Bogdan Matuszewski

Photochemistry of 3,6-dihydro-1,2-oxazines: A versatile route to substituted pyrroles

A high yield pyrrole synthesis which employs a sequence of cyclo-addition, photoextrusion of H2O and Birch reduction has been developed.

Photolysis of some dichloroacetic acid esters in cyclohexane

Quantitative analysis has been performed on the products of direct as well as benzene- and acetone-sensitized photolysis of ethyl and methyl dichloroacetates in cyclohexane. Their quantum yields have been determined. It has been found that the primary mechanism is similar to that for corresponding α-monochloroacetic acid esters and consists mainly in homolytic cleavage of the CCl bond. The short-lived excited state of these esters, most likely the singlet state, is responsible for this reaction. The lack of fluorescence and the efficiency of the CCl rupture approaching unity, indicates that other decay processes for these esters does not play a considerable role. The maximum lifetime of the excited singlet state for these ester molecules is τs < 2 × 10−11 s. Photolysis of dichloroacetic acid esters can be sensitized by benzene and acetone. In the case of benzene the energy transfer occurs from the excited singlet of the donor, and both the classical energy transfer, [singlet(benzene) → singlet(ester)], and the formation of an exciplex between the excited donor molecule and the acceptor molecule in the ground state, are possible.

Photolysis of ethyl α-chlorophenylacetate in cyclohexane

Of many possible primary processes of photolysis of the compouds containing a carboxyl group, the primary reaction of ethyl α-chlorophenylacetate (I) proceeds outside the carboxyl group and is in practice limited only to a homolytic CCl bond rupture. Because of large differences in the rate of hydrogen abstraction by the PhĊHCOOEt and Cl radicals, the main photolysis product of I in cyclo-hexane is PhCH(C6H11)COOEt (II) and not PhCH2COOEt (III) (as it was in the case of α-haloesters). Besides HCl and II, the other products of photolysis of I in cyclohexane are: III, C6H11Cl (IV), diethyl α,α-diphenylsuccinate (PhCHCOOEt)2 (V), traces of p-ClC6H4CH2COOEt (VI), as well as a number of secondary products, of which PhC(C6H11)2COOEt (VII) was identified. The quantum yields of main photolysis products are determined. The Norrish type II reaction does not play any significant role in the photolysis of I. The excited singlet state of I is probably responsible for the CCl bond homolysis. The lifetime of this state has been estimated to be less than or equal to 1 × 10−11 s, and the quantum yield of fluorescence of I as < 1 × 10−5. The photolysis of I is sensitized by benzene, and the products and quantum yields of sensitized photolysis are the same as in direct photolysis. The energy transfer takes place from the singlet state of benzene to the ester with the rate constant 7.0 × 1010 M−1 s−1. The mechanism of intramolecular energy transfer in I is considered.