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–C₆H₂O)₂PCl (19) with DEAD/DIAD leads to the chlorophosphoranes CH₂(6-t-Bu-4-Me–C₆H₂O)₂PCl[N(COOR)–NC(OR)O–] [R = Et (21a), i-Pr (21b)]. Treatment of 21a–b with pyrazole or imidazole leads to CH₂(6-t-Bu-4-Me–C₆H₂O)₂P(NRR′)[N(COOR)–NC(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)–NC(OR)O–] (15), but S(6-t-Bu-4-Me–C₆H₂O)₂PCl (20a) gives the hexacoordinate phosphorane S(6-t-Bu-4-Me–C₆H₂O)₂PCl[N(COOR)–NC(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–C₆H₂O)₂PPh[N(COOR)–NC(OR)O–] (17) is prepared similarly. Reaction of 16 with imidazole gives S(6-t-Bu-4-Me–C₆H₂O)₂P(imidazolyl)[N(COOR)–NC(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.