Simple ZnEt₂as a catalyst in carbodiimide hydroalkynylation: structural and mechanistic studies

Abstract

Expanding the possibilities of the use of simple and available ZnEt as a catalyst, the hydroalkynylation of carbodiimides with a variety of alkynes to obtain unsaturated substituted amidines is described in this work. Different stoichiometric studies allow proposing that amidinate complexes are intermediates in this catalytic process, produced by easy activation of the C-H bond of the alkyne and formation of alkynyl derivatives followed by a carbodiimide insertion step. Kinetics studies allowed the generation of a rate law for the hydroalkynylation of N,N'-diisopropylcarbodiimide with phenylacetylene which is second order in [carbodiimide], first order in [catalyst] and zero order in [alkyne], with a negligible PhC[triple bond, length as m-dash]CH/PhC[triple bond, length as m-dash]CD isotopic effect, consistent with a rate-determining state involving carbodiimide insertion. The hydroalkynylation reaction has been coupled with isocyanate (and isothiocyanate) insertion and intramolecular hydroamination to obtain imidazolidin-2-ones (or thione). The structures of different plausible intermediates have been determined by X-ray diffraction studies.

ZnEt2 as a Precatalyst for the Addition of Alcohols to Carbodiimides

“…Then, [2 + 2] cycloaddition of a molecule of carbodiimide and a Zn-OR moiety of A would give rise to a N,O -isoureato intermediate ( C in Scheme 1 ), which may isomerize or be in equilibrium with a more stable N,N ′-isoureato species ( C ′ in Scheme 1 ). Although these intermediates could not be isolated, previously described N,N′ -monoguanidinato, 17h propiolamidinato, 18 and phosphaguanidinato 19 zinc compounds structurally related to C′ displayed a dimeric structure in the solid state, as evidenced by X-ray diffraction studies. Moreover, these compounds were all proposed as intermediates in the addition of amines, terminal alkynes and phosphines to carbodiimides catalyzed by ZnEt 2 .…”

“…16 Since 2010, our group has successfully used simple, commercially available ZnEt 2 as a precatalyst in other hydroelementation reactions involving carbodiimides and (i) amines to give guanidines (N−H addition), 17 or (ii) terminal alkynes to give the corresponding propiolamidines (C−H addition). 18 Additionally, in 2021, Ma, Roesky, and co-workers reported the use of this compound for the hydrophosphination (P−H addition) of carbodiimides and related heterocumulenes. 19 In an attempt to expand the catalytic hydroelementation repertoire of ZnEt 2 , we focus now our attention on the hydroalkoxylation of carbodiimides.…”

Recent Advances in the Catalytic Synthesis of Imidazolidin-2-ones and Benzimidazolidin-2-ones

“…The mechanism involves the formation of acyclic ureas; then, the intramolecular hydroamination step leads to the construction of the five-member cyclic urea. A similar approach was reported in 2017, replacing alkaline earth catalysis with zinc [154].…”

“…Palladium was found to be a suitable candidate by Wolfe et al, who deeply studied this transformation. In 2006, they published a new synthetic strategy for the synthesis of imidazolidin-2-ones by a palladium-catalyzed carboamination from allylic ureas and aryl bromides [154]. They managed to obtain several products from both terminal and internal double bonds.…”

An efficient catalytic method for hydrophosphination of heterocumulenes with diethylzinc as precatalyst without a solvent

“…However, ZnEt 2 being used in the reaction without auxiliary ligands are rare. 17 Organophosphorus compounds are becoming significant in modern industrial chemistry and academics as they are widely used in the organic modification of materials, 18 syntheses of pesticides 19 and catalysts, 20 and design of pharmaceuticals. 21 The synthesis of organophosphorus compounds has become a hot research area.…”