Bimolecular nucleophilic displacement at primary and secondary carbon atoms

The ease of bimolecular nucleophilic replacement of substituents generally follows the expected order primary secondary. Primary substituents may often be replaced in polar solvents, but secondary substituents are only satisfactorily replaced in dipolar aprotic solvents since adverse steric and dipolar interactions hinder the approach of the azide ion to the secondary carbon Although 

Displacement of secondary sulphonates usually proceeds with inversion and significant exceptions are stereochemically informative. When the reaction of 107 with azide ion in dimethylformamide was carried out over 24 hours at 85°, the triazide (109) was isolated. Substitution at C(4j proceeds with the expected Walden inversion, but at C(3) the azido function is introduced with retention of configuration. This result has been rationalized in terms of neighbouring group 

Hanessian has described a case in which an azido substituent is involved in neighbouring group participation . 5-Azido-5-deoxy-4-0-methanesulphonyl-D-arabinose-2,3-0-isopropylidene di- 

Such neighbouring group participation is rare with azide ion, although it heis been well established with other nucleophiles (e.g. I , SCN ). Azide ion is a moderately powerful nucleophile and in dipolar aprotic solvents its strength as a nucleophile is enhanced. In consequence, normal bimolecular substitution with inversion generally proceeds to the exclusion of neighbouring group participation. Thus, 

Since direct displacement with azide ion competes favourably with anchimeric reactions, less direct routes such as the double inversion procedure illustrated for conversion of 119 into the triazide 122 in equation (62), must often be used to achieve substitution with overall retention of configuration . 

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