C O M M U N I C A T I O N S
Scheme 3
(63:37 at 120 °C) (eq 5). The low selectivity observed suggests
that the terminal methyl group has a limited effect on the
torquoselectivity.
The products of the ring closure reaction have a doubly-allylic
boron substituent, which is a competent partner in allylation
chemistry. When 2a was heated at 140 °C in xylene (5 h) and then
at 80 °C with benzaldehyde (48 h), sequential ring-closing reaction
and allylation took place, yielding homoallylic alcohol 13 with high
diastereoselectivity (eq 6). The allylation occurs exclusively at the
endocyclic position of 5a to avoid the formation of an antiaromatic
cyclobutadiene. Although the relative configuration was not
determined, we predict, on the basis of a six-membered cyclic
chairlike transition state, the major isomer as shown in eq 6.
electrocyclic ring-closing reaction. Furthermore, a large difference
between the activation energies for the cis- and trans-stereoisomers
was observed. These results can be understood by assuming
electronic participation of the vacant p orbital of the boryl
substituent, in accord with the prediction made by Houk for the
ring-opening reaction of 3-borylcyclobutene.12 The vacant p orbital
interacts with the frontier orbitals of the transition states as an
electron acceptor. Scheme 3 shows the second highest occupied
molecular orbitals (SHOMOs) of the transition states A from
vinylallene 8 to methylenecyclobutene 9, B from cis-borylvinyl-
allene 10 to borylmethylenecyclobutene 11, and C from trans-
borylvinylallene 12 to 11.13 A significant part of the SHOMO of
A is concentrated along the σ-bond axis developing between C2
and C5, which is still distorted as exemplified by the schematic
depiction.14 With the SHOMOs of B and C, there is extensive
mixing of the distorted σ-orbital with the vacant p orbital of boron.
This mixing leads to a stabilizing two-electron interaction and thus
results in stabilization of the transition states. Furthermore, the
mixing of the distorted σ-bond with the boron p orbital is greater
at B than that at C, because the boron p orbital of B is in much
closer proximity to the orbital sketched on the remote C2. Therefore,
the reaction of the cis-isomer is more accelerated than that of the
trans-isomer.
In summary, we have shown that boryl substituents have
pronounced effects on the ring-closing reaction of vinylallenes.
These effects can be accounted for by considering electronic
participation of the vacant boron p orbital.
Supporting Information Available: Experimental details and
selected spectral data for new compounds. This material is available
References
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If the allene terminus is unsymmetrically substituted, torquose-
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closed product.4a
(7) Delas, C.; Urabe, H.; Sato, F. J. Am. Chem. Soc. 2001, 123, 7937.
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(10) Alternatively, the trans-isomer 4 could be obtained as a separable mixture
with 2a (45:55) by photochemical isomerization of the cis-isomer 2a.
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(12) The computational study by Houk predicted exclusive inward rotation
for the boryl group of 3-borylcyclobutene during the thermal ring-opening
reaction as a result of electron delocalization from the transition state
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The ring-closing reaction of vinylallene 2b having one methyl group
at the allene terminus furnished a mixture of (Z)- and (E)-isomers
(13) Frontier molecular orbitals were generated with restricted Hartree-Fock
calculations with the 6-31G(d) basis set using Spartan.
(14) The transition state HOMO is essentially the bonding π-orbital of the
C1-C2 double bond.
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