PENTACOORDINATION AND PSEUDOPENTACOORDINATION VIA SULFUR DONOR ACTIONIN CYCLIC PHOSPHATES AND PHOSPHITES

New cyclic chlorophosphites S[(t-Bu)(2)C6H2O](2)PCl (3) and CH2[(t-Bu) MeC6H2O](2)PCl (6) and the cyclic phosphate S[(t-Bu)MeC6H2O](2)P(O)Cl (4) were synthesized from the reaction of PCl3 or POCl3 with the appro priate diol. The aminophosphite S[(t-Bu)MeC6H2O](2)PNMe2 (2) was prepa red by a chlorine displacement reaction from S[(t-Bu)MeC6H2O](2)PCl (1 ) with Me2NSiMe3 while S[(t-Bu)MeC6H2O](2)P(O)(OC6Cl4-o-OH) (5) result ed as a hydrolysis product of S[(t-Bu)MeC6H2O](2)PCl(O2C6Cl4) (7). All of the cyclic compounds of tri- and tetracoordinated phosphorus compo unds 1-5 contained a sulfur atom as part of a flexible eight-membered ring while 6 incorporated a methylene group in place of the sulfur ato m. X-ray studies on 1-6 revealed that the cyclic phosphites 1-3 underg o an increase in coordination geometry to a pseudo trigonal bipyramid (TBP) as a result of sulfur donor action while 1 and 5 experience a si milar increase in forming a trigonal bipyramid. This is the first seri es where sulfur donor action results in an increase in coordination ge ometry for tri- and tetracoordinated phosphorus compounds. The structu ral displacement toward a pseudo-TBP or TBP increased from 30.9% to 54 .5% as the P-S distance decreased from 3.177(2) Angstrom for 5 to 2.81 6(2) Angstrom for 1. P-31 NMR data support the retention of solid stat e structures in solution. These results are compared with series of cy clic phosphoranes which become octahedral provided by donor action by sulfur atoms present in similar ring systems. The degree of sulfur int eraction in the lower coordinate series presented in this study is les s than that for the phosphorane series in line with an increase in pho sphorus atom electrophilicity expected for the more highly coordinated series. Comparison is made with other series with nitrogen and oxygen donor action leading to hexacoordination for pentacoordinate phosphor us compounds. The prevalence of donor action by sulfur, nitrogen, and oxygen atoms suggests that mechanistic criteria for nucleophilic displ acement reaction may be subject to such action, e.g., at active sites of phosphoryl transfer enzymes.