Acetylene radical cations

The dissociation process forming 3ir CX+ + 4S CY and its associated bond strength D is the most uniform for the acetylene radical cation cleavage reactions. Encouragingly, this corresponds most closely with the initial "Simonized dissociation of ethylenes — recall that neither we, nor Simons, considered the formation of two carbenes, one apiece in their n2 and o2 states upon the dissociation of any ethylene. 

Acetylene radical cations dissociation process, 49 energetics, 49,50 ionization potentials, 49,50 ... 

The oxygen transfer to the -acetylenic carbon results in the very intense benzoyl cation, whereas the transfer to the a-carbon, via a series of fragments corresponding to the loss of OH , CO and CO2, respectively, leads to annelated heterocycles such as the radical cation of carbazole, as a result of elimination of CO81. The generation of the benzoyl cation was rationalized as shown in Scheme 1281. 

Remarkable enhancements of the unimolecular c-t isomerization of c-S with p-MeO and oxidation of S with -MeO are explained by charge-spin separation in such S Unimolecular c-t isomerization of such c-S proceeds with a chain mechanism, while regioselective oxidation occurs in such S because of the spin localization. Cycloreversion of t,c,t-TPCB occurs to give a a-St 2, while the photochemical cycloreversion of TPCB and t,t,t-TPCB gives Tr-St 2 and t-St /t-St pair, respectively. Radical cations of phosphorus compounds (9 and 10 form intramolecular rr-dimer between two Nps from which Np 2 forms. Formation of intermolecular a-dimer of aromatic acetylene (11 - and 12 -) and intramolecular dimer of 13 and diarylmethanoT was observed, and the n = 3 rule is not effective for intramolecular dimer -. 

Other olefinic substrates known to dimerize through photo-induced electron transfer sensitization include enamines (72), diarylethylenes (73-75), vinyl ethers (76), styrenes (77,78), and phenyl acetylenes (79). Alternate ring closures (besides cyclobutanes) are sometimes observed, probably via 1,4-radical cationic intermediates. For example, a tetrahydronaphthalene is formed from the radical cation of 1,1-diphenylethylene, eq. 

The AMI-calculated structure and charge distribution of radical the trication of [1-carotene have been reported and its UV absorption spectrum estimated from INDO/S methods.158 The decomposition of the ftiran radical cation proceeds by two separate pathways according to a recent theoretical study, one via formation of propene radical cation and CO, the other a lower energy process via acetylene and a ketene radical cation.159 As a result of a reflection mass spectrometric study, a likely mechanism is... 

An interesting application of PET mediated bond cleavage reaction from azirine 63 has been reported by Mattay et al. [67] for synthesizing N-substituted imidazoles (65) via the (3 + 2) cycloaddition reaction of resultant 2-azaallenyl radical cations with imines 64. Synthesis of pyrrolophane 3,4-dimethyl ester (68) has been reported recently [68] by the ring opening of 66 followed by inter-molecular cycloaddition with dimethyl acetylene dicarboxylate (67) as shown in Scheme 13. 

A quantum-chemical interpretation of the MS fragmentation of organic molecules has been reported <1987CHE512>. Based on the bond orders, the weakest bond is the C(3)-C(4) bond and hence should fragment first and to a much greater extent than the S-C or the C(2)-C(3) bond. This then forms the divinyl sulfide radical cation which cyclizes to intermediate 94 and then eliminates acetylene to form thiirene 93 (Figure 16). On the other hand, it is also possible that the bicyclic intermediate 95 is formed first, which could fragment to form radical cation 96 and/or intermediate 94 both of which lose acetylene to form thiirene 93. 

An exciting cycloaddition which is not pericyclic in nature has been developed on the basis of PET opening of azirines to 2-azaallenyl radical cations [373] these add readily to substrates such as imines and acetylenes. This paves the way for the synthesis of imidazoles [373a, e], heterophanes [373b, e], and even porphyrins (cf. the preparation of 34) [373c, e]. 

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