Reactions between alkylaminobis(trifluoromethyl)phosphines and boron trihalides

Abstract

The reaction between alkylaminobis(trifluoromethyl)phosphines, RNH·P(CF₃)₂, and boron trihalides is complex and the products depend on the reaction conditions and on the nature of both the alkyl group and the boron trihalide. At low temperatures 1:1 adducts are formed but at room temperature the predominant reaction is cleavage of the N–P bond to give halogenobis(trifluoromethyl)phosphines, (CF₃)₂PX, and alkylaminoboron–halogen compounds: At –78°: RNH·P(CF₃)₂+ BX₃= RNH·P(CF₃)₂,BX₃ At 25°: RNH·P(CF₃)₂+ BX₃=(CF₃)₂PX + RNH·BX₂ where R = Bu^t and X = F or Cl. When R = Me or Et the aminoboron product of the second reaction disproportionates into a boron trihalide adduct and the corresponding borazine: 2RNH·BX₂= RNH₂,BX₃+⅓(RNBX)₃

A second reaction sequence results in elimination of hydrogen chloride and the formation of the new alkylbis(trifluoromethyl)phosphinylaminoboron dichlorides (CF₃)₂PN(R)BCl₂ where R = Me or Bu^t. These compounds are unstable liquids and have been characterised by analysis, molecular weight, and infrared spectra, and by proton, boron-11, and phosphorus-31 n.m.r. spectra. The methyl compound is dimeric and the t-butyl compound monomeric, and possible structures are discussed.