Amide-Directed Formation of Five-Coordinate Osmium Alkylidenes from Alkynes

Abstract

The amide-directed synthesis of five-coordinate osmium alkylidene derivatives from alkynes is reported. These types of complexes, which have been elusive until now because of the tendency of osmium to give hydride alkylidyne species, are prepared by reaction of the dihydride OsH2Cl2(PiPr3)2 (1) with terminal alkynes containing a distal amide group. Complex 1 reacts with N-phenylhex-5-ynamide and N-phenylhepta-6-ynamide to give OsCl2{=C(CH3)(CH2)nNH(CO)Ph}(PiPr3)2 (n = 3 (2), 4 (3)). The relative position of carbonyl and NH groups in the organic substrates has no influence on the reaction. Thus, treatment of 1 with N-(pent-4-yn-1-yl)benzamide leads to OsCl2{=C(CH3)(CH2)3NHC(O)Ph}(PiPr3)2 (4). The new compounds are intermediate species in the cleavage of the C–C triple bond of the alkynes. Under mild conditions, they undergo the rupture of the Cα–CH3 bond of the alkylidene, which comes from the alkyne triple bond, to afford six-coordinate hydride–alkylidyne derivatives. In dichloromethane, complex 2 gives a 10:7 mixture of OsHCl2{≡C(CH2)3C(O)NHPh}(PiPr3)2 (5) and OsHCl2{≡CCH(CH3)(CH2)2C(O)NHPh}(PiPr3)2 (6). The first complex contains a linear separation between the alkylidyne Cα atom and the amide group, whereas the spacer is branched in the second complex. In contrast to the case for 2, complex 4 selectively affords OsHCl2{≡C(CH2)3NHC(O)Ph}(PiPr3)2 (7). In spite of their instability, these compounds give the alkylidene–allene metathesis, being a useful entry to five-coordinate vinylidene complexes, including the dicarbon-disubstituted OsCl2(=C=CMe2)(PiPr3)2 (8) and the monosubstituted OsCl2(=C=CHCy)(PiPr3)2 (9).

Formation of Osmium Alkylidene, Alkylidyne, and Dinitrogen Complexes from Reactions of OsCl2(PPh3)3 with Diazoalkanes

“…Unfortunately, complex 5 is only sparingly soluble in common organic solvents. As a result, the 13 C{ 1 H} NMR spectrum of 5 could not be recorded.…”

“…One might expect that the R group may also affect the equilibrium shown in eq , although such an effect has not yet been described in the literature. In comparison with hydrogen migration, reactions involving migration of the hydrocarbyl group (X = hydrocarbyl) are still rare and are only limited to osmium complexes. − …”

“…Our group has been working on transformations of osmium alkylidene/alkylidyne complexes. In our efforts to study the R effect on α-H migration as well as α-hydrocarbyl migration between osmium alkylidene and alkylidyne complexes, we have searched for a more universal strategy to prepare five-coordinate alkylidene complexes of the type OsCl 2 (alkylidene)­(PR 3 ) 2 , which are still rare in the literature. ,, …”

“…In our efforts to study the R effect on α-H migration as well as α-hydrocarbyl migration between osmium alkylidene and alkylidyne complexes, we have searched for a more universal strategy to prepare fivec o o r d i n a t e a l k y l i d e n e c o m p l e x e s o f t h e t y p e OsCl 2 (alkylidene)(PR 3 ) 2 , which are still rare in the literature. 5,13,14 Reactions of diazoalkanes with transition-metal complexes represent some of the most common routes to transition-metal alkylidene complexes. For instance, the first-generation Grubbs catalysts (Scheme 1, top) could be synthesized by direct treatment of the five-coordinate 16e − starting material RuCl 2 (PPh 3 ) 3 with diazoalkanes N 2 CHR (R = Ph, p-C 6 H 4 Cl, Me, Et) followed by phosphine exchange.…”

Rhenium-catalysed reactions in chemical synthesis: selected case studies

“…[38][39][40][41][42][43][44][45] Classical approaches for the synthesis of transition metal vinylidene complexes exploit 1,2-alkyl or 1,2-hydrogen shifts of internal, [46][47][48][49] or terminal alkynes. [50][51][52][53] These activation pathways are also often invoked in many associated transitionmetal-catalysed reactions employing alkyne substrates. [54][55][56][57] Jia and co-workers demonstrated the first example of C-C bond cleavage of internal propargyl alcohols to access rhenium viny-lidene complexes in 2018 (Scheme 1B).…”