The tri-π-methane rearrangement: Mechanistic and exploratory organic photochemistry

Article abstract of DOI:10.1021/ol006141n

(equation presented) The di-π-methane rearrangement is firmly established as a mode of synthesizing three-membered-ring compounds. We now report the tri-π methane counterpart.

Full text of DOI:10.1021/ol006141n

ORGANIC
LETTERS
2000
Vol. 2, No. 15
2365-2367
The Tri-π-methane Rearrangement:
Mechanistic and Exploratory Organic
Photochemistry1
Howard E. Zimmerman* and Vladim´ır C´ırkva
Chemistry Department, UniVersity of Wisconsin, Madison, Wisconsin 53706
zimmerman@bert.chem.wisc.edu
Received May 31, 2000
ABSTRACT
The di-π-methane rearrangement is firmly established as a mode of synthesizing three-membered-ring compounds. We now report the tri-π-
methane counterpart.
The di-π-methane rearrangement has become a well-
understood organic reaction that is of considerable value in
the synthesis of three-membered-ring compounds. We now
report the tri-π-methane counterpart. Scheme 1 shows the
mechanism of the di-π-methane rearrangement and its
potential diversion to a tri-π-methane pathway. In this, on
excitation, the singlet or triplet bridges to give cyclopropyl
dicarbinyl diradical 2. Opening of this species tends to give
transoid (bond a-b) allylic diradical 3 as a consequence of
less steric interference. This species closes to afford π-sub-
stituted cyclopropane product 5 characteristic of a di-π-
methane rearrangement, but closure to form a five-membered
ring would result in a trans double bond in that ring. In
contrast, if cyclopropyl dicarbinyl diradical opening were
to lead to cisoid allylic diradical 4, closure to tri-π-methane
product 6 may compete with 1,3-closure to di-π-methane
product 5. We can document two instances of tri-π-methane
rearrangement. In one case we successfully employed crystal
lattice photochemistry,2 wherein the lattice enforced the
cisoid conformation in the cyclopropyl dicarbinyl diradical;
note eq 1. The only solution example, curiously, is found in
the barrelene to semibullvalene rearrangement, where in the
final step of the mechanism symmetry resulted in equal
proximity of the two ends of the allylic moiety to the single
odd-electron carbon. The consequence was a di-π-methane
Scheme 1. Competition between Di-π-methane and
Tri-π-methane Rearrangements. Note That the Allylic Diradical
May Be Transoid Above Bond a-b as in 3 or Cisoid as in 4
(1) This is Paper 256 of our general series. For paper 255, see:
Zimmerman, H. E.; Nesterov, E. E. Org. Lett. 2000, 2, 1169-1171.
(2) Zimmerman, H. E.; Zuraw, M. J. J. Am. Chem. Soc. 1989, 111, 7974-
7989.
10.1021/ol006141n CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/24/2000

Scheme 2. Kinetic Problem. A Represents the Tri-π Reactant,
B Is the Di-π-methane Three-Membered-Ring Compound, and
C Is the Tri-π-methane Five-Membered-Ring Photoproduct
rearrangement which could be construed as arising by two
routessthe di-π and the tri-π.3
However, now we report finding a solution tri-π-methane
rearrangement, depicted in eq 2. This occurs in competition
f C are well-known, the kinetics have not been solved
analytically for the situation in Scheme 2. The solution is
given4a in eq 4. It needs to be recognized that the three rate
constants, in actuality, are operational values which give the
rate at which each process occurs under the photolysis
conditions employed. Each constant is proportional to the
product of the quantum efficiency of that reaction multiplied
by the efficiency of formation of the reacting excited state.
Because of differential light absorption, A and B compete
unequally for light. From the operational viewpoint, this is
irrelevant, but the relative utilization of the two pathways is
a function of the reaction conditions.
As a second mode of excitation, it was of interest to use
singlet sensitization by naphthalene, since in this case energy
transfer rather than relative light absorption is a factor.
Fitting the expression for the concentration of B as a
function of time, as given in eq 4, to the experimentally
obtained4b values leads to relative values for k1, k2, and k3.
Table 1 gives the relative rate constants obtained for direct
with the di-π-methane rearrangement. On direct irradiation,
tris-diphenylvinyl methane 9 led to 52% of tri-π-methane
product 11 in addition to the ordinary di-π-methane photo-
product 10, with both cis and trans isomers being formed
(31 and 17%, respectively).
However, a complication soon became apparent. It was
observed that di-π-methane photoproduct 10 rearranged
photochemically to afford five-membered-ring isomer 11.
The consequent question then was whether there really was
a direct pathway to tri-π-methane product 11 or, instead, just
an indirect route via the well-known di-π-methane rear-
rangement followed by a 1,3-sigmatropic shift, via diradical
12 or its concerted equivalent; note eq 3.
Table 1. Relative Rate Constants
sensitizer
none
naphthalene
k1
k2
k3
0.223
0.099
0.143
0.028
0.024
0.014
irradiation and for naphthalene sensitization. Under both
conditions the direct tri-π-methane pathway dominates
relative to the indirect route. For direct irradiation there is a
factor of 6 while in the naphthalene-sensitized runs there is
a factor of 2. The sources of the differences in utilization of
the two pathways seem likely to be greater light absorption
by the tri-π-methane reactant in the direct irradiations and a
potentially lower efficiency of singlet energy transfer to the
di-π-methane three-membered-ring photoproduct in the
sensitized runs.
The kinetic situation is depicted in Scheme 2. The question
is just how large is the direct rate constant k2 relative to the
constant k3 for the indirect formation of the five-membered-
ring product? It seems that although the kinetics of A f B
Hence the direct route, the tri-π-methane rearrangement,
is dominant and the indirect, two-step mechanism plays a
lesser role.
(3) (a) Zimmerman, Binkley, R. W.; Givens, R. S.; Sherwin, M. A. J.
Am. Chem. Soc. 1967, 89, 3932-3933. (b) There is a reaction utilizing all
three π-bonds in a different way in a barrelene derivative; note Pokkuluri,
P. R.; Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc. 1990, 112, 3676-3677.
(4) (a) The kinetic derivation is given in the Supporting Information.
(b) The details of curve fitting and the programming involved will be
described in our full paper.
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Org. Lett., Vol. 2, No. 15, 2000

Additionally, from the preparative viewpoint, at extended
photolysis times, the five-membered-ring photoproduct domi-
nates as a consequence of both routes affording this product.
Scheme 3. Multiplicity Dependence
Another interesting facet is the multiplicity dependence.
On acetophenone sensitization, tri-π-methane reactant 9
afforded only di-π-methane rearrangement (eq 5). One
pathways. Relevant to the present reactions, homopolar
diradicals have been categorized as a “large K” type with K
increasing as the odd-electron centers become more separated
as in the transoid diradicals.
In conclusion, it now is apparent that a five-membered-
ring counterpart, the tri-π-methane rearrangement, of the
established di-π-methane rearrangement is present. We are
pursuing the reaction to determine its generality and mech-
anisms.
interpretation is that triplet cyclopropyl dicarbinyl diradical
2T opens selectively to transoid allylic carbinyl diradical 3
while singlet cyclopropyl dicarbinyl diradical 2S opens to
afford cisoid allylic carbinyl diradical 4. Multiplicity depen-
dence in organic photochemistry is common, and a theoretical
basis has been provided5 in terms of “exchange integral
control”.
The exchange integral K in SCF-MO theory gives a
measure of the energetic splitting between S1 and T1. The
generalization has been suggested that triplets prefer “large
K” reaction routes while singlets prefer “small K” reactions.
Scheme 3 shows the source of the preference, namely each
reacting excited state selecting the lower energy of two
Acknowledgment. Support of this research by the
National Science Foundation is gratefully acknowledged with
special appreciation for its support of basic research.
Supporting Information Available: The derivation of
eq 4 is given along with information on some of the details
of irradiation wavelength. This material is available free of
charge via the Internet at http://pubs.acs.org.
(5) (a) Zimmerman, H. E.; Penn, J. H.; Johnson, M. R. Proc. Natl. Acad.
Sci. U.S.A. 1981, 78, 2021-2025. (b) Zimmerman, H. E.; Armesto, D.;
Amezua, M. G.; Gannett, T. P.; Johnson, R. P. J. Am. Chem. Soc. 1979,
101, 6367-6383. (c) Zimmerman, H. E.; Factor, R. E. Tetrahedron 1981,
37, Supplement 1, 125-141.
OL006141N
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