Acetylene complexes migration

The ligand migration or insertion reaction is a fundamental mechanistic process of organometallic chemistry. As relating to acetylene complexes, it may be represented as shown in [Eq. (58)]. The reaction thus may be viewed either as an addition of M—L to the acetylene or, alternatively, as an insertion of the acetylene into the M—L bond. In cases where L = carbon monoxide, alkene, acetylene, or other potentially bidentate ligands, L may remain bonded to the metal, resulting in metallocycle formation [Eq. (57)]. 

Both MeMn(CO)5 and PhMn(CO)5 react with acetylenes to yield vinyl ketone tetracarbonyl complexes, most likely via a pathway involving CO insertion [Eq. (18)] 14, 36). Reactions of these same alkyls with 1,3-dienes may proceed similarly 16, 95, 96). The chelating ligand o-styryldiphenyl-phosphine (L—L) converts MeMn(CO)j into two products 25) whose structures (XXII and XXIII) were elucidated by X-ray crystallography 24). An unusual migration of COMe onto L—L occurs subsequently to the initial insertion step. 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

If the pKa of the corresponding acid R1 - H from the stabilized carbanion is smaller than 35, the migration of R1 fails in (dichloromethyl)borate complexes. Failure to convert pinanediol [(phenylthio)methyl]boronate to an a-chloro boronic ester has been reported15. Reaction of (dichloromethyl)lithium with an acetylenic boronic ester resulted in loss of the acetylenic group to form the (dichloromethyl)boronate, and various attempts to react (dichloromethyl)boronic esters with lithium enolates have failed17. Dissociation of the carbanion is suspected as the cause, but in most cases the products have not been rigorously identified. 

This was explained by the involvement of a vinylidene complex that is also in agreement with the migration of the acetylenic hydrogen to C-2 observed by deuterium labeling. The stereoselective reaction requires the use of EtjN and a slight excess of the alkyne. 

Acetylene-vinylidene rearrangements of silylacetylene-iron carbonyl complexes have been observed,537 while iron-acetylide hydride complexes of the type [Fe(H)(C=CR)(dmpe)2], where dmpe=l,2-bis(dimethylphosphino)ethane, have been found to react with anions to afford substituted alkenyl complexes. It has been proposed538 that a likely reaction course for this latter rearrangement involves initial protonation of the cr-bound acetylide ligand at the carbon (I to the metal centre to form a vinylidene complex. Metal-to-carbon hydride migration in this vinylidene complex with attack by the anion would then lead to the neutral complex (see Scheme 106). A detailed mechanistic investigation has been carried out539 on the novel metathetical... 

Scheme 8). Although the insertion step forming the intermediate acetylene hydride complex appears feasible, the migration of hydride from the metal to the /3 carbon is energetically too costly for this to be a significant pathway for the reaction (69). 

Stoichiometric or catalytic use of W(CO)s-THF leads to successful cycloalkenylation of iu-acetylenic silyl enolates such as 42a in the presence of water (Scheme 23).321 321a The W(CO)s-promoted cyclization of iu-iodoacetylenic silyl enolate 42b affords the iodine-migrated product in good yield.322 This observation is indicative of the presence of the vinylidene complex intermediate 43. 

One important example of this class of rearrangement was reported for a rhenium complex in 199367 and, in retrospect, took advantage of the lower barrier of hydrogen migrations as compared to alkyl migrations. Mayer and co-workers prepared the tris-acetylene hydroxide complex 11, as well as the oxo hydride 12. Upon standing at room temperature for 5 days in benzene solution, the hydroxide spontaneously forms the oxo hydride with loss of one equivalent of acetylene, as shown in Eq. (8). 

It has been found that monosubstituted acetylenes can be obtained by Brown s method (equation 164) only when lithium acetylide is replaced by the lithium acetylide-ethylenediamine complex. High yields of terminal alkyl and cycloalkyl-acetylenes are obtained. It has also been demonstrated that the migration of an alkyl group proceeds with retention of configuration, as shown in equation (168). ... 

Iwasawa et al. have demonstrated that the endo-seleclive cycloalkenylation of co-acetylenic SEE is successfully achieved by stoichiometric or catalytic use of W(C0)5 THF (Scheme 10.101) [269]. In the reaction of SEE 99 fhe mode of cyclization endo or exo can be controlled by appropriate choice of the silyl group, the amount of W(CO)5, and the solvent. The W(CO)5-promoted cyclization would proceed by nucleophilic addition of fhe enolate to a W(CO)5-coordinated alkyne and/ or a vinylidene W(CO)5-complex. Quite recently, the cyclization of co-iodoacetylenic SEE such as 100 has been found to afford fhe iodine-migrated products in good yields [270]. This observation indicates the presence of the vinylidene complex intermediate 101. 

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