The molecular structure of 1-(2′-triphenylphosphonio-3′,3′,4′,4′-tetrafluorocyclo-butenyl) (3,3-difluorocyclobutene-2,4-dionate)•chloroform solvate

The title compound, C26F15O2F6P(0.5CHCl3) (A), crystallizes as pale yellow crystals in the monoclinic system C2/c with a = 29.624(6), b = 10.224(3), c = 18.576(4) Å; β = 116.26(2)°, Z = 8. On the basis of 1652 observed reflections (I ≥ 2.3σ(I)) in the range 3° ≤ 2θ ≤ 42° the structure was solved and refined by least-squares methods to R = 0.042 and shown to be a zwitterion consisting of a phosphonium cation and a delocalised anion. The bond parameters of the cation are normal; P—C distances range from 1.773(5)–1.786(5) Å and C—P—C angles range from 106.4(2)–111.7(2)°. Within the delocalised anionic fragment of the molecule C—C distances range from 1.388(7)–1.440(8) Å and C—O distances are 1.216(7) and 1.208(7) Å.

Mechanism of SNi reactions. The effect of the base strength of the anionic fragments

The kinetics and stereochemical aspects of the thermal decomposition of aralkyl carbonates, thiocarbonates, and carbamates were examined. The rates for the decompositions as well as the rates of loss of optical activity followed first-order kinetics. The decompositions appear to involve the heterolysis of the aralkyl–oxygen bond followed by the breakdown of the subsequent ion pair via a cyclic mechanism. However, it was found that this ion pair could return to covalency without completely decomposing to products leading to the racemization of the starting materials. This type of racemization occurred more readily in the case of the carbamates than in the thiocarbonates and carbonates. The dependence of the decomposition rates and the loss of optical activity on the nature of the hetero atom of the anionic fragment of the starting material is discussed.