Unexpected Cyclization Reaction of an Overcrowded 2-Phosphinophenylmethanimine Derivative Leading to the Formation of the First Stable 2-Phospha-2H-isoindole Derivative

The reaction of overcrowded 2-phosphinophenylmethanimine derivative 1 bearing a 2,4,6-tri-t-butylphenyl (Mes*) group on the phosphorus atom and a 2,6-diisopropylphenyl (Dip) group on the nitrogen atom with potassium hydride afforded 2-phospha-2H-isoindole 2 along with Mes*H.# Dedicated to Prof. Dr. Ryoji Noyori on the occasion of his 70th birthday.Recently, β-diketiminato ligands bearing resonance structures with amido and imino moieties have attracted much attention from the viewpoint of unique monovalent bidentate ligands toward transition metals and main group elements. 1On the other hand, the coordination chemistry of sp 2 -hybridized phosphorus compounds has drawn a great deal of recent attention due to their unique electronic properties of π-electron systems containing a phosphorus atom, 2 whose characteristic low-lying atomic orbital make it possible to work as a good π-accepter toward the metal center. 3Although there have been much interest in the chemistry of phosphorus analogues of a β-diketiminato ligand, which should exhibit unique properties reflecting characteristic features of an sp 2 -hybridized phosphorus atom, 4,5 it is difficult to isolate and handle such a low-coordinated phosphorus compound.It is well known that a double bond containing a heavier atom such as a P=C double bond is highly reactive to undergo facile oxidation, hydrolysis, and self-oligomerization. 2 Since the first isolation of stable phosphaalkene (Bickelhaupt et al.) 6 and diphosphene (Yoshifuji et al.) 7 around 1980's, the idea of kinetic stabilization afforded by bulky substituents has been proven to be very effective for the construction of doubly bonded systems