Hydrido-alkynyl-intermediate

Another theoretical study also showed that the third pathway (bl +b3+b4), 1,3 hydrogen shift, through a hydrido-alkynyl intermediate could compete with the 1,2 hydrogen shift pathway (bl+b2) when the metal center is electron-rich enough [29, 30]. Indeed several hydrido-alkynyl intermediates have been detected or even isolated during the q -l-alkyne-to-vinylidene rearrangement on electron-rich metal centers, such as Co(I), Rh(I) and Ir(I) [73-78]. The ab initio M P2 calculations by Wakatsuki, Koga and their coworkers on the transformation of the model complex RhCl(PH3)2(HC=CH) to the vinylidene form RhCl(PH3)2(C=CH2) indicated that the transformation proceeded via the oxidative addition intermediate RhCl(PH3)2(H) (C CH) [30]. 

The main species in solution has been identified to be the hydrido-alkynyl complex [IrH(C2Ph)(cod)(//2-iPrPCH2CH2OMe)]+BF4 (23). This is, however, only a sink that results from direct reaction of 22 with the 1-alkyne, draining the active catalyst from the system. The catalysis proceeds via the dihydrido-diene intermediate [IrH2(cod)(//2- PrPCH2CH2OMe)]+ BF4 (24), which reacts reversibly with the alkyne to yield the hydrido-iridium-styryl complex 25, followed by a rate-determining reaction of this hydrido-vinyl species with hydrogen to re-... 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

Scheme 4.5 shows several possible pathways from r -acetylene metal complexes RE to metal vinylidenes PR. In the first pathway (al + a2), metal vinylidenes PR can be obtained from an intermediate (INI) with a 1,2 hydrogen shift from C to Cp. The second pathway (bl + b2) is through an intermediate (IN2) with an r agostic interaction between the metal center and one C—H bond, which undergoes a 1,2 hydrogen shift to PR. The third pathway (bl + b3 + b4) also starts from IN2 but then goes into another intermediate, the hydrido-alkynyl IN3, which leads to PR with a 1,3 hydrogen shift from the metal center to Cp. 

Acetylenes are also oligomerized to mono- or divinylacetylenes, or dienyl-acetylenes by Ni(0) (112), Rh(I) (118), or Pd(II) (114) complexes (Scheme 5). Meriwether et al. (112) proposed hydrido-a-alkynylnickel complexes as active intermediates in the catalytic linear oligomerization. Subsequent insertion of acetylene into an M-o--alkynyl bond has been assumed. 

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