The Diverted Di-π-Methane Rearrangement; Mechanistic and Exploratory Organic Photochemistry

Article abstract of DOI:10.1021/ol025564h

matrix presented An unusual diversion of the di-π-methane rearrangement has been encountered. The reaction is characteristic of di-π-methane systems having one vinyl group bearing one or two carbonyl groups and provides a synthesis of dihydrofurans.

Full text of DOI:10.1021/ol025564h

ORGANIC
LETTERS
2002
Vol. 4, No. 7
1155-1158
The Diverted Di-π-Methane
Rearrangement; Mechanistic and
Exploratory Organic Photochemistry1
Howard E. Zimmerman* and Wensheng Chen
Chemistry Department, UniVersity of Wisconsin, Madison, Wisconsin 53706
zimmerman@chem.wisc.edu
Received January 15, 2002
ABSTRACT
An unusual diversion of the di-π-methane rearrangement has been encountered. The reaction is characteristic of di-π-methane systems having
one vinyl group bearing one or two carbonyl groups and provides a synthesis of dihydrofurans.
The di-π-methane rearrangement has become a well under-
stood reaction with a large number of examples.2 Thus it
was surprising when we encountered a different reaction
course. The photochemical reaction of diene 1 led to an
unexpected photoproduct, 2, rather than the anticipated vinyl-
cyclopropane. We term this a “diverted di-π-methane re-
arrangement”, eq 1.
necessary for the reaction to proceed in this fashion. The
example of cyanoester 3 is depicted in eqs 2 and 3.
Indeed, in the case of reactants lacking keto carbonyl
groups, such as the cyanoester in eqs 2 and 3, intersystem
crossing is anticipated to be slower relative to the rate of
rearrangement, and the diverted di-π-methane rearrangement
is obtained only upon sensitization.
In the case of the diester 6 where there are two ester
carbonyl groups, the diverted di-π-methane photoproduct 8
is obtained along with the normal three-membered ring
product, even without sensitization. However, sensitization
doubles the relative amount of the diverted photoproduct.
Subsequently, a number of further examples were found
for reactants with similar structural features as noted in the
following. However, equally interesting was evidence on the
reaction multiplicity. Thus, in the irradiation of those systems
where intersystem crossing might be anticipated to be less
efficient than in the case of ketones, triplet sensitization was
(1) This is paper 265 of our general series. For paper 262 see:
Zimmerman, H. E.; Wang, P. HelV. Chim. Acta 2001, 65, 1342-1346.
(2) Zimmerman, H. E.; Armesto, D. Chem. ReV. 1996, 96, 3065-
3112.
10.1021/ol025564h CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/06/2002

results from fission of bond a rather than bond b and yet
originates from the triplet. Indeed it is the sensitized reactions
(i.e., triplet conditions) that afford the diverted di-π-methane
products throughout.
Hence we conclude that, in the systems presently under
study, there is an anomaly in which the triplet cyclopropyl-
dicarbinyl diradical species partitions differently. The forma-
tion of normal di-π-methane products follows the usual
regioselectivity with fission of bond b, whereas the formation
of the diverted di-π-methane products involves the unusual
fission of bond a.
One possibility is that the less regioselective triplet
cyclopropyldicarbinyl diradical 10 ring opening, with di-
methyl substitution, merely results from smaller energy
differences slightly favoring bond a fission. However,
additionally there is the point that bond a scission leads to
the five-membered ring product and not the usual three-ring
di-π-methane one.
In direct and sensitized control runs, the di-π-methane
cyclopropanes 7 and 9 did not afford the five-membered ring
product and thus are not reaction intermediates.
The mechanism of the diverted di-π-methane rearrange-
ment is given in Scheme 1. We note that the cyclopropyl-
In earlier studies,3b we investigated a similar reactant 15
with phenyl groups at the central carbon rather than the
methyls as in the present reactant 6. In that case the same
multiplicity dependent regioselectivity of cyclopropyldi-
carbinyl ring opening was observed in formation of di-π-
methane photoproducts; note Scheme 2. However, no
Scheme 1. Mechanism of the Di-π- and Diverted
Di-π-Methane Rearrangements
Scheme 2. Mechanism in the Tetraphenyl Case
dicarbinyl diradical 10 opens regioselectively in formation
of the ordinary di-π-methane three-membered ring photo-
products. This regioselective preference as a function of the
diradical multiplicity has been observed in a number of our
earlier studies.3 Thus bond a fission (leading to photoproduct
9) is expected to be preferred by the singlet S1, whereas bond
b cleavage (leading to 7) is characteristic of the triplet T1 of
the cyclopropyldicarbinyl diradical 10. These triplet-singlet
differences in reactivity result from exchange integral
control.3
products corresponding to the diverted di-π-methane re-
arrangement were encountered.
The chief difference in that earlier study was the absence
of any three-ring opening of bond a from the triplet of the
cyclopropyldicarbinyl diradical 16. Hence intermediate 12
in the present study is a species not encountered previously.
Thus it was of interest to obtain ROHF/6-31G* energies
of the triplet diradicals resulting from cyclopropyldicarbinyl
ring opening in the tetraphenyl and dimethyldiphenyl cases.
These were obtained from Gaussian984 computation; note
Tables 1 and 2. It is seen that the preferred triplet diradical
Surprisingly, as noted in Scheme 1, species 14 (the
precursor of 8 before hydrolytic workup) is a product that
(3) (a) Zimmerman, H. E.; Armesto, D.; Amezua, M. G.; Gannett, T.
P.; Johnson, R. P. J. Am. Chem. Soc. 1979, 101, 6367-6383. (b)
Zimmerman, H. E.; Factor, R. E. Tetrahedron 1981, 37, Supplement 1,
125-141. (c) Zimmerman, H. E.; Cirkva, V. J. Org. Chem. 2001, 66, 1839-
1856.
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Org. Lett., Vol. 4, No. 7, 2002

Table 1. Relative Energies of Triplet Diradicals from Bond a and Bond b Openings: Tetraphenyl 15a
a Three conformations of 17T are included. 18T is a global mininum.
in the case of the tetraphenyl reactant is species 18, which
has benzhydryl odd-electron centers. Thus the computation
is in accord with experimental observation.
In the case of the dimethyl reactant the alternative triplet
diradicals, 12T and 13T, resulting from cyclopropyldi-
carbinyl ring opening are within 1 kcal/mol of one another
and the diminished regioselectivity encountered is in accord
with the computations.
Still, as noted above, the difference between the presently
studied dimethyl example and the earlier diphenyl counterpart
may be ascribed to an effect other than just relative stability
of the diradicals formed in cyclopropyldicarbinyl ring
opening. Moreover, the behavior of diradical 12 (note
Scheme 1) is suggestive of singlet, rather than triplet,
multiplicity; in any case, the triplets do need to intersystem
cross to ring close.
ally there is bond a opening and this leads finally to the
ground state, S0, of diradical 12, a species which is best
pictured as a zwitterion. This zwitterion then undergoes ring
closure to afford the dihydrofurans 14.
In the case of the direct irradiation of reactant 6, there is
one constraint. There cannot be intersystem crossing of S1
to T1 prior to or during ring opening of cyclopropyldicarbinyl
diradical 10, since no bond b opening is observed. Thus the
ground-state product 9 must be formed by internal conversion
of the S1 excited singlet 11, perhaps via a conical inter-
section.5
Scheme 3. Photochemical Mechanism of the Benzoyl
Reactant 21
It seems more reasonable that control is by intersystem
crossing of T1 of the dimethyl cyclopropyldicarbinyl diradical
10 to S0. This may occur concomitantly with ring opening
and leads directly to species 12 as a ground-state zwitterion
rather than to species 12 as a triplet. Actually, intersystem
crossing may occur at any point between triplet diradical 10
and ground-state singlet zwitterionic diradical 12.
This intersystem crossing is enhanced by electron-pair
proximity and decreased with separation of electron-pair
centers. In the opening of triplet diradical 10 to afford 12,
there is an increased odd-electron density at the carbonyl
oxygen that enhances ISC resulting from py-π interaction.
Thus, we conclude that the triplet diphenyl cyclopropyl-
dicarbinyl diradical 16 opens to the triplet diradical 18 while
the corresponding triplet dimethyl cyclopropyldicarbinyl
diradical 10 opens in two competitive ways. Bond b fission
proceeds in the same way as its diphenyl relative. Addition-
With a single carbonyl substituent as in the benzoyl
reactant 21 (Scheme 3) the diverted di-π-methane rearrange-
ment did not occur. Rather an unanticipated product 24 was
Table 2. Relative Energies of Triplet Diradicals from Bond a and Bond b Openings: DiMethyl-Diphenyl 6a
a Three conformations of 12T are included. 13T is a global minimum.
Org. Lett., Vol. 4, No. 7, 2002
1157

obtained. As indicated in the mechanism of Scheme 3, the
reaction results from cyclization of the cyclopropyldicarbinyl
diradical 22. In virtually all previously studied examples of
the di-π-methane rearrangement this intermediate undergoes
ring opening more rapidly than competing processes.
In this case, the cyclopropyldicarbinyl diradical is trapped
intramolecularly to afford species 23, which then undergoes
tautomerization to afford final product 24. The stereochem-
istry at the benzoyl-bearing carbon of 23 is dictated by the
product stereochemistry as determined by X-ray.6
We are pursuing the reaction to determine its generality
and mechanisms as well as computationally to determine the
points of maximum spin-orbit coupling and intersystem
crossing and the energies of the diradical species involved
in the reactions.
Acknowledgment. Support of this research by the
National Science Foundation is gratefully acknowledged with
special appreciation for its support of basic research.
(4) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(5) Typical Procedure. Preparative Sensitized Irradiation of 1,1-
Dimethoxycarbonyl-3,3-dimethyl-5,5-diphenylpentadiene 6. A solution
of 200 mg (0.55 mmol) of 1,1-dimethoxycarbonyl-3,3-dimethyl-5,5-
diphenyl-1,4-pentadiene and 8.5 g of acetophenone (70.8 mmol) in 210
mL of benzene was irradiated for 40 min through a 0.20 M cupric sulfate
filter, which cuts off below 300 nm. Concentration in vacuo gave a yellow
oil that was subjected to bulb-to-bulb distillation (0.05 Torr, 40 °C) to
remove acetophenone. The remaining yellow oil was chromatographed on
Supporting Information Available: Experimental details
and X-ray data. This material is available free of charge via
the Internet at http://pubs.acs.org.
OL025564H
a 4 cm × 70 cm column eluted with 5% ether/hexane. Fraction 1 gave
0.717 g of acetophenone. Fraction 2 gave 10 mg of overlap material. Fraction
3 gave 52 mg of 1,1-dimethyl-2,2-diphenyl-3-(2,2-dimethoxycarbonylvinyl)-
cyclopropane 7, mp 143-144 °C. Fraction 4 gave 110 mg of trans-2-
methoxycarbonyl-3-(2,2-diphenylvinyl)-4,4-dimethyltetrahydrofuran-2-
one 8, mp 108-109 °C
(6) The structures of compounds 2, 7-9, and 24 were determined by
single-crystal X-ray diffractometry and the remainder by NMR analysis;
see Supporting Information for tables and data.
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Org. Lett., Vol. 4, No. 7, 2002