Structural investigations of ylides. Part VI. Crystal and molecular structures of two resonance-stabilized Wittig reagents 2-carboxy-1-methoxycarbonylethyltriphenylphosphorane and its t-butyl ester

Abstract

The structures of the title compounds (3) and (4) have been determined by three-dimensional X-ray analyses by use of 3356 (3) and 2883 (4) independent reflexions measured by diffractometer. Crystals of both compounds are monoclinic, space group P2₁/n; for (3)Z= 4, unit cell dimensions a= 8.579(1),b= 13.893(4),c= 16.768(3)Å, β= 97.31(3); for (4),Z= 4, cell dimensions a= 15.731(5),b= 11.941(2), c= 13.194(2)Å, β= 97.22(1)°. The structure of (3) was solved by the heavy-atom method, and of (4) by the symbolic addition method. Both structures have been refined by least-squares calculations to R0.054(3) and 0.076(4). The results show that the structures are stabilised in part by delocalisation of the formal negative charge on the P⁺–C^– group into the carbomethoxy-function.