Tetrazoles acidity

An acidified solution of the sodium salt was allowed to evaporate during 3 days and spontaneously exploded 2 weeks later. The nature of the explosive species, possibly the aci-tetrazolic acid, was being sought [1]. The silver and mercury salts are explosive [2], The chemical properties of the free nitrotetrazole have been studied. The sodium salt tetrahydrate loses water above 50°C, greatly increasing its friction and shock sensitivity [3],... 

An alternate approach, pioneered by Kunz and MacMillan, proceeds by reaction of sulfur ylides with a,(3-unsaturated iminium ions, formed by reaction of a,p-enals with enantiomerically pure amines. In this approach the sulfur ylide (9.87) reacts with a range of (3-aryl- and P-alkyl-substituted enals, including (9.88), in the presence of the dihydroindole (9.89) to give the cyclopropane with ees ranging from 89 to 96%. More recently, it has been shown that replacement of the carboxylate functional group in (9.89) with an isosteric tetrazolic acid gives an improved catalyst that effects cyclopropanation with 99% ees in all cases studied. 

Hartikka A, Arvidsson PI (2004) Rational Design of Asymmetric Organocatalysts - Increased Reactivity and Solvent Scope with a Tetrazolic Acid. Tetrahedron Asymmetry 15 1831... 

Hartikka, A., Arvidsson, P. I. (2007). Tetrazolic acid functionalized dihydroindol rational design of a highly selective cyclopropanation organocatalyst. Journal of Organic Chemistry, 72, 5874-5877. 

A second-generation catalyst in which the carboxylic acid of (S)-(-)-indoline-2-carboxylic acid was replaced by tetrazolic acid was used to improve enantioselectiv-ity as a consequence of increased steric bulk while retaining important structural functionahty associated with the proposed directed electrostatic activation mode [101], Combination of the iminium catalysis with arsonium yhdes also provided access to cyclopropanes with high enantioselectivity [102]. 

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