Alkyne metal complexes

Complexes in Which the Acceptor Is a Metal Ion and the Donor is an Alkene or an Aromatic Ring. Note that n donors do not give EDA complexes with metal ions but form covalent bonds instead. Many metal ions form complexes, which are often stable solids, with alkenes, dienes (usually conjugated, but not always), alkynes, and aromatic rings. The generally accepted picture... 

Finally, the reaction of (L)AuX complexes with metallated alkynes in an inert solvent and at low temperature is a particularly safe pathway to complexes with more delicate components (X = halogen M = alkali metal Equation (21)).74 75... 

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... 

W. D. Wulff, Transition Metal Carbene Complexes Alkyne and Vinyl Ketene Chemistry, in Comprehensive Organometallic Chemistry II, Vol. 12, L. S. Hegedus, Ed., Pergamon, Tarrytown, NY, 1995, Chapter 5.3, pp. 469/. 

Following the proposals of Rooney et al. [85—87], it has generally been assumed that, as with monoolefins, the adsorbed state of an alkyne active in hydrogenation is a 7r-complex formed by the interaction of the 7r-orbitals of the acetylenic bond with two metal atoms. The 7r-complexed alkyne may be represented as structure L. 

Pyrrole and indole rings can also be constructed by intramolecular addition of nitrogen to a multiple bond activated by metal ion complexation. Thus, 1-aminomethyl-l-alkynyl carbinols (obtained by reduction of cyanohydrins of acetylenic ketones) are cyclized to pyrroles by palladium(II) salts. In this reaction the palladium(II)-complexed alkyne functions as the electrophile with aromatization involving elimination of palladium(II) and water (Scheme 42) (81TL4277). 

The simplest alkyne complexes, the metal acetylenes, resemble those of ethylene. For example, there are analogues of Zeise s salt in which an acetylene molecule ts bound to platinum(l)) and occupies a position like that of ethylene in Zeise s salt. In addition, there are L,Pt(RC=CR) complexes that have structures paralleling that of LjWHjC CHj) (Fig. 15.24). For both of these PtfO) complexes, an approximate square planar arrangement around the metal is found. Alkynes are more electronegative than ulkenes and are therefore better it acceptors. Thus it is appropriate to view them as metal lacyclopropenes 79... 

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). 

Varghese V, Saha M, Nicholas KM (1988) Org Synth 67 141 Nicholas KM (1987) Acc Chem Res 20 207 Caffyn AJM, Nicholas KM (1995) Transition metal alkyne complexes transition metal stabilized propargyl system. In Abel EW, Stone FGA, Wilkinson G (eds) Comprehensive organometallic chemistry II, vol 12. Pergamon Press, Oxford, p 685 Schreiber SL, Klimas MT, Sammakia T (1987) J Am Chem Soc 109 5749 Nakamura T, Matsui T, Ta-nino K, Kuwajima I (1997) J Org Chem 62 3032... 

In the same way that terminal oxo or nitrene units create a pyramidal distortion in M(0)L5 131) and M(NR)L5 132) complexes, alkynes also tend to pull the metal out of the plane of the four adjacent ligands. In W(CO)(HC=COAlCl3)L3Cl the two PMe3 ligands cis to the alkyne form an L—W—L angle of 161° 130), and in the molybdenum structure the... 

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). 

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