(Z)‐Bis(alkylzinc)imido‐1,2‐di(2‐pyridyl)ethene: Intramolecular Stabilization of a Bis(alkylzinc)imide

Abstract

Dedicated to Professor Nils Wiberg on the occasion of his 70th birthdayThe oxidative metal-mediated C-C coupling reaction of (2-pyridylmethyl)(trialkylsilyl)amines succeeds smoothly with dialkylzinc [Eq. (1)] or tin(ii) bis[bis(trimethylsilyl)amide].[1]The influence of the standard potential as well as the role of the steric demand of the nitrogen-bonded trialkylsilyl and zinc-bonded alkyl groups have been investigated.[2] The Nbonded trialkylsilyl substituent is obligatory to prevent side reactions. The reaction of dialkylzinc with 2-py-CH 2 NH 2 (py = pyridyl) yields several products which can be explained by a combination of oxidative C-C coupling and transamination reactions. After protolysis of this reaction mixture a small amount of cis-1,2-di(2-pyridyl)aminoethene (1; Scheme 1) can be isolated in a reproducible procedure.[3] However, we were not able to detect the zincated 1,2-di(2-pyridyl)aminoethene during the reaction. Amine 1 crystallized as the eneamine tautomer, an equilibrium with the imine tautomer was not detectable with spectroscopic methods.Even 30 years ago, doubly zincated primary amines of the type [RN(ZnR') 2 ] were of interest as catalysts for polymerization reactions, [4] however, these compounds were poorly characterized and more recent attempts to prepare bis(alkylzinc)alkyl-and -arylimides by zincation of primary alkylamines failed. [5,6]

Ligand Exchange Reaction Between an Alkylzinc Primary Amide and a Dialkylmagnesium Compound: Crystal Structures of [(thf)MeMg-μ-N(H)SiiPr3]2 and (tmeda)Mg[N(H)SiiPr3]2

“…Furthermore, no metalation reactions have occurred between the alkylzinc triisopropylsilylamides and the excess dimethylzinc; 1 bis-(alkylzinc)imides are not accessible by a 2-fold deprotonation of primary amines but by a rather complex reaction. 26 Therefore, more reactive metalation reagents such as dialkylmagnesium were applied. The reaction sequence is shown in eq 2.…”

“…The metalation power of the zinc-bonded alkyl groups is strongly reduced, and the isolation of the amine adducts of these compounds is favored compared to a second deprotonation reaction. Furthermore, no metalation reactions have occurred between the alkylzinc triisopropylsilylamides and the excess dimethylzinc; bis(alkylzinc)imides are not accessible by a 2-fold deprotonation of primary amines but by a rather complex reaction …”