Issue 38, 2014

Understanding the directed ortho lithiation of (R)-Ph2P([double bond, length as m-dash]NCO2Me)NHCH(Me)Ph. NMR spectroscopic and computational study of the structure of the N-lithiated species

Abstract

A multinuclear magnetic resonance (1H, 7Li, 13C, 15N, 31P) and DFT computational study at the M06-2X(SMD,THF)/6-311+G(d,p)//B3LYP/6-31G(d) level of the structure of a N-lithiated phosphinimidic amide (R)-Ph2P([double bond, length as m-dash]NCO2Me)NHCH(Me)Ph 13 has been performed. In THF solution it exists as an equilibrium mixture of monomers and dimers. The monomers consist of a six-membered ring formed by coordination of the lithium atom with the deprotonated nitrogen and the oxygen atom of the carbonyl group. This coordination mode is in contrast to the standard N,N-chelation observed in N-lithiated N,N′-bis(trimethylsilyl)phosphinimidic amides. The calculations showed that the metallacycle adopts a twist-boat conformation and that the lithium atom is in a tetrahedral environment involving O,N-chelation by the ligand and coordination to two/one THF molecules in the monomer/dimer. Dimerization takes place through O–Li bridges. For all species two series of isomers have been identified, which originated by restricted rotation of the methoxy group and ring inversion. The twist-boat conformational interconversion seems to be operating for explaining the pattern of signals observed in the 7Li and 31P NMR spectra. The structure found for the most stable dimer is analogous to the molecular structure reported for a related Cα-lithiated phosphazene 20. The structural study revealed that the chiral side-arm of the N-lithiated species is oriented to the outer face of the pro-S P-phenyl ring, which shows one ortho-proton very close to the nitrogen atom of the carbamate moiety. In this conformation, proton abstraction by a base is highly favoured, in agreement with the experimental results.

Graphical abstract: Understanding the directed ortho lithiation of (R)-Ph2P( [[double bond, length as m-dash]] NCO2Me)NHCH(Me)Ph. NMR spectroscopic and computational study of the structure of the N-lithiated species

Supplementary files

Article information

Article type
Paper
Submitted
28 Mar 2014
Accepted
29 Apr 2014
First published
29 Apr 2014

Dalton Trans., 2014,43, 14291-14301

Author version available

Understanding the directed ortho lithiation of (R)-Ph2P([double bond, length as m-dash]NCO2Me)NHCH(Me)Ph. NMR spectroscopic and computational study of the structure of the N-lithiated species

M. Casimiro, J. García-López, M. J. Iglesias and F. López-Ortiz, Dalton Trans., 2014, 43, 14291 DOI: 10.1039/C4DT00927D

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