Substituent R-Dependent Regioselectivity Switch in Nucleophilic Addition of<i>N</i>-Phenylbenzamidine to Pd^II- and Pt^II-Complexed Isonitrile RN≡C Giving Aminocarbene-Like Species

Abstract

Metal-mediated coupling between one or two isonitrile ligands in cis-[MCl2(C≡NR)2] [M = Pd, Pt; R = 2,6-Me2C6H3 (Xyl), But, cyclohexyl (Cy)] and N-phenylbenzamidine, HN═C(Ph)NHPh, proceeds with different regioselectivity upon varying R group. When the aromatic isonitrile is used (R = Xyl), N-phenylbenzamidine is coordinated to a metal by the HN═C moiety, and the nucleophilic attack proceeds via the NHPh center of the benzamidine giving [MCl{C(N(Ph)C(Ph)═NH)═NXyl}(C≡NXyl)]. For R = But, HN═C(Ph)NHPh is coordinated to a metal by the NHPh center, and the addition occurs via the HN═C center of the nucleophile to afford [MCl{C(NC(Ph)═NPh)═NBut}(C≡NBut)]. With R = Cy, a mixture of two products that are derived from the addition of N-phenylbenzamidine by two nucleophilic centers was detected. The substituent R dependent reactivity was explored using theoretical (DFT) methods and interpreted as a result of the steric repulsions in one of the regioisomers of the addition products, when R = Cy and But. All prepared species were fully characterized by elemental analyzes (C, H, N), high resolution ESI+−MS, IR, 1D (1H, 13C{1H}) and 2D (1H,1H-COSY, 1H,13C-HMQC/1H,13C-HSQC, 1H,13C-HMBC) NMR spectroscopies, and by X-ray diffraction for four complexes. The latter studies indicate that in the case of XylNC, the corresponding carbene species possess distinct consequence of double and single bonds in the MCN2 fragment and, therefore, belong to a novel family of aminocarbene-like ligands, while in the case of ButNC, the carbene ligand is of the classical diaminocarbene type with bond delocalization in the MCN2 functionality. The catalytic properties of systems based on two representative species, i.e., [PdCl{C(N(Ph)C(Ph)═NH)═NXyl}(C≡NXyl)] (10) and [PdCl{C(NC(Ph)═NPh)═N(H)But}(C≡NBut)] (16)—that are derived from the addition of HN═C(Ph)NHPh to a PdII-bound isonitrile by its different nucleophilic centers — in Sonogashira cross-coupling of 4-NO2C6H4I with oct-1-yne (in EtOH as a solvent, K2CO3 as a base, 60 °C) yielding 1-nitro-2-(oct-1-ynyl)benzene, were evaluated indicating that the system involving 16 exhibit slightly higher catalytic efficiency (yields up to 99%, TONs up to 2000; TOFs up to 280) as compared to the system based on 10 (yields up to 99%, TONs up to 1400; TOFs up to 120). Moreover, catalytic activities of both systems are substantially higher than the conventional one based on [PdCl2(PPh3)2] (yield 40%, TON 400; TOF 22). We also employed 16 for the synthesis of 1-(dodeca-1,3-diyn-1-yl)-2-nitrobenzene from 1-iodo-2-nitrobenzene and dodeca-1,3-diyne (in EtOH as a solvent, K2CO3 as a base, 50 °C), and found that aminocarbene complex 16 is significantly more efficient precatalysts for Sonogashira reaction with diynes [as compared to the previously used Pd(OAc)2] providing 1-(dodeca-1,3-diynyl)-2-nitrobenzene in 97% yield with TON up to 1400.