Solid -Sta te Dip h otocycliza tion of Iso- a n d Ter ep h th a la ld eh yd es
via Dih a logen Su bstitu tion
J . Narasimha Moorthy,*,† P. Venkatakrishnan,† Prasenjit Mal,† and P. Venugopalan*,‡
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India, and
Department of Chemistry, Panjab University, Chandigarh 160 014, India
moorthy@iitk.ac.in
Received J une 14, 2002
The supramolecular nonbonded C-H‚‚‚X interactions between formyl hydrogens and ortho-halogen
atoms (Br/Cl) have been exploited to achieve conformational control in the solid state of dimethyl-
substituted iso- and terephthaladehydes (1-3) for unprecedented diphotocyclization. It is shown
that the dihalogen substitution also contributes to the stability of the benzocyclobutenols relative
to their precursor photoenols, so that the solid-state photolysis of dialdehydes 2b, 2c, and 3b leads
to diphotocyclization to afford respectable yields of bis-benzocyclobutenols.
In tr od u ction
variety of products, while their solid-state photochemistry
is marked by intriguing photochromism due to enoliza-
tion,7 cyclization, and polymerization. Our recent mecha-
nistic investigations have shown that the benzocy-
clobutenols, the products of photocyclization, derive from
the precursor photoenols by a thermal conrotatory closure
(vide infra, Scheme 1); consequently, the relative energies
of the photoenols and the benzocyclobutenols determine
the observation of photochromism (photoenolization) or
formation of the latter.6 Given this scenario, a rational
execution of photocyclization of o-alkyl aromatic alde-
hydes in the solid state necessitates the need to take into
account not only the appropriate conformation of the
formyl group for H-abstraction but also the relative
energies of the E-photoenol and the benzocyclobutenol.
Herein, we demonstrate how the dihalogen substitution
may be exploited to achieve not only the conformational
control via weaker C-H‚‚‚X interactions but also the
promotion of the reactivity of the enols of iso- and
terephthalaldehydes (1-3, Chart 1) to undergo unprec-
edented solid-state diphotocyclization to bis-benzocy-
clobutenols.
Our rationale for selection of the aldehydes 1-3 was
the following. The Cambridge Structural Database analy-
sis by Koppenhoefer has revealed that the carbonyl
oxygen points toward the halogen atom in all of the
o-bromoaroyl compounds (ketones, carboxylic acids, es-
ters, etc.) with the exception of o-haloaldehydes.8 In the
latter, an entirely opposite scenario, i.e., the carbonyl
oxygen pointed away from the o-halogen atom, has been
found to result as a consequence of stabilization through
Crystal engineering is concerned with the design and
synthesis of materials with predetermined physical and/
or chemical properties.1 With regard to the chemical reac-
tivity in the solid state, various approaches for steering
molecules into reactive orientations for bi- and polymo-
lecular [2 + 2] photocycloadditions have been elegantly
demonstrated.2 Interestingly, rational strategies for en-
gineering the molecules into their “reactive conforma-
tions” for unimolecular photoreactions are rather limited
despite a vast amount of literature.3 Photocyclization of
o-alkyl ketones is one of various unimolecular reactions
that has been extensively investigated;4 both solution and
solid-state photolyses lead invariably to photocyclization
yielding synthetically very important intermediates,
namely, benzocyclobutenols.5 In contrast, there exists
only scant literature on the solution and solid-state photo-
reactivity of o-alkyl aromatic aldehydes;6 the solution-
state photochemistry is frustrated by the formation of a
* To whom correspondence should be addressed. Fax: 91-512-
597436/-590007.
† Indian Institute of Technology.
‡ Panjab University.
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Solids; Elsevier: Amsterdam, The Netherlands, 1989.
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Coates, G. W.; Dunn, A. R.; Henling, L. A.; Dougherty, D. A.; Grubbs,
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10.1021/jo026055w CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/19/2002
J . Org. Chem. 2003, 68, 327-330
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