Nitrilimines:  Evidence for the Allenic Structure in Solution, Experimental andAb InitioStudies 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)H2 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 PSR2 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.

Carbenic Nitrile Imines: Properties and Reactivity

“…Similar results are obtained with all correlated methods. − However, substituents can have a profound effect on the relative stabilities. Thus, based on the matrix IR spectra and/or calculations we found allenic structures for HCNNH ( 1 ), MeCNNMe ( 2 ), PhCNNH ( 3 ), HCNNPh (4 ), PhCNNCH 3 ( 5 ), and Ph 3 C–CNN–CPh 3 , and propargylic structures, possessing a CN triple bond, for PhCNNSiMe 3 ( 6 ), PhCNNPh ( 7 ), and diborylnitrile imines exemplified by the model compound H 2 B–CNN–BH 2 8 (Chart ) …”

Asymmetric 1,3-Dipolar Cycloaddition Reactions

“…Only very few studies have been performed in the field of asymmetric 1,3-DC reactions involving nitrile imines. The bis(trityl)nitrile imine 420 is found to undergo a diastereoselective 1,3-DC reaction with ( R )-α-(acyloxy)-β,β-dimethyl-γ-butyrolactone 421 to give 2-pyrazoline 422 which is formed in 60% yield and with a diastereomeric excess of 50% according to 1 H NMR spectroscopy (eq 72) …”

Hydrazones as substrates for cycloaddition reactions

“…210, 264 ± 268 Diastereoselective synthesis of pyrazolines 185 is exemplified by the reaction between bis(trityl)nitrile imine (186) and (R)-a-(acryloxy)-b,b-dimethylg-butyrolactone 187 (Scheme 78). 210 …”