Crystal lattice photochemistry often proceeds in discrete stages. Mechanistic and exploratory organic photochemistry

Article abstract of DOI:10.1021/ol0057838

While the solid-state photochemistry of crystals has been assumed to proceed to some critical point beyond which the crystal is destroyed, it has now been found that crystal lattice photochemistry often occurs in stages or regimes. In each regime different chemistry generally results. The reaction in different stages may be followed kinetically.

Full text of DOI:10.1021/ol0057838

ORGANIC
LETTERS
2000
Vol. 2, No. 8
1169-1171
Crystal Lattice Photochemistry Often
Proceeds in Discrete Stages.
Mechanistic and Exploratory Organic
Photochemistry1,2
Howard E. Zimmerman* and Evgueni E. Nesterov
Department of Chemistry, UniVersity of WisconsinsMadison,
Madison, Wisconsin 53706
zimmerman@chem.wisc.edu
Received March 9, 2000
ABSTRACT
While the solid-state photochemistry of crystals has been assumed to proceed to some critical point beyond which the crystal is destroyed,
it has now been found that crystal lattice photochemistry often occurs in stages or regimes. In each regime different chemistry generally
results. The reaction in different stages may be followed kinetically.
Solid state photochemistry provides a powerful synthetic tool
of particular mechanistic interest. Often products, otherwise
not accessible, may be obtained. The one question most often
posed relative to a solid-state photochemical reaction, is to
what conversion the reaction is practical. Thus, it is usually
assumed that crystal-lattice reactions proceed consistently
only to a given point followed by ill-defined results.3
proceed with one reaction to very high conversions, the
process can still be quite practical.
This investigation was prompted by our observations in a
study of dimorphs.4 A discontinuity was observed in the
reactivity as the reaction proceeded and it proved possible
to deal with the reaction kinetics. It now has been found
that the phenomenon extends beyond dimorphs and is
common, and we have generalized the kinetic treatment.
For the present study we selected the Type B Enone
Rearrangement5 since both stereo- and regioselectivity are
involved, and thus several products are possible in each case.
Thus, in the first reaction, that of R-naphthyl-â-naphthyl
enone 1, four stereoisomers are observed in solution pho-
tolyses6 and three in the solid-state reaction. In the second
reaction studied, that of 2-methyl enone 2, while one
photoproduct results in solution, in the early stage of the
We now report that for a large fraction of crystal lattice
photochemical reactions, reactivity occurs in definite stages
with each regime having its own characteristic course and
that this is a general phenomenon. Additionally, we have
succeeded in analyzing the kinetics involved. There are
several important consequences of our findings. Perhaps the
most important is that in a stage beyond the initial one, a
reaction can often be obtained which is not accessible initially
from any ordinary crystal reactant. Another consequence is
that for solid-state photochemical processes which do not
(4) Zimmerman, H. E.; Alabugin, I. V.; Chen, W.-C, Zhu, Z. J. Am.
Chem. Soc. 1999, 121, 11930-11931.
(5) (a) Zimmerman, H. E.; Wilson, J. W. J. Am. Chem. Soc. 1964, 86,
4036-4042. (b) Zimmerman, H. E.; Hancock, K. G. J. Am. Chem. Soc.
1968, 90, 3749-3760. (c) For a review, see: Schuster, D. I. In Rearrange-
ments in Ground and Excited States; P. DeMayo, Ed.; Academic Press:
New York, 1980; Vol. 3.
(1) This is Paper 255 of our general series.
(2) For Paper 254 see Zimmerman, H. E.; Alabugin, I. V. J. Am. Chem.
Soc. 2000, 121, 952-953.
(3) (a) Hitherto, there have been literature examples where a reaction
has been described as proceeding with continuous product variation. (b)
For example, note Teng, M.; Lauber, J. W.; Fowler, F. W. J. Org. Chem.
1991, 56, 6840-6845.
(6) Zimmerman, H. E.; St. Clair, J. D. J. Org. Chem. 1989, 54, 2125-
2137.
10.1021/ol0057838 CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/29/2000

crystal photolysis an additional product also results.7a In more
extended photolysis, the solution product appears.7b In a third
example, that of phenyl R-naphthyl enone 3, again the solid-
state chemistry contrasts with that observed in solution.8
The examples we report presently are those with two
regimes. In two cases, regime two affords a single product
while regime one gives two products. In the third case, both
regimes afford several photoproducts but with different
distributions. We also anticipate reactions where further
regimes occur.
Scheme 2. The Solution and Solid State Rearrangement of
2-Methyl-4,4-diphenylcyclohexenone 2.
In Schemes 1, 2, and 3 we show the behavior of (a) 4-R-
naphthyl-4-â-naphthylcyclohex-2-enone 1, (b) 2-methyl-4,4-
diphenylcyclohex-2-enone 2, and (c) 4-R-naphthyl-4-phen-
ylcyclohex-2-enone 3.
In a general treatment, if one plots separately the amounts
of two photoproducts, A and B, versus the conversion, it
can be shown that at any point with Ao and Bo concentrations,
the ratio of slopes is given by eq 1. Here the slope of the
plot of A is R, and that of B is â. The conversion is C and
R gives the ratio of A to B. A0, B0, C, and R are variables.9
Scheme 2. The last example, presented in Scheme 3, involves
the competition between phenyl and R-naphthyl migration.10
R /â ) [CR - A0(R + 1)]/[C - B0(R + 1)]
(1)
Scheme 3. The Solution and Solid State Rearrangement of
4-R-Naphthyl-4-phenylcyclohexenone 3.
If we have, for example, several regimes, each with its
own selectivity, then within a given regime as product
concentrations increase, the incremental ratio, Rinc, of product
A to B becomes constant and is given by the ratio of slopes.
Note eq 2.9
R /â ) Rinc
(2)
The first of the three examples is depicted in Scheme 1.
This example is that of 4-R-naphthyl-4-â-naphthyl enone 1
whose reactivity in solution and in the crystal lattice differs
markedly. The second example is that of the 2-methyl-4,4-
diphenylcyclohexenone 2 whose chemistry is dealt with in
Scheme 1. The Solution and Solid State Rearrangement of
4-R-Naphthyl-4-â-naphthylcyclohexenone 1.
In each of the three examples, the observed multistage
reactivity is depicted quantitatively in Figures 1, 2, and 3.
The most remarkable facet is the linearity seen for each
product within each stage of the reaction and the different
rates of formation of these products in the first and second
regimes. A particularly striking further observation was made
in the reaction of the dinaphthyl enone 1. Within each of
(7) (a) Zimmerman, H. E.; Zhu, Z., J. Am. Chem. Soc. 1995, 117, 5245-
5262. (b) It is clear that this did not result from melting as evidenced by
monitoring the crystal shape and melting point throughout the photolysis.
The minimum melting point was 80 °C while the run was at 0 °C.
(8) Interestingly, in the case of the enone 3, two polymorphs, P21/n and
P212121 were found, and only the latter, that described here, was found to
be reactive.
(9) This is derived in the Supporting Information.
(10) Regime 2 is more complex than in the first two examples in that
cis-trans isomerization of the product of stage 1 occurs. This does not
affect the interpretation.
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Org. Lett., Vol. 2, No. 8, 2000

Figure 1. Stages and changes in reactivity with extent conversion
of reactant 1. Plot 1 corresponds to cis-5-R-naphthyl-6-â-
naphthylbicyclo[3.1.0]hexan-2-one 1c, plot 2 corresponds to trans-
5-R-naphthyl-6-â-naphthylbicyclo[3.1.0]hexan-2-one 1d, plot 3
corresponds to cis-5-â-naphthyl-6-R-naphthylbicyclo[3.1.0]hexan-
2-one 1a.
Figure 3. Stages and changes in reactivity with extent conversion
of reactant 3. Plot 1 corresponds to the sum of products of
R-naphthyl migration (trans- and cis-bicyclics) 3a and 3b; plot 2
corresponds to the sum of products of phenyl migration (trans- and
cis-bicyclics) 3c and 3d.
molecule adjacent. The occurrence of a sharp melting point
at the point of discontinuity strongly supports the interpreta-
tion of a newly ordered molecular arrangement at this
juncture. In contrast, in the case of the 2-methyl enone 2,
despite our finding two discrete stages, the melting point
remained sharp but decreased until 60% reaction and then
became broad until the final photoproduct was formed at
which point it again was sharp.
the two stages broad melting behavior was found. However,
at the point where the first stage ended and the second began,
a sharp melting point was encountered. Here we know from
X-ray analysis that each reactant molecule is surrounded by
three neighbors. The point of discontinuity between the two
stages occurs just as every reactant molecule has a product
We ascribe the existence of stages to the different reactivity
of a molecule surrounded by one or more product molecules
compared with the original situation where a reactant is in
an environment with neighboring reactant molecules. In the
dinaphthyl enone 1 reaction, it seems that the presence of
one product neighbor diminishes reactivity so that isolated
reactant molecules react preferentially. Similarly, in the case
of the 2-methyl enone 2, there are eight molecules surround-
ing any single one. Thus, as before, if the reactivity is
diminished by the presence of one product neighbor, then at
ca. 10% conversion, only a slower and different reaction is
possible and stage two begins. With the large ratio of
unreacted neighbors to reacted molecules, one anticipates a
delayed change in melting behavior.
Conclusion. Crystal lattice photochemistry often proceeds
in stages, each stage with its own reactivity. A most critical
aspect is the chemistry of subsequent stages which by
definition cannot be obtained directly in a photolysis but only
by utilizing a subsequent stage of solid-state reaction.
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: Experimental de-
tails, and spectroscopic and X-ray data for compounds
described. Also, mathematical derivation of eqs 1 and 2, as
well as a generalized treatment of stage reactivity. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 2. Stages and changes in reactivity with extent conversion
of reactant 2. Graph b shows the area of graph a corresponding to
low conversions. Plot 1 corresponds to 1-methyl-trans-5,6-
diphenylbicyclo[3.1.0]hexanone 2b, plot 2 corresponds to 2-methyl-
3-(diphenylmethyl)cyclopent-2-enone 2a.
OL0057838
Org. Lett., Vol. 2, No. 8, 2000
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