Nitrilimines: Evidence for the allenic structure in solution, experimental and ab initio studies of the barrier to racemization, and first diastereoselective [3+2]-cycloaddition

The lithium salts of [bis(diisopropylamino)thioxophosphoranyl]diazomethane (2) and [(diisopropylamino)(dicyclohexylamino)thioxophosphoranyl]diazomethane diazomethane (5) react with the (diisopropylamino)(dicyclohexylamino)chlorophosphine (3) and bis(diisopropylamino)chlorophosphine (6), leading to the nitrilimines 4 (63% yield) and 7 (69% yield), respectively. The lithium salt of 2 also reacts with the bis(dicyclohexylamino)phosphenium ion, affording nitrilimine 9 in 51% yield. Using the presence of a chiral substituent either at the carbon or at the nitrogen terminus of the CNN skeleton, variable-temperature solution NMR studies of nitrilimines 4 and 7 demonstrate that they possess a bent allenic structure. The free energy of activation for racemization is ca. 30 kJ mol(-1). Ab initio/DFT studies performed on nitrilimine with C-P(S)H-2 and N-PH2 substituents 10 show that the most likely pathway for the racemization process is inversion at the carbon atom, followed by rotation of the PSR(2) fragment. Bis(trityl)nitrilimine 17 reacts with (R)-alpha-(acryloxy)-beta,beta-dimethyl-gamma-butyrolactone, leading to diastereomeric pyrazolines 18a,b (3/1 ratio) in 60% total yield; this is the first example of a diastereoselective [3 + 2]-cycloaddition reaction involving a nitrilimine.