David M. Read

Medium-ring diphosphines: synthesis and transannular chemistry

Abstract Medium-ring diphosphines offer unique opportunities to study the interactions between phosphorus atoms. In addition, monocyclic medium-ring diphosphines may be very useful ligands in transition metal complexes. A number of useful synthetic routes to these compounds have been developed, but further efforts in this direction might be rewarding.

Conformational analysis of saturated heterocycles. Part 78. Passing pyramidal nitrogen inversions in some perhydro-1,3-oxazines and -1,3-diazines

Variable temperature proton n.m.r. studies allowed the direct observation of pyramidal nitrogen inversion hindered by an adjacent equatorial alkyl group in some 2-alkyl-3-methylperhydro-1,3-oxazines and 2-alkyl-1,3-dimethylperhydro-1,3-diazines. The axial–equatorial free energy differences for the N-methyl groups and the conformer populations have been measured.

The conformational analysis of saturated heterocycles. Part LXII. cis–trans-Equilibria of 4-phospha-analogues of 2,6-dimethylcyclohexanone

4-Phospha-analogues of 2,6-dimethylcyclohexanones exist in three (isolable) stereoisomeric forms. These have been investigated for the 4-phenylphospha-compound and its oxo-, thioxo-, and selenoxo-derivatives. Equilibrium constants have been determined and corresponding ΔG° differences are compared with those for other heteraanalogues of 2,6-dimethylcyclohexanone.

Bridgehead phosphorus chemistry: in–out inversion, intrabridgehead P ⋯ P bonding, and reactivity

Propellane diphosphonium dications undergo addition reactions with a range of nucleophiles, and the products often have in,out-geometry with some P–P interaction, as shown by 1JPP values ranging from 46–253 Hz. X-Ray structures of 1-benzyl-6-phospha-1-phosphoniabicyclo[4.4.4]tetradecane trifluoromethanesulfonate, 1-benzyl-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate, and 1-hydroxy-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate are reported; these show that there is no simple relationship between P–P distance and 1JPP values, although the latter do correlate with the apicophilicity of the attached group. Hydride adducts, although formally protonated phosphines, can react as hydride sources, and salts of the 6-phospha-1-phosphoniabicyclo[4.4.3]tridecane ion, while stable in solution, undergo ring opening to the 1-propyl-1-phosphonia-6-phosphabicyclo[4.4.0]decane ion in the solid state. Addition of hydroxide to the 1,6-diphosphoniatricyclo[4.4.4.01,6]tetradecane ion leads, via a non-isolable hydroxide adduct, to out,in-1λ5,6-diphosphabicyclo[4.4.4]tetradecane 1-oxide. Addition to the 1,6-diphosphoniatricyclo[4.4.3.01,6]tetradecane dication leads to isolable 1-hydroxy-6-phospha-1-phosphoniabicyclo[4.4.3]tridecane hexafluorophosphate, converted by excess hydroxide to the diphosphine monoxide which exists as a dynamic mixture of out,in- and out,out-isomers, the equilibrium being dependent on solvent polarity, the water content of the solvent, and the presence of metal ions. Addition of hydroxide to the 1,6-diphosphoniatricyclo[4.3.3.01,6]tetradecane dication leads to an out,in-hydroxide adduct which is rapidly converted to the out,out-monoprotonated phosphine oxide.

Quantitive dependence of N-methyl inversion barriers in six-membered rings on electronic and steric effects. Direct observation of the conformational equilibria in tetrahydro-1,3-oxazines and hexahydropyrimidines

Low-temperature 1H n.m.r. spectra of N-methyl and N,C-polymethyl derivatives of tetrahydro-1,3-oxazine and hexahydropyrimidine allow calculation of conformational equilibria and the rationalisation of steric and electronic effects on N-methyl inversion barriers.

THE CONFORMATIONAL ANALYSIS OF SATURATED HETEROCYCLES. PART 72. TETRAHYDRO-1,3,4-OXADIAZINES

Variable-temperature proton n.m.r. spectroscopy has been used to study the inversion barriers and conformations of 3.4-dialkyltetrahydro-1.3.4-oxadiazines. The predominant conformer of 3,4-dimethyltetrahydro-1.3,4-oxadiazine was found to be 3ax, 4eq.

The conformational analysis of saturated heterocycles. Part 72. Tetrahydro-1,3,4-oxadiazines

Variable-temperature proton n.m.r. spectroscopy has been used to study the inversion barriers and conformations of 3.4-dialkyltetrahydro-1.3.4-oxadiazines. The predominant conformer of 3,4-dimethyltetrahydro-1.3,4-oxadiazine was found to be 3ax, 4eq.

The conformational analysis of perhydropyrido[1,2-c][1,3]oxazines, perhydropyrido[1,2-c][1,3]thiazines, and perhydropyrido[1,2-c] pyrimidines

Dipole moments of perhydropyrido[1,2-c][1,3]oxazine, 2-t-butylperhydropyrido[1,2-c]pyrimidine, and syn-perhydrodipyrido[1,2-c : 2′,l′-f] pyrimidine indicate their exclusive existence in the trans-fused conformation. Perhydropyrido[1,2-c][1,3]thiazine and 2-methylperhydropyrido[1.2-c]pyrimidine also exist to a significant extent in this conformation. The effects of cis(4a-H, 5-H)-5-methyl, cis(4a-H, 6-H)-6-methyl and cis and trans(4a-H, 7-H)-7-ethyl groups in the perhydropyrido[1,2-c][1,3] oxazine system are discussed, and the conformational behaviour of this series is rationalized. These results are compared with estimates of the positions of conformational equilibria based on n.m.r. data.