Synthesis, characterization, and transition-metal complexes of 3,4-diazaphospholanes

The steric, electronic, and synthetic characteristics of 3,4-diazaphospholanes are reported. Crystallographic structures of free and metal-complexed 3,4-diazaphospholanes provide steric metrics (cone angle, solid angle, etc.). Diazaphospholanes span a wide range of sizes with cone angles varying from 135 to 188 degrees. The electron-donating abilities of diazaphospholanes have been estimated using the carbonyl infrared stretching frequencies, nu(CO), of [trans-Rh(diazaphospholane)2(CO)Cl] complexes. Frequencies for the CO stretches range from 1975 to 2011 cm(-1), thus indicating that 3,4-diazaphospholanes may be as electron rich as dialkylarylphosphines or as electron deficient as trialkyl phosphites. Reduction of N,N'-phthalamido-3,4-diazaphospholanes with BH3 center dot SMe2 yields diazaphospholanes that not only are more electron rich but also show a reorientation of the phospholane substituents that may affect catalytic properties. Diazaphospholanes readily react with many Rh and Pd catalyst precursors to form complexes. Metal complexes of 3,4-diazaphospholanes exhibit reactivities different from those of common phosphine complexes, presumably due to the generally greater steric bulk and electron deficiency of 3,4-diazaphospholanes relative to phosphines. Cationic Rh(I) complexes of 3,4-diazaphospholanes abstract chloride ligands from chlorinated solvents to afford chloride-bridged dimers. The complex [(rac-17)Pd(Me)Cl] rearranges in solution-stereoselectively transferring methyl from palladium to phosphorus while simultaneously opening a diazaphospholane ring. Many of the 3,4-diazaphospholane-metal complexes have extremely close Cl center dot center dot center dot H-C(P)(N) contacts, suggesting Cl center dot center dot center dot H hydrogen bonding.