M. Teresa Garland

Comparison of Conformations of Diesters of Stabilized Phosphonium Ylides in Solution and in the Crystal

Computed bond lengths and angles of methyl ethyl and dimethyl triphenyl phosphonium ylidic diesters, 1b, c , respectively, are similar to those in the crystal, as for the diethyl ester, 1a , where both acyl oxygens are anti to phosphorus. The 1 H and 13 C NMR spectra of the methyl ethyl diester, 1b , where one acyl oxygen is syn and the other anti to phosphorus, are as expected in terms of the conformation in the crystal, but the dimethyl ester, 1c , in the crystal is an equimolar mixture of conformers. For a given ylidic diester the different conformers have similar energies from B3LYP//6-31G(d) computations, interconversions of conformers should not be slow at ambient temperatures and 1 H and 13 C NMR signals in solution are sharp. Estimation of Natural Atomic Charges indicates significant cationoid character on phosphorus and the acyl carbons, and anionoid character on the ylidic carbon and the ester oxygens depending on orientations towards phosphorus.

A Common Conformation of Stabilized Triphenyl Phosphonium Ylidic Diesters with Bulky Alkoxy Groups

The phosphonium ylidic diesters, methyl and ethyl isopropyl and, methyl and ethyl t-butyl triphenylphosphoranylidene malonates, 1a,b and 2a,b , respectively, have the syn-anti conformation in solution, as in the crystal, and the bulkier alkoxy group is oriented towards phosphorus. The 1 H NMR spectra show that in 1a,b , the isopropyl group is oriented towards the face of a phenyl group, consistent with π shielding in the 1 H signals, and examination of the 1 H coupled 13 C NMR spectra allows assignment of the acyl carbon signals. Computed bond lengths and angles for isolated molecules are similar to those in the crystal, and the geometry and the NMR spectra indicate extensive ylidic resonance. Estimated partial atomic charges on the ester oxygens are more negative when they are oriented towards, rather than away from, phosphorus.

Conformational Differences in Stabilized Diketo and Diester Phosphonium Ylides in a Solid and in Solution

The diacyl ylid, 3-triphenylphosphoranylidene-2,4-pentanedione 2 can adopt conformations with one oxygen syn- and the other anti- to phosphorus 2a as in the crystal, or with both oxygens syn- to phosphorus 2b . In solution, 1 H and 13 C NMR signals are sharp, but 13 C signals in the solid confirm that the two acyl groups are in different positions, and a comparison of the 13 C signals in solution and a solid shows that conformations differ. Ab initio geometrical optimizations give, for 2a , a geometry similar to that in a crystal, and structures of both conformers permit extended ylidic resonance through near planar ylidic and acyl moieties. In crystalline diethyl 2-triphenyl phosphoranylidene malonate 3 , the torsional angles of the ester moieties to the plane of the near trigonal ylidic moiety are −149.2° and 170.4°, due to packing forces and interactions with adjacent molecules, which limit ylidic resonance. The 13 C NMR spectrum in the solid differs from that in solution, with the two ester groups in different positions, but in solution, 13 C and 1 H NMR signals are sharp. Ab initio geometrical optimizations indicate that for an isolated molecule, the conformer with torsional angles as in the crystal is energetically unfavored but energetic barriers to interconversion are small, and rapidly equilibrating conformers probably exist in solution.

Solvent switchable CuII complexes

A new tetranuclear complex Cu4, [Cu2L(OH)]2·2CH3OH·H2O (1), with the functionalized Schiff base ligand 2,6-bis((2-(acetylamino)phenylimino)methyl)-4-tert-butylphenol (H3L) has been obtained and characterized in the solid state by X-ray diffraction. The formation of the tetranuclear species is solvent dependent, the presence of water being a determinant in its isolation. Based on the mass-spectrometric evidence, the behaviour of the H3L–CuII system in the presence of water was investigated. Namely, water can switch the nuclearity of the CuII cluster from dinuclear to tetranuclear. The redox behaviour of this species in DMSO solution, showing two cathodic metal-centred peaks at EP = −0.80 and −1.35 V and an irreversible ligand-centred anodic peak at EP = 1.03 V, was found to be similar to that of a pristine dinuclear complex. The tetranuclear species was also characterized in the solid state by magnetic measurements, showing a dominating bulk antiferromagnetic behaviour, with a singlet ground state at approximately 2 K. DFT calculations permitted us to estimate the strong intradimer antiferromagnetic exchange interaction (J1 = −440 cm−1), together with two weak interdimer ferromagnetic exchange interactions (J2 = +0.5 and J3 = +1.7 cm−1).

Rearrangement of 18-iodo- and 20-iodopregnanes mediated by iodosyl derivatives

Conversion of 20-acetoxy-18-iodopregn-4-en-3-one 1 to the 18-iodosyl derivative by MCPBA resulted in a Wagner–Meerwein-type rearrangement with regioselective migration of the C13–C17 bond to give, in high yield, an abeo-pregnane in which C-18 was incorporated into ring D. The rearranged steroid was epoxidized in situ yielding a mixture of β and α 13,14-epoxides (3 and 4) which were characterized spectroscopically and by X-ray crystallography. When (20R)-20-iodopregn-4-en-3-one 9a was used as substrate, regioselective migration of the C16–C17 bond gave the D-homoandrostane with incorporation of C-20 into ring D in up to 95% yield. The 20S epimer 9b however, gave a mixture of substitution and rearrangement products. The crystal structures of the deacetylated β-epoxide 3 (5), the methanolysis product of α-epoxide 4 (7) and 20-iodopregnanes 9a and 9b are reported.