Pentacoordination and Pseudopentacoordination via Sulfur Donor Action in Cyclic Phosphates and Phosphites1

Abstract

New cyclic chlorophosphites S[(t-Bu)2C6H2O]2PCl (3) and CH2[(t-Bu)MeC6H2O]2PCl (6) and the cyclic phosphate S[(t-Bu)MeC6H2O]2P(O)Cl (4) were synthesized from the reaction of PCl3 or POCl3 with the appropriate diol. The aminophosphite S[(t-Bu)MeC6H2O]2PNMe2 (2) was prepared by a chlorine displacement reaction from S[(t-Bu)MeC6H2O]2PCl (1) with Me2NSiMe3 while S[(t-Bu)MeC6H2O]2P(O)(OC6Cl4-o-OH) (5) resulted as a hydrolysis product of S[(t-Bu)MeC6H2O]2PCl(O2C6Cl4) (7). All of the cyclic compounds of tri- and tetracoordinated phosphorus compounds 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 atom. X-ray studies on 1−6 revealed that the cyclic phosphites 1−3 undergo an increase in coordination geometry to a pseudo trigonal bipyramid (TBP) as a result of sulfur donor action while 4 and 5 experience a similar increase in forming a trigonal bipyramid. This is the first series where sulfur donor action results in an increase in coordination geometry for tri- and tetracoordinated phosphorus compounds. The structural displacement toward a pseudo-TBP or TBP increased from 30.9% to 54.5% as the P−S distance decreased from 3.177(2) Å for 5 to 2.816(2) Å for 1. 31P NMR data support the retention of solid state structures in solution. These results are compared with series of cyclic phosphoranes which become octahedral provided by donor action by sulfur atoms present in similar ring systems. The degree of sulfur interaction in the lower coordinate series presented in this study is less than that for the phosphorane series in line with an increase in phosphorus 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 phosphorus compounds. The prevalence of donor action by sulfur, nitrogen, and oxygen atoms suggests that mechanistic criteria for nucleophilic displacement reaction may be subject to such action, e.g., at active sites of phosphoryl transfer enzymes.