Radical ring-opening benzocyclobutene

The electron transfer photochemistry of benzocyclobutenes in the presence of dienophiles exhibits another interesting feature of CIP reactions [93]. For example, charge transfer irradiation of the EDA complex composed of 1,2-diphenylbenzocy-clobutene (36) and TCNE (IS) leads to the CIP (36 15 ), of which 36 spontaneously ring-opens to the o-xylylene (37) radical cation, followed by the collapse of the new CIP (Eqs. (20, 21)), leading to the Diels-Alder adduct 39 in high yield and with high efficiency. 

Further studies on the stereochemistry of these processes indicate an allowed conrotatory ring opening for the benzocyclobutene radical ration, similar to the neutral process (there is no evidence for 38 as intermediate either uncharged or as radical cation). However, diffusional quenching by means of chloranil (9) as acceptor leads to SSIP s (Eqa. (22—25)). 

The interconversion of butadiene radical cations and ionized cyclobutene represents a model case for a formal pericyclic process. Much work has been invested to study not only the distinguishability of these isomers and their derivatives by mass spectrometry, but also to check the role of orbital symmetry in the ionic species. Hass has addressed the latter problem in depth in a review on pericyclic reactions in radical cations in both the gas and condensed phases and no further survey on the papers mentioned there will be given here. The topic pertains also to the ring-opening of ionized benzocyclobutene to ionized ortho-quinodimethane (cf Section V) and various otha- phenyl-, methyl- and carboxy-substituted derivatives. In this context, we restrict ourselves hwe mentioning that an upper limit of 7 kcalmol only has been detemined by CE mass spectrometry for the activation barrier of the cycloreversion of the parent cyclobutene radical cations. The energy requirement for the cycloreversion of ionized 1- and 3-substituted cyclobutenes were found, by experiment, to be markedly different. Obviously, dissociation of the (in a sense bis-allylic) strained C—C bond is much more facile when the substituent is at C-3,... 

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