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42.17.1 Product Subclass 1: Schwesinger Bases

DOI: 10.1055/sos-SD-042-01089

Urgaonkar, S.; Verkade, J. G.Science of Synthesis, (200942923.

Schwesinger bases are strong, nonmetallic, non-ionic, and non-nucleophilic phosphazene bases in which the pentavalent phosphorus is connected to a basic nitrogen center via a double bond.[‌6‌‌9‌] The simplest representative bases of this subclass are the P1-phosphazenes, e.g. P1-t-Bu (1, N-tert-butyl-N,N,N,N,N,N-hexamethylphosphorimidic triamide), BEMP [2, 2-(tert-butylimino)-N,N-diethyl-1,3-dimethyl-1,3,2λ5-diazaphosphinan-2-amine], and BTPP [3, (tert-butylimino)tripyrrolidin-1-ylphosphorane] (Scheme 1). The successive homologization of a peralkylated phosphorimidic triamide gives rise to P2-, P3-, P4-, etc. phosphazene bases with successively increasing basicity. Examples of P2-bases (P2-t-Bu, 4 and P2-Et, 5) and a P4-base (P4-t-Bu, 6) are also shown in Scheme 1. The pKa values of these bases range from 26 to 42 and their increasing basicity is derived from the additional resonance structures that can be drawn for species in which the terminal imido nitrogen is protonated.

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