane
into the corresponding silyl cyclopropanes. The effect of the
silyl substituents on the outcome of the reaction was investi-
gated. The scope of this methodology was further extended
by the use of diazoethane and diazobutane, which enabled
diastereoselective formation of substituted silyl cyclopro-
panes. The cyclopropanation of a divinyl siloxane derivative
provided some insight into the coordination sphere of the
active species and revealed a Pd0 resting state in the catalyt-
ic cycle. The use of a homoleptic Pd0(vinyl silane)3 complex,
as an extremely efficient catalyst, at À358C, provided fur-
ther evidence for a Pd0 resting state. The stabilization of the
active Pd0 species by vinyl silanes was corroborated by a
DFT study, and the reaction pathway with this class of ole-
fins was calculated.
[a] Reaction conditions: vinyl silane (1 equiv), PdACTHNUGTRENUNG(OAc)2 (0.5 mol%),
CH2N2 (6 equiv based on the amount of N-methyl-N-nitroso urea used),
08C, Et2O (1.0m), 10 min. [b] Yield of isolated product (GC conversion).
[c] 4 equiv of CH2N2 used. [d] Volatile compound.
remain in solution for up to 1 h (as attested by titration)
without detrimental side effects if the mixture is kept at
58C. In contrast to previous reports, it appears that decom-
position of diazomethane to ethylene (or polymethylene) is
not catalyzed efficiently by the active species involved in the
cyclopropanation reaction.[41] On the other hand, diazome-
thane decomposes quickly in the presence of the Pd black
formed during the cyclopropanation of other olefins.
We were especially interested in vinyl siloxanes 5 and 6
(Table 1, entries 5 and 6), since they are considerably more
stable towards moisture and less toxic than their alkoxy
vinyl silane counterparts. They were converted in excellent
yields to the desired cyclopropane adducts 11 and 12, which
were easily purified by chromatography on silica gel without
any hydrolysis. These results are complementary to the Sim-
mons–Smith cyclopropanation of vinyl silanes and vinyl sila-
nols.[50–52]
Results and Discussion
Cyclopropanation of alkenyl silanes by diazomethane: We
explored the influence of electronically diverse substituents
on the cyclopropanation of monosubstituted vinyl silanes
(Table 1). To the best of our knowledge, only the cyclopro-
panation of triethoxyvinylsilane (4) by this method had
been previously described.[29] Standard conditions, that is,
addition of a solution of diazomethane in Et2O (6 equiv,
1.0m) to a solution of vinyl silane in Et2O (1.0m) containing
At this stage of our investigations, we observed no signifi-
cant influence of the nature of the substituents on silicon on
the course of the reaction.
The scope of this process was extended by subjecting
trans-alkenyl silanes to the above reaction conditions
(Table 2). The unsaturated substrates were obtained by our
recently reported regioselective Pt-catalyzed hydrosilylation
of alkynes.[53] These olefins were also cyclopropanated
smoothly and with excellent yields. However, the conversion
drops significantly when sterically hindered, trisubstituted
vinyl silanes are employed (Table 2, entry 4). In these cases,
formation of Pd black particles is also observed. Interesting-
a suspension of PdACHTNUGTRNEUNG(OAc)2 (0.5 mol%) at 08C (Table 1),
were applied. The cyclopropanation of vinyl silanes was vis-
ually very different from that of other classes of alkenes, in
which palladium black typically appears immediately after
initial addition of diazomethane. In marked contrast, with
vinyl silanes the light-orange reaction medium became per-
fectly clear and homogeneous after the addition of diazome-
thane, and this suggests that vinyl silanes may be able to sta-
bilize reactive palladium species. Monosubstituted vinyl si-
lanes reacted smoothly and quantitatively to afford the de-
sired silyl cyclopropanes without any byproducts detectable
by GC analysis. This reaction is essentially a titration; the
color remains yellow after all of the vinyl silane has been
converted to the corresponding silyl cyclopropane. The
excess diazomethane, added at the end of the reaction, can
ly, 1,2-bisACTHNUTRGNEUG(N silyl) alkene 17 (Table 2, entry 5) is also cyclopro-
panated efficiently. Thus, the electronic activation provided
by the two silyl groups overcomes the steric hindrance they
create around the double bond in the substrate.
Surprised by the relative stability of the active catalyst,
we lowered the amount of PdACHTNUGRTENUNG(OAc)2 and diazomethane
used. We were able to obtain high conversion with a palladi-
um loading as low as 5ꢀ10À3 mol%. Only 3 equiv of CH2N2
2924
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 2923 – 2931