Ring-opening of cyclopropanes by "frustrated Lewis pairs"

Article abstract of DOI:10.1039/c0cc02862b

Reactions of phosphine/borane frustrated Lewis pairs with cyclopropanes result in the ring opening, yielding phosphonium borate products.

Full text of DOI:10.1039/c0cc02862b

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Ring-opening of cyclopropanes by ‘‘frustrated Lewis pairs’’w
Jason G. M. Morton, Meghan A. Dureen and Douglas W. Stephan*
Received 28th July 2010, Accepted 1st October 2010
DOI: 10.1039/c0cc02862b
Reactions of phosphine/borane frustrated Lewis pairs with
cyclopropanes result in the ring opening, yielding phosphonium
borate products.
Frustrated Lewis pairs (FLPs) have been shown to react with a
variety of reagents other than H2.1–5 These include terminal
olefins,6 alkynes,7 B–H bonds,8 disulfides,9 the C–O bonds of
cyclic ethers,10 CO2 and N2O.12–15 But for the heterolytic
11
cleavage of disulfides,9 these reactions of FLPs generally
involve molecules with either a multiple bond and/or markedly
Scheme 1 Synthesis of 1 and 2.
polar character. In the activation of molecules with hetero-
atoms or p bonds, intuitively one can imagine electron donation
subsequently confirmed crystallographically (Fig. 1),z the
to the Lewis acid of the FLP polarizes the substrate and
prompts attack by the nucleophilic base. In the case of the
heterolytic cleavage of H2, computational studies16–21 of this
intriguing reaction infer that the field generated by the sterically
restricted approach of an electron donor to the electron
deficient Lewis acid is sufficient to polarize the otherwise
non-polar H2 molecule and thus induce heterolytic cleavage.
The above observations prompt questions about the ability
of FLPs to effect the heterolytic cleavage of other bonds. In
this regard our attention turned to C–C bonds. A number of
studies have exploited transition metal catalyzed or Lewis
acid activators to effect the ring-opening of functionalized
cyclopropanes such as donor-acceptor cyclopropanes22 and
vinylidene cyclopropanes.23–26 In contrast, alkyl or aryl
substituted cyclopropanes are much less studied as such
systems are less activated as they do not possess a ‘‘handle’’
for activation by a metal or a Lewis acid. Herein, we demonstrate
that FLPs effect the heterolytic ring-opening of such cyclo-
propanes, affording zwitterionic products derived from C–C
bond scission.
metric parameters were unexceptional.
The analogous reaction of B(C6F5)3 and tBu3P with the
cyclopropane Ph2CQCCHC3H5 resulted in the formation of
the zwitterionic phosphonium borate tBu3PCH(CHQCPh2)-
CH2CH2B(C6F5)3 2 in 64% yield. This species results from a
similar ring-opening of the cyclopropane, with P–C bond
formation at the substituted carbon atom (Fig. 2).z
It is clear that the formation of products 1 and 2 occurs via
regio-specific cyclopropane ring opening, with formation of
the B–C bond at the unsubstituted methylene carbon and the
P–C bond to the phenyl-substituted carbon. The regiochemistry
is consistent with attack of P at the substituted carbon which
stabilizes the developing cationic charge.
To probe substituent effects on such reactions, the cyclo-
propanes PhHCQCHC3H5 and H2CQCHC3H3Ph2 were also
reacted with B(C6F5)3 and tBu3P (Scheme 2). In the former
case, an approximately 1 : 1.3 mixture of two phosphonium
borate products 3a and 3b was obtained. The spectroscopic
The cyclopropane PhC3H5 was added to a toluene solution
of tBu3P and B(C6F5)3. On standing overnight this reaction
afforded colorless crystals of a new species 1 which were
subsequently isolated in 69% yield. The 11B NMR spectrum
of 1 gave a sharp singlet at ꢀ13.5 ppm. This together with the
19F NMR data were consistent with the formation of a borate
unit. The corresponding 31P{1H} NMR signal was observed at
54.1 ppm consistent with a phosphonium center. Among
the 1H NMR signals are signals at 3.6, 2.1, 1.8, 1.1 and
0.8 ppm with the appropriate couplings indicative of an
alkyl chain. These data lead to the formulation of 1 as
tBu3PCH(Ph)CH2CH2B(C6F5)3 (Scheme 1). While this was
Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, Ontario, Canada, M5S 3H6.
E-mail: dstephan@chem.utoronto.ca
w Electronic supplementary information (ESI) available: Preparative
and spectroscopic data; Crystallographic data: CCDC 786848-786849.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c0cc02862b
Fig. 1 POV-ray depictions of 1, H-atoms have been deleted for
clarity, C: black, F: pink, B: yellow-green, P: orange.
c
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Chem. Commun., 2010, 46, 8947–8949 8947

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acids was observed via low temperature matrix-isolation
spectroscopy.27 Nonetheless, reactions of methylenecyclo-
propanes with nucleophiles such as alcohols in the presence
of Lewis acids are thought to proceed via initial protonation of
the cyclopropane species.28 Conversely, while nucleophilic ring
opening of activated cyclopropanes have been described,23,28–32
no reaction is observed for the combination of the cyclo-
propanes and tBu3P. It is noteworthy that prior theoretical
work,16–21 examining the reactions of FLPs with H2 and
olefins, suggests polarization of the substrate upon interaction
with the ‘‘encounter complex’’ formed by the approach of the
Lewis base with B(C6F5)3. In the case of cyclopropanes, it is
suspected that the process may involve initial Lewis acid
interaction with the cyclopropane prompting a cooperative
nucleophilic interaction with the Lewis base. While this notion
would account for the formation of the products 1, 2 and 3a,
the precise details of the mechanism await a thorough theoretical
examination. In contrast, the formation of 4 suggests that
sterically accessible olefinic fragments are more susceptible to
reaction with FLPs than cyclopropanes.
Herein, the reactions of FLPs with cyclopropanes have been
shown to result in ring opening. For sterically accessible
cyclopropanes, P and B add across one of the C–C bond to
afford three carbon linked phosphonium–borate zwitterions.
The utility of these products and the further reactivity of FLPs
continues to be the focus of efforts in our laboratories.
DWS gratefully acknowledges the financial support of
NSERC of Canada and the award of a Canada Research
Chair and a Killam Research Fellowship.
Fig. 2 POV-ray depictions of 2, H-atoms have been deleted for
clarity, C: black, F: pink, B: yellow-green, P: orange.
Notes and references
z Crystallographic data. 1: P21/n, a = 10.2236(5) A, b = 16.6138(7) A,
c = 21.9968(9) A, b = 100.4010(10)1, V = 3674.8(3) A3, data = 8462,
var = 568, R (>3s): 0.0712, Rw (all) = 0.2410, GOF = 1.030. 2(1.5
CH2Cl2): Pbca, a = 19.5216(6) A, b = 21.1148(6) A, c = 21.3064(7)
A, V = 8782.4(5) A3, data = 9905, var = 602, R (>3s): 0.0601, Rw
(all) = 0.1590, GOF = 1.036.
1 P. A. Chase, A. L. Gille, T. M. Gilbert and D. W. Stephan, Dalton
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Scheme 2 Synthesis of 3 and 4.
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data for 3a are consistent with the ring opening of the
cyclopropane ring, akin to that seen for 1 and 2. The species
3b results from attack of the olefinic carbon with subsequent
migration of the double bond and ring opening of the cyclo-
propane to give the borate fragment. These two products are
thus consistent with SN2 and SN20 processes.
The product of the reaction of B(C6F5)3, tBu3P and
H2CQCHC3H3Ph2 was isolated in 69% yield and
spectroscopically identified as [tBu3PH][Ph2CQCHCHQ
CHCH2B(C6F5)3] 4. The formation of this species suggests
a modified mechanism where steric demands about the cyclo-
propane shut down direct attack at this site. Rather, Lewis
acid interaction with the exocyclic olefin results in enhanced
acidity of the methylene protons of the cyclopropane, prompting
deprotonation by phosphine and the cascade rearrangement to
the butadiene-borate anion.
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¨
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¨
S. Fromel, G. Kehr, R. Frohlich, S. Grimme and G. Erker,
¨
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¨
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In considering the mechanism of interaction of the FLP
with cyclopropane, it is important to note that it has been
previously reported that no evidence of a discrete molecular
interaction between cyclopropane and boron-based Lewis
´
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Chem. Commun., 2010, 46, 8947–8949 8949