Suggestion of a "twist" mechanism in the oligomerisation of a dimeric phospha(III)zane: insights into the selection of adamantoid and macrocyclic alternatives.
The reaction of the dimeric phospha(III)zane [ClP(mu-Npy)]2 (1) (py = 2-pyridyl) with pyNHLi (2:1 equivalents, respectively) in THF/Et3N leads to rapid formation of the bicyclic nona-phospha(III)zane [[ClP(Npy)2]2-[P2(Npy)]] (2). This novel rearrangement can be rationalised by a mechanism involving "twisting (or swivelling)" of the central P(mu-Npy)P fragment of the presumed intermediate [[ClP(mu-Npy)2P]2(mu-Npy)] (3), a process that provides a fundamental mechanistic relationship between the majority of previously reported imidophosphospha(III)zanes. This process is fundamentally reliant on relief from ring strain on going from the four-membered ring units of 3 to the six-membered units of 2. The rearrangement observed for 1 is suppressed on steric grounds by Me-substitution of the pyridine ring at the 6-position, the dimeric phosphazane [ClP(mu-N-6-Me-py)]2 (4) (6-Me-py = 6-methyl-2-pyridyl) being formed almost exclusively in the 1:1 reaction of PCl3 with 6-Me-pyNHLi. The syntheses and X-ray structures of 1, 2 and 4 are reported, together with 31P NMR spectroscopic and DFT calculational studies of the conversion of models of 1 into 2. The combined studies pinpoint relief from ring strain as the key factor dictating the rearrangement.
Bashall
Alan
A
School of Chemistry, University of North London, London N78DB, UK.
Doyle
Emma L
EL
García
Felipe
F
Lawson
Gavin T
GT
Linton
David J
DJ
Moncrieff
David
D
McPartlin
Mary
M
Woods
Anthony D
AD
Wright
Dominic S
DS