Nitrilimines:  Evidence for the Allenic Structure in Solution, Experimental and<i>Ab Initio</i>Studies of the Barrier to Racemization, and First Diastereoselective [3 + 2]-Cycloaddition

Abstract

The lithium salts of [bis(diisopropylamino)thioxophosphoranyl]diazomethane ( 2 ) and [(diisopropylamino)(dicyclohexylamino)thioxophosphoranyl]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 -PH 2 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 )-α-(acryloxy)-β,β-dimethyl-γ-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.