Alkynes metalated

With an electrophilic transition metal complex, it is believed that the hydration of an alkyne occurs through a trans-addition of water to an 72-alkyne metal complex (Scheme 15, path A),380 although the m-pathway via hydroxymetallation has also been proposed (path B).381,382 However, distinguishing between the two pathways is difficult due to the rapid keto-enol tautomerization that renders isolation of the initial water adduct challenging. 

Complex condensation products are obtained by reaction of the alkyne complex with excess of 3,3-dimethyl-l-butyne, which yields two isomeric products of formulas Ru3(CO)6[HC2C(Me)3-COCH2CMe3][HC2CMe3]2 (125). The X-ray structure of one of those adducts (Fig. 19) shows that both dimerization of two alkyne molecules and the insertion of carbon monoxide into the alkyne metal bonds have occurred. The Ru-Ru distances of 2.820,2.828, and 2.686 A in the ring are of interest. The value of 2.686 A is one of the shortest found in a... 

Scheme 1. Alkene-and alkyne-metal complexes may be formulated with metallacyclo-propane (A) or -propene (B) structures, respectively. Similarly, complexation of a metal atom with an alkylidene or alkylidyne group gives rise to a dimetallacyclopropane (C) or -propene (D) ring system.

Although alkynes are highly reactive toward a wide range of transition metals, very few instances of metal-catalyzed reactions of nucleophiles with alkynes are known. This is, in part, because most stable alkyne-metal complexes are inert to nucleophilic attack, while most unstable alkyne-metal complexes tend to oligomerize alkynes faster than anything else. Hence synthetic methodology involving this process is quite limited. 

Carbometallation is a term coined for describing chemical processes involving net addition of carbon-metal bonds to carbon-carbon Jt-bonds [1] (Scheme 4.1). It represents a class of insertion reactions. Whereas the term insertion per se does not imply anything chemical, the term carbometallation itself not only explicitly and clearly indicates carbon-metal bond addition but also is readily modifiable to generate many additional, more specific terms such as carboalumination, arylpalladation, and so on. In principle, carbometallation may involve addition of carbon-metal double and triple bonds, that is, carbene- and carbyne-metal bonds, as well as those of metallacycles. Inasmuch as alkene- and alkyne-metal Jt-complexes can also be represented as three-membered metallacycles, their ring expansion reactions via addition to alkenes and alkynes may also be viewed as carbometallation processes (Scheme 4.1). 

In the idealized ethylene-acetylene model complex the HOMOl is the olefin stabilized dxz while the HOM02 orbital, dxy, reflects alkyne w overlap. The M—C alkyne distances employed in the calculation increase overlap responsible for the alkyne-metal v interactions relative to the olefin which is further from the metal and overlaps less (60). The dir bonding contribution of the single-faced 7r-acid olefin is to stabilize the lone filled d tr orbital which is independent of the alkyne. This role is compatible with the successful incorporation of electron-poor olefins cis to the alkyne in these d4 monomers. It may well be that the HOMOl and H0M02 orbitals in isolated complexes are reversed relative to the model complex as a result of electron-withdrawing substituents present on the olefins. 

Bisalkyne bisdithiocarbamate derivatives are both harder to reduce and harder to oxidize than their carbonyl analogs (53). Factors discussed in the Section VI and used to rationalize visible absorption spectra are also applicable here. The potentials required for oxidation and reduction reflect the strength of the alkyne-metal dn interactions. Their properties, as well as chemical behavior, no doubt reflect the complementary nature of the complete set of a and 77 metal-ligand bonds in these happy d4 M(RC=CR)2(S2CNEt2)2 complexes. 

Alkyne ligands have been removed from metal complexes intact, in oxidized form, and in reduced form. In spite of accomplishing alkyne-metal cleavage with different reagents in specific cases, there is no general method for systematically separating an alkyne ligand from a d4 metal center. 

The cluster valence electron (c.v.e.) count usually corresponds to 12 + 22 electrons. Bonding of the C2 unit involves stabilization of a, a, and ji orbitals by interaction with radial metal MOs of the same symmetry, together with overlap of ji orbitals with filled metal MOs, i.e., a similar synergic interaction to the familiar bonding mode found in alkyne-metal complexes. For the model [Co8(C2)(n-L)(L)8]4 based on two trigonal... 

Complexes containing coordinated tertiary phosphines or arsines of the type [Rh2(PF3)4L2(alkyne)] and [Rh2(PF3)2L2(alkyne)] (L = mono-dentate, L = bidentate ligand) are readily obtained by displacement of PF3 from [Rh2(PF3)6(alkyne)] under mild conditions (method D). There are also reports of displacement of CO by PF3 from an alkyne metal carbonyl complex (method E) e.g., 113). 

At the outset of the subsection, it should be emphasized that distinction between metallacyclopropenes and q -alkyne-metal complexes or that between metallacyclopropanes and q -alkene-metal complexes is largely a matter of semantics or convenience. Most or perhaps all of these species should probably be viewed as hybrids of three-membered metallacycles and ir-complexes. 

Alkynyllithium reagents add readily to metal carbonyls, leading to thermally unstable anionic alkynic metal carbonylates (8), which give mainly the 1,3-diyne, by treatment with iodine-methanol in THF (Scheme... 

The Pauson-Khand reaction gives the same product as the group 4 metal-mediated reductive coupling and carbonylation, and both reactions proceed by essentially the same mechanism formation of an alkyne-metal tt complex, insertion of an alkene, insertion of CO, and reductive elimination. Some details differ, however. When an alkyne is added to Co2(CO)g, CO evolves, and an isolable, chromatographable alkyne-Co2(CO)6 complex is obtained. This butterfly complex contains four Co(II)-C bonds, and the Co-Co bond is retained. The formation of the alky n e-C o2 (C O) 6 complex involves the formation of an ordinary tt complex of the alkyne with one Co(0) center, with displacement of CO. The tt complex can be written in its Co(II) cobaltacyclopropene resonance structure. The tt bond of the cobaltacyclopropene is then used to form a tt complex to the other Co center with displacement of another equivalent of CO. This second tt complex can also be written in its cobaltacyclopropene resonance structure. The alkyne-Co2(CO)6 complex has two 18-electron Co(II) centers. 

As outlined in 5.8.2.3.4, coordination of an alkene to a neutral or cationic transition metal activates the alkene toward nucleophilic attack, leading to an alkylmetal product. Development of the analogous synthesis of alkenylmetal complexes starting with /y -alkyne-metal complexes is more recent. An early reaction of this type is ... 

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