Structurally diverse penta- and hexacoordinate phosphorus compounds from the reaction of diethyl or diisopropyl azodicarboxylates with phosphorus(III) compounds

Abstract

The reaction of diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxylate (DIAD) with cyclic phosphites/phosphoramidites has been examined in an effort to delineate the structural preferences in spirocyclic penta- and tricyclic hexacoordinate (amino)oxyphosphoranes. It is shown that the familiar Bent's or apicophilicity rules referred to in standard books give an oversimplified picture. Thus the reaction of CH2(6-t-Bu-4-Me-C 6H2O)2PCl (19) with DEAD/DIAD leads to the chlorophosphoranes CH2(6-t-Bu-4-Me-C6H2O) 2PCl[N(COOR)-N=C(OR)O-] [R = Et (21a), i-Pr (21b)]. Treatment of 21a-b with pyrazole or imidazole leads to CH2(6-t-Bu-4-Me-C 6H2O)2P(NRR′)[N(COOR)-N=C(OR)O-] [NRR′ = pyrazolyl (12a-b), imidazolyl (13a-b)] that have trigonal bipyramidal phosphorus with 'reversed apicophilicity'. Compound S(6-t-Bu-4-Me-C6H2O)2P(NH-i-Pr) (20b) affords the pentacoordinate derivative S(6-t-Bu-4-Me-C6H2O) 2P(NH-i-Pr)[N(COOR)-N=C(OR)O-] (15), but S(6-t-Bu-4-Me-C 6H2O)2PCl (20a) gives the hexacoordinate phosphorane S(6-t-Bu-4-Me-C6H2O)2PCl[N(COOR)-N= C(OR)O-] (16) with the shortest known S→P coordinate bond. Compound 15 also exhibits the 'reversed apicophilicity' phenomenon, but the disposition of substituents is different from that in 12-13. The compound S(6-t-Bu-4-Me-C 6H2O)2PPh[N(COOR)-N=C(OR)O-] (17) is prepared similarly. Reaction of 16 with imidazole gives S(6-t-Bu-4-Me-C6H 2O)2P(imidazolyl)[N(COOR)-N=C(OR)O-] (18). Both 17 and 18 show distorted octahedral geometry with S→P coordination, but the sulfur is trans to the phenyl/imidazolyl group (while it is cis to -Cl in 16). The variable temperature 31P NMR spectra of 15 exhibit four totally distinct signals showing a dynamic behaviour with isomeric pentacoordinate species present. Theoretical calculations suggest that the compound as isolated is the favoured one for S(6-t-Bu-4-Me-C6H2O) 2P{(N-t-Bu)[N(CO2Et)NH(CO2Et)]} (2, previously reported) as well as 15 and 16. © the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.