Cyclobutene cation radical

Radical Cations Derived From Cyclobutene Systems. . . . 211... 

In this context it is also of interest that the parent cyclobutene radical cation can be observed at 77 K and, upon warming to 110 K, undergoes stereoselective ring opening to transoid butadiene radical cation [270]. A cyclopropylcarbindiyl radical cation has been proposed as an intermediate for this transformation [271]. 

Radical Cations Derived From Cyclobutene Systems... 

The radical cations of cyclobutene and derivatives constitute the simplest system... 

A case where both effects are important is the ring opening of the cyclobutene radical cation l +.13 This reaction was studied experimentally by Bally and coworkers, who found that, in contrast to the thermal reaction, only the trans-1,3-butadiene radical cation, trans-2 +, is formed in a matrix isolation experiment.14,15 This surprising result can be explained by considering the electronic states of the species involved in the reaction, which are shown in Fig. 1. 

In subsequent argon matrix isolation studies, similar bands were found when hexatriene or cyclohexadiene are ionized , and eventually, five of the six possible rotamers of hexatriene radical cation were identified by selective, wavelength-specific in ter conversions . Similar results were later obtained for octatetraene , where six of the twenty possible rotamers are formed on ionization in argon (Figure 31) which could be interconverted and identified by selective photolysis . Interestingly, in the case of the butadiene radical cation the s-cis rotamer could not be detected, even if the diene radical cation was formed from the cyclobutene radical cation . In contrast, in a recent resonance Raman study, some weak bands were detected and assigned to the s-cA-butadiene radical cation which might have escaped detection in the earlier ESR and EA experiments . ... 

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,... 

For some time it has been known that many pericyclic reactions can be greatly accelerated if they are run under single electron transfer (SET) conditions (also known as electron transfer catalysis, ETC). Examples include Diels-Alder reactions, electrocyclic openings of cyclobutenes, and retro [2-1-2] cycloadditions. From the beginning it has been debated whether these SET reactions really are concerted processes with aromatic transition states, or whether they are better thought of as stepwise processes involving radical cation intermediates. 

The second example comes from recent calculations on the ring opening of the cyclobutene radical cation Correlation diagrams show... 

G. N. Sastry and T. Bally,/. Ant. Ghent. Soc., 120, 9323 (1998). The C4Hj Potential Energy Surface. 1. The Ring-Opening Reaction of Cyclobutene Radical Cation and Other Rearrangements. 

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