Bridged intermediates

The major conclusion from the smdies of the mechanism of O2 reduction by CcO is that formation of a peroxo-level intermediate bridging two metal ions is not a prerequisite for four-electron reduction, at least in molecular complexes. 

The ionic intermediates bridged bromonium ions or open /3-bromocarbocations 220... 

Finally, in contrast to the reactions reported above, 49 reacts131 with dimethyl(methyl-thio)sulfonium fluoroborate (DMTSF) and triethylamine tris(hydrofluoride) in dichlorome-thane to give only 5% of the 1,2-addition product 122. The main products, present in 73 27 ratio, are the exo-exo and endo-exo adducts 123 and 124, formed by exclusive exo attack of the electrophile on the double bond, followed by transannular -participation in the intermediate bridged cation and final addition of fluoride to the nortricyclic cation from both the exo or endo side (equation 120). 

The preparative oxidation of vinyl sulfides is thought to proceed via an intermediate bridged sulfonium cation that can be attacked by an external nucleophile from the less encumbered side to give two isomers (Scheme 21) [87]. 

The nature of the intermediate cationic species, whether it is a classical methyl cation or a nonclassical bridged cation, has also been determined. The deamination of optically active e c/o-2-aminobicyclo[2.2.1]hcplane-exo-2-carbonitrile gave 2-hydroxybicyclo[3.1. l]-heptane-1-carbonitrile (19) in 9% yield with 92% retention of the optical activity. Similarly, deamination of e.vo-2-trifluoromethylbicyclo[2.2.1]heptyl-e Jo-2-amine gave 1-trifluoromethyl-bicyclo[3.1.1]heptan-2-ol (20) in 11 % yield with 88% retention of the optical activity. These results are interpreted in terms of intermediate bridged cations.87... 

The combination of steps 1 and 2 corresponds to a 1,2 hydride shift (R = H) or a Wagner-Meerwein rearrangement (R = alkyl). The intermediate bridged nonclassical structure... 

Z/-l-Benzothiopyrans with catalytic amounts of acid undergo disproportionation to thiochromans and benzothiopyrylium salts. In the case of 3,4-dimethyl-2//-l-benzothiopyran, intermolecular hydride transfer yields an 85 15 mixture of cis- and stereochemical control of this reaction.276 Bromination of 2H-1-... 

Alternative mechanisms for the OH transfer process in enzyme-coenzyme B12-catalysed dehydration of 1,2-dihydroxyethane, to give acetaldehyde and water, have been explored using ah initio MO calculations.75 Transfer within an (HOCH— CH2OH) radical was ruled out because the activation energy is too high, and no intermediate bridge structure could be found to facilitate conversion of 1,2-dihydroxyethyl cation (if it could be formed from the radical) to 2,2-dihydroxyethyl cation. The radical cation (HOCH—CH20H2)+ transformed rapidly to a stable... 

In the controversy that developed, the point under attack was not so much the existence of the intermediate bridged ion—although this was questioned, too— as its mode of formation. The 3-phenyl-2-butyl tosylates undergo solvolysis at much the same rate as does unsubstituted e c-butyl tosylate formolysis a little faster, acetolysis a little slower. Yet, as depicted by Cram, phenyl gives anchimeric assistance to the reaction. Why, then, is there no rate acceleration ... 

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