Nitrenium ions nucleophilic additions

A different method of generating a nitrenium ion has been demonstrated the sulfur-nitrogen bond in (55) cleaves and then a novel intramolecular hydride shift to the arylnitrenium ion centre takes place. The nitrenium species (56) undergoes two nucleophilic additions to the double bond to give the product (57). Two studies concerning the physiological effects of nitrenium ions in vivo are reported. Products are formed from both the ion-paired nitrenium ion (59) and the free ion (60)... 

In addition to solvolysis and nitrenium ion formation, Af-aLkoxy-A-chloroamides (2) also undergo bimolecular reactions with nucleophiles at nitrogen. Not only is the configuration destabilized by the anomeric effect, it also parallels that of a-halo ketones, where halogen on an sp carbon is activated towards reactions by the adjacent carbonyl. This rate-enhancing effect on 8 /2 processes at carbon is well-known, and has been attributed to conjugation of the p-orbital on carbon with the carbonyl jr-bond in the S 2 transition state stabilization of ionic character at the central carbon as outlined by Pross as weU as electrostatic attraction to the carbonyl carbon. The transition states are also affected by the nature of the nucleophile. ... 

The situation is different for alkylarylnitrenium ions. Haley first demonstrated that a A -l-adamantyl-A -(2-benzoylphenyl) nitrenium ion 62 (Fig. 13.36) could be generated by photolysis of an anthranilium ion." In this case, rearrangement of the adamantyl ring, to give products from hydrolysis of 63, competes with addition reactions, giving 64. Similar experiments carried out on A -tert-butyl-A -arylni-trenium ions also showed that the 1,2-shift of a methyl group competes with additions of nucleophiles to the aryl ring. In the latter cases, the formation of the alkylarylnitrenium ion was verified by LFP. 

The only well-characterized addition of an n nucleophile to a nonaromatic nitrenium ion is the reaction of singlet NH2 with water or methanol to provide, respectively, NH2OH and NH20Me (Fig. 13.39). ... 

In some cases, the initial adduct can combine with an additional nucleophile to give a diadduct. This process is particularly likely when the driving force for aro-matization of the monoadduct is weak or absent. For example, Novak has shown that heteroarylnitrenium ion 85 is trapped at first by water and then by an additional nucleophile to give the diadduct 87 (Fig. 13.45). Likewise, Bose et al. reported the dihydroxylation of the nitrenium ion 88 derived from stilbene... 

The addition of nitrenium ions to n nucleophiles (arenes and alkenes) has attracted attention as a potentially useful synthetic transformation. Takeuchi et and... 

Phenyl azide was also found to react with alkenes in TFA (Scheme 11). The reaction was assumed to go via an aziridinium ion generated from attack of the alkene on the nitrenium ion because of the overall /rans-addition to the alkene noted in product 29. An alternative Sn2 mechanism was disfavored because phenyl azide decomposes at 21 °C in 50 vol% cyclohexene/TFA with an almost identical first-order rate constant as in 50vol% benzene/TFA even though cyclohexene is a considerably stronger nucleophile than benzene. ... 

In the course of studies on cyclopropanone—)5-lactam conversions, Wasserman and coworkers developed a route to the nocardicins by taking advantage of the reactivity of primary amines with cyclopropanone. The unusual susceptibility of the carbonyl group of cyclopropanone to attack by nucleophiles is well exemplified in this synthesis which involves the addition of the highly hindered malonate derivative (156) to generate the cyclopropanol adduct (157). The hindered amine (156) was previously found to be completely unreactive as a nucleophile in a displacement reaction with dibromoester (158) in an attempt to form the azetidine carboxylate (159). The further conversion of the amino malonate adduct (157) to the -lactam through a nitrenium ion rearrangement is illustrated in Scheme 59. 

---