Carbodiimide Synthesis via Ti-Catalyzed Nitrene Transfer from Diazenes to Isocyanides

Abstract

Simple Ti imido halide complexes such as [Br2Ti­(N t Bu)­py2]2 are competent catalysts for the synthesis of unsymmetrical carbodiimides via Ti-catalyzed nitrene transfer from diazenes or azides to isocyanides. Both alkyl and aryl isocyanides are compatible with the reaction conditions, although product inhibition with sterically unencumbered substrates sometimes limits the yield when diazenes are employed as the oxidant. The reaction mechanism has been investigated both experimentally and computationally, wherein a key feature is that the product release is triggered by electron transfer from an η2-carbodiimide to a Ti-bound azobenzene. This ligand-to-ligand redox buffering obviates the need for high-energy formally TiII intermediates and provides further evidence that substrate and product “redox noninnocence” can promote unusual Ti redox catalytic transformations.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

“…17,18 Since then, further work in investigating reactions across terminal imido groups in high-valent early transition metal complexes has led to the development of a number of catalytic processes involving nitrene transfers, 19-21, including imine and azide metatheses, [22][23][24][25] hydroaminations of alkenes and alkynes, [26][27][28][29][30][31] and carboaminations to give a,b-unsaturated imines. [32][33][34] In addition, Ti imido, [35][36][37] Zr imido, 38 and V bis(imido) complexes 14 have recently been shown to carry out multicomponent coupling to give carbodiimides, pyrroles, pyrazoles, and other nitrogen-containing heterocycles.…”

Synthesis, Reactivity, and Insights into the Lewis Acidity of Mononuclear Titanocene Imido Complexes Bearing Sterically Demanding Terphenyl Moieties

“…1 In this context, imido complexes of group 4 metals have been utilized in cycloaddition reactions, 2 hydroaminations, 3 and, more recently, nitrene transfer reactions. 4 A combination of cycloaddition and nitrene transfer reactions continues to gain momentum from Tonks' pyrrole, substituted imine, and pyrazole syntheses. 5 Moreover, transition-metal imido complexes have been comprehensively explored in olefin polymerization 1e,6 and are of industrial relevance in the production of acrylonitrile.…”

Phosphorus and Arsenic Atom Transfer to Isocyanides to Form π‐Backbonding Cyanophosphide and Cyanoarsenide Titanium Complexes

“…Ap reliminary attempt at converting AdNC with Na(OCP) in THF over one day resulted in unconverted starting materials as verified by IR spectroscopy.E ncouragingly,early transition metals can trigger spectacular reactivity of isocyanides, [13] such as the catalytic transfer of nitrenes to form carbodiimides. [14] We recently reported the tetrahedral Ti II complex, [(Tp tBu,Me )TiCl] (1 Ti ,T p tBu,MeÀ = hydrotris(3tert-butyl-5-methylpyrazol-1-yl)borate), to capture AdNC, forming paramagnetic [(Tp tBu,Me )TiCl(CNAd)] (2 Ti ), which bears areduced isocyanide ligand (vide infra). [15] Treatment of maroon 2 Ti with Na(OCP) in THF resulted in short-lasting (< 1min) effervescence and anew species,which was isolated in 72 %y ield as ag reen crystalline material (Scheme 2).…”