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°. The electron-donating abilities of diazaphospholanes have been estimated using the carbonyl
infrared stretching frequencies, ν(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·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···H−C(P)(N) contacts, suggesting Cl···H hydrogen bonding.