Abstract
The mechanism of the addition of indazole (Ind)—a bifunctional aromatic N,NH-nucleophile—to cyclohexyl isocyanide coordinated to the palladium(II) center in the model complex cis-[PdCl2(CNMe)(CNCy)] (1) to give the corresponding aminocarbene ligand was investigated in detail by theoretical (DFT) methods. The most plausible mechanism of this reaction is that of the associative type involving nucleophilic attack of Ind by its unprotonated N atom at the isocyanide carbon atom followed by the stepwise proton transfer from the nucleophile molecule to the isocyanide N atom via deprotonation/protonation steps. Two reaction channels based on two tautomeric forms of indazole were found. The channel leading to the experimentally isolated aminocarbene product is based on the less stable tautomeric form. Another channel based on the more stable tautomer of Ind is slightly kinetically more favorable but it is endergonic. Thus, the regioselectivity of this reaction is thermodynamically rather than kinetically driven. The bonding situation in key species was analyzed.
Highlights
The nature of all transition states was investigated by analysis of the vectors associated with the imaginary frequency and by the calculations of the intrinsic reaction coordinates (IRC) by using the method developed by Gonzalez and Schlegel [70,71,72]
The dissociative mechanism starts with deprotonation of the nucleophile by a base that is present in the reaction mixture
The concerted mechanism occurs in one step via a cyclic transition state, which may be either 4- or 6-membered with the participation of a third molecule playing the role of a proton shuttle
Summary
The coupling of N-nucleophiles and isocyanides (C≡NR) is an important type of organic transformation yielding a great variety of chemical systems with newly formed C–N bonds [1,2,3,4]. Despite intensive investigations of NA of N-nucleophiles to metal coordinated [15] experimental and amidines [18,61]). NA of N-nucleophiles to metal coordinated isocyanides, theintensive mechanism of this addition started of to become clear only recently. NA of N-nucleophiles to metal isocyanides, the mechanism of this addition started to become clear only recently. Indicated a negative activation entropy for the reaction between amines and isocyanides, the mechanism of thisaaddition to become only recently. Asfor recently as 2017, it was established kinetic studies negative activation entropy the as reaction amines and isocyanides ligated to the Pt(II) or Pd(II). Nucleophilic addition (NA) of amines, imines, and hydrazones to isocyanides coordinated to the center. Indazole molecule has two adjacent imineand andamine aminenucleophilic nucleophilic centers The former formercenter centerexhibits exhibitsstronger stronger nucleophilic two adjacent imine and amine nucleophilic centers.
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