Ring opening in the hydrostannation of methylenecyclopropanes: Effect of the catalyst and substrate

Full text of DOI:10.1021/ja962582j

10676
J. Am. Chem. Soc. 1996, 118, 10676-10677
Table 1. Palladium-Catalyzed Hydrostannations of
Methylenecyclopropanes
Ring Opening in the Hydrostannation of
Methylenecyclopropanes: Effect of the Catalyst and
Substrate
substrate
product (yield)a
Mark Lautens,* Christophe Meyer, and Alexander Lorenz
Department of Chemistry, UniVersity of Toronto
Toronto, Ontario Canada M5S 3H6
ReceiVed July 26, 1996
The generation of organometallic species by hydrometalation
of carbon-carbon multiple bonds is a fundamental reaction in
synthetic organic chemistry.1 Of particular interest in this field
is the hydrostannation reaction, due to the numerous possibilities
of further manipulation of the newly formed carbon-tin bond.2
In general, the hydrostannation of carbon-carbon multiple
bonds has been achieved under radical conditions3 or by using
transition metal catalysis.4 However, the methods described
were found to be unsuitable for a wide range of simple
unactiVated alkenes. Indeed, the reversibility of the addition
of the tributyltin radical onto a carbon-carbon double bond in
the absence of a stabilizing group generally restricts this reaction
to activated alkenes.3b On the other hand, little information
concerning the metal-catalyzed processes was available and
essentially dealt with conjugated reductions of alkenes.4a
We recently reported the hydrostannation of oxabicyclic
compounds using soluble palladium catalysts5 and found that,
in the case of unactivated alkenes, the only reaction observed
was the disproportionation of tributyltin hydride to hexa-
butylditin and molecular hydrogen.3b,4 Fortunately, a heterog-
enous catalyst, such as Pd(OH)2/C, successfully catalyzed the
hydrostannation of a wide variety of alkenes, providing a general
route to synthetically useful alkylstannanes.6
In the course of our studies, we were interested to assess the
reactivity of other types of alkenes in hydrostannation reactions,
and we now report our results with methylenecyclopropanes.
The three reactive sites in a methylenecyclopropane are the
vicinal and distal cyclopropane bonds and the olefin. The
majority of the palladium-catalyzed reactions (including [3 +
2] cycloadditions with alkenes and alkynes,7 (trimethylsilyl)-
cyanation of 2-aryl or 1-substituted methylenecyclopropanes,8
and chloro-9 and carbopalladation10)10 occur at the distal
cyclopropane bond or the exocyclic olefin. Therefore, it was
of interest to see at which site the palladium-catalyzed hy-
drostannation of methylenecyclopropanes would occur.
a Isolated yields of analytically pure products. b Bu3SnH (1.5 or 3
equiv) was added over 1 or 1.5 h to a 0.1 M solution of the substrate
in THF containing Pd(PPh3)4 (3 to 5 mol %) or Pd(OH)2/C (5 mol %).
c See text.
We report herein the different pathways in the hydrostanna-
tion of methylenecyclopropane derivatives in the presence of
homogeneous and heterogeneous catalysts.
When (methylenecyclopropyl)carbinols 111 were treated with
a slight excess (1.5 equiv) of tributyltin hydride in THF in the
presence of a catalytic amount of tetrakis(triphenylphosphine)-
palladium (3-5 mol %), the corresponding ring opened ho-
moallylstannanes 2 were obtained in good yields and as single
diastereoisomers (Table 1). We could not isolate the products
resulting from simple hydrostannation of the double bond even
by lowering the temperature of the reaction mixture to -20 °C,
indicating that methylenecyclopropanes were significantly more
reactive than “simple” olefins.6 Nevertheless, as a precaution,
tributyltin hydride was usually added slowly over 1 h or more,
in order to prevent its rapid decomposition by the palladium
catalyst.
Reaction of 1 performed under heterogeneous conditions, with
Pd(OH)2/C as a catalyst, gave a mixture of homoallylstannanes
2 and diorganostannanes 3 resulting from further hydrostanna-
tion of 2. Increasing the amount of tin hydride to 3 equiv
enables the selective formation of 3a-e in good yields. We
independently showed that 2a-e can be hydrostannated under
heterogeneous conditions which confirms that they are inter-
mediates in the formation of the distannanes 3a-e.
For 1e, the corresponding homoallylstannane 2e could not
be isolated as a pure compound because, on silica gel, it suffers
stannodemethoxylation12 to give the corresponding dienyl
alcohol.13
(1) Labinger, J. A.; Smith, K.; Pelter, A.; Eisch, J. J.; Hiyama, T.;
Kusumoto, T. In ComprehensiVe Organic Synthesis; Trost, B. M., Ed.;
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S0002-7863(96)02582-6 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10677
Scheme 1
Scheme 2
noticed.4f,h Under heterogeneous conditions, further hydrostan-
nation of the less reactive double bond of 2 can readily occur
as previously mentioned to give the diorganostannane 3.6
We then turned out our attention to the case of methylene-
cyclopropanes bearing substituents on the double bond. Me-
thylenecyclopropane 410d gave exclusively the (E)-homo-
allylstannane 5 in very good yield, irrespective of the choice of
the catalyst (Table 1). The high reactivity of 4 is noteworthy,
given the reluctance of unactivated trisubstituted or R,â-
disubstituted alkenes such as 5 to undergo hydrostannation, even
using a heterogeneous catalyst.6
The mono- and the diorganostannanes 2 and 3 are all obtained
as single diastereosiomers. Diastereoisomers 1c and 1c′ gave
the diastereoisomeric homoallylstannanes 2c and 2c′. Moreover,
we have checked that the relative stereochemistry of the
homoallylstannane 2a corresponds to that of the parent com-
pound 1a, establishing that there is no loss of the stereochemical
information initially present in the (methylenecyclopropyl)-
carbinol 1 throughout the course of the reaction. Indeed,
transmetalation of the organostannane 2a to the organo-
lithium,14,15 followed by protonolysis afforded the corresponding
known syn homoallylic alcohol.16
Reactions with benzylidenecyclopropane 617 deserve further
comment (Scheme 2). When the latter compound was treated
with a slight exceess (1.5 equiv) of tributyltin hydride at room
temperature, a 60/40 mixture of the (E)-homoallylstannane 7
and the diorganostannane 8 was obtained, since under these
conditions, the activated double bond of the intermediate 7 is
hydrostannated with the homogeneous catalyst. However,
lowering the temperature of the reaction mixture to -23 °C
enables the selective formation of 7, which was isolated in 50%
yield.18 The very mild conditions required for this reaction as
well as the formation of a single isomer (E) are noteworthy
compared to the rhodium-catalyzed hydrosilylation of 6.19 The
most surprising result is that the heterogeneous palladium-
catalyzed hydrostannation gave the benzylstannane 9 as the
major product and the bisorganostannane 8, isolated in 49 and
16% yields, respectively.18 Clearly, in this case, the reductive
elimination of the intermediate benzyl tributyltin palladium
postulated species is somewhat faster than the ring opening
reaction.
To explain the formation of compounds 2 and 3, we propose
the mechanism shown in Scheme 1.
Oxidative addition of a zero-valent palladium into the tin-
hydrogen bond of tributyltin hydride first generates a stan-
nylpalladium hydride species which hydropalladates the meth-
ylenecyclopropane derivative, thus affording a (cyclopropyl-
methyl)palladium stannane. The latter undergoes a highly
regioselective ring opening reaction, generating a primary
homoallylpalladium stannane rather than a secondary or a
tertiary one, which subsequently undergoes reductive elimination
to yield the homoallylstannane 2. The fact that there is no loss
of the stereochemical information present in the parent com-
pound 1 indicates that the reductive elimination of the homo-
allylstannylpalladium intermediate is faster than the correspond-
ing â-elimination observed in related carbopalladation reac-
tions.10 Under homogeneous conditions, the reaction stops at
this stage and the palladium complex subsequently catalyzes
the disproportionation of tributyltin hydride, which is announced
by a sudden darkening of the reaction mixture as previously
In conclusion, we have described the first examples of
palladium-catalyzed hydrostannation of methylenecyclopropane
derivatives. This reaction provides a regioselective and ste-
reospecific entry to a variety of homoallylstannanes that can
be useful for organic synthesis.
Acknowledgment. The E. W. R. Steacie Fund administered by the
Natural Science and Engineering Research Council (NSERC) of
Canada, the Merck Frosst Centre for Therapeutic Research, Upjohn/
Pharmacia (USA), and Eli Lilly (USA) are thanked for financial support.
C.M. thanks the Ministe`re Franc¸ais des Affaires Etrange`res for a
Lavoisier fellowship. The authors thank Eric Fillion and Shawn
Johnstone for helpful discussions.
(12) Koerber, K.; Gore´, J.; Vatele, J.-M. Tetrahedron Lett. 1991, 32,
1187.
(13) Filtration through a short plug of neutral alumina enabled us to
characterize 2e. In contrast, the corresponding diorganostannane 3e could
be purified by rapid chromatography on silica gel, which suggests the greater
instability of 2e is attributed to the presence of the double bond enabling
the formation of a conjugated diene.
Supporting Information Available: General experimental proce-
dures, specific details for representative reactions, and isolation and
spectroscopic information for the compounds prepared (9 pages). See
any current masthead page for ordering and Internet access instructions.
(14) (a) Meyer, N.; Seebach, D. Chem. Ber. 1980, 113, 1290. (b)
Newman-Evans, R. H.; Carpenter, B. K. Tetrahedron Lett. 1985, 26, 1141.
(c) Brieden, W.; Ostwald, R.; Knochel, P. Angew. Chem., Int. Ed. Engl.
1993, 32, 582.
(15) See Supporting Information. Quenching the organolithium with
AcOD gave the corresponding monodeuterated homoallylic alcohol with
g95% deuterium incorporation.
JA962582J
(17) (a) Utimoto, K.; Tamura, M.; Sisido, K. Tetrahedron 1973, 29, 1169.
(b) Stafford, J. A.; McMurry, J. E. Tetrahedron Lett. 1988, 29, 2531.
(18) Separation of each of these nonpolar compounds and hexabutylditin
by chromatography on silica gel is not readily accomplished, therefore the
yields reported have not been optimized.
(16) (a) Yamamoto, Y.; Yatagai, H.; Naruta, Y.; Maruyama, K. J. Am.
Chem. Soc. 1980, 102, 7109. For a review on allylic organometallics, see:
(b) Yamamoto, Y.; Asao, N. Chem. ReV. 1993, 93, 2207.
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I. P. Metalloorg. Khim. 1993, 6, 30. (b) Blinov, K. A.; Tveritinova, E. V.;
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