Carbocations formation from diazonium ions

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

This diazo ester is formed because loss of N2 from the diazonium ion results in formation of a quite unfavorable carbocation. 

Like electrophilic addition to diazo compounds [7] from which diazonium ions and, subsequently, carbocations are generated, transition-metal compounds that can act as Lewis acids are potentially effective catalysts for metal carbene transformations. These compounds possess an open coordination site that allows the formation of a diazo carbon-metal bond with a diazo compound and, after loss of dinitrogen, affords a metal carbene (Scheme 5.2). 

The first step in the decomposition of nitrosoamides 123) is formation of the diazo ester 125) which fragments to a diazonium ion pair (128)129 The ion pairs thus produced differ from those obtained in the reaction of diazoalkanes with acids. The ratio of ester to ether formed in the decomposition of rV-nitroso-fV-benzhydrylbenz-amides in alcohol is lower than that found in the reaction of diphenyldiazomethane 132) with acids, and in the solvolysis of benzhydryl benzoate (I35)135,136 This effect has been attributed to the intervention of trans-diazo ester in the decomposition of 125) which leads to a greater distance between carbocation and carbox-ylate anion. In the diazoalkane reaction attack of the acid occurs at the electron-rich carbon atom to generate the carboxylate in the immediate vicinity of the incipient carbocation. 

Since nitrogen-free products result from the formation and decomposition of diazonium ions, these reactions are often referred to as deamination reactions. Alkyl diazonium ions are rarely used in synthetic work but have been studied extensively to probe the behavior of carbocations generated under conditions in which the leaving group is lost rapidly and irreversibly. 

Kirmse et al. (1991) came more closely to a differentiation between classical and nonclassical carbocation intermediates in deamination by the photolytic formation and nucleophilic substitution of diazonium ions from the 4-toluenesulfonyl hydrazones of 6,6-dimethyl- and 5,5,6,6-tetraalkylnorbornan-2-one 7.117, 7.118 and 7.119 (Scheme 7-36). By running the reactions in D20/Na0D, the diazonium ions were labeled with deuterium in the 2-position. For all three reagents, the alkylated exo-norbornanols (from 7.117 7.120 and 7.121) showed a distribution of deuterium in positions 1 and 2 that was very close to 1 1 (49.5 50.5 to 47.6 52.4). If these deviations from an equal distribution were caused by two classical ion intermediates. 

Formation of carbocations from aliphatic diazonium ions... 539... 

There is evidence that carbocations do play a role in the reaction of some diazonium ions. For example, the products of nucleophilic addition to f-butyl carbocations are essentially the same whether the carbocations are produced from aliphatic amines or from solvolysis of the corresponding alkyl halide. The decomposition of the diazonium ion to form a carbocation should become less favorable as the stability of the carbocation decreases, and the formation of an ethyl cation from ethyldiazonium ion in the gas phase has been calculated to be endothermic by 13.9kcal/mol. The decomposition of a 1° alkyldiazonium ion to N2 and a 1° carbocation in solution may be energetically more favorable, perhaps even exothermic, but it is questionable... 

One of the strongest arguments against the formation of 1° carbocations in the deamination of 1° amines comes from stereochemical studies. Reaction of nitrous acid with optically active l-[ H]-l-butanamine (36) produced the diazonium ion 37, which then formed 1-butanol (38) with essentially complete inversion of configuration (equation 10.69). This result suggested that the formation of 38 does not occur by capture of the 1° butyl cation by water but is, instead, the product of an Sn2 reaction on 3 ... 

Reviews this year include the collation of evidence in favour of the o-bridged 2-norbornyl cation,the use of aliphatic diazonium ions as means for the production of carbocations, terpene rearrangements in superacid media,and the formation of carbocations by the silver-assisted reactions of chloroformates. A convenient procedure, using syringe techniques, has been described for the preparation of ca. IM solutions of alkyl cations in SbF5-S02ClF from suitable precursors that are soluble in SO2CIF at —78 Since conversions are essentially quantitative, the... 

Scheme 10.43. A representation of the formation and decomposition of the diazonium ion produced from optically active (but of unknown absolute stereochemistry ) 1- H-l-aminobutane in acetic acid. It has been reported that 87% of the product is the primary acetate and, of that, 69 + 7% had inverted configuration. Some hydride migration occurred and the secondary carbocation underwent acetolysis. A small amount of 1-[ H]-1-nitrobutane was also detected (see Streitwieser, A. Jr. Schaeffer, W. D. J. Am. Chem. 5 oc., 1957,79,2888).

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