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The crystal structures of four alkyl 3-oxo-2-(triphenylphosphoranylidene)butyrates, where the alkyl group is methyl (C 23 H 21 O 3 PÁ0.5C 6 H 6), (II), ethyl (C 24 H 23 O 3 P), (III), isopropyl (C 25 H 25 O 3 P), (IV), or tert-butyl (C 26 H 27 O 3 P), (V), show all of them to have the same conformation. They present a tetrahedral P atom and an sp 2 ylidic C atom, with the carbonyl groups adopting anti conformations with respect to the keto groups located close to the P atom. PÐCÐCÐO torsion angles, bond lengths and angles indicate an effective electronic delocalization toward the keto groups. In each case, one H atom of the alkoxy group is close to one of the phenyl rings. These preferred conformations are evaluated as the result of attractive and repulsive intramolecular interactions.

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Hydrogen bonding and crystal packing play major roles in determining the conformations of ethyl methyl 2-(triphenylphosphoranylidene)malonate, Ph(3)P=C(CO(2)CH(3))CO(2)CH(2)CH(3) or C(24)H(23)O(4)P, (I), and dimethyl 2-(triphenylphosphoranylidene)malonate, Ph(3)P=C(CO(2)CH(3))(2) or C(23)H(21)O(4)P, (II). In (I), the acyl O atom of the ethyl ester group is anti to the P atom, while the O atom of the methyl ester group is syn. In (II), the dimethyl diester is a 1:1 mixture of anti-anti and syn-anti conformers.