Molybdenum complexes reaction with alkynes

Molybdenum and tungsten carbyne (alkylidyne) complexes frequently undergo 2+2 cycloaddition reactions with alkynes to give the corresponding metallacyclobutadiene... 

Other late transition metals used in special cases of hydrosilation include cobalt, iron, ruthenium (vide infra for reactions with alkynes), osmium, chromium, molybdenum, tungsten and copper. Metallocenes (see Metallocene Complexes) of early transition metals and lanthanides have also been found to catalyze the hydrosilation of a number of unsaturated compounds including alkenes and esters (vide infra). 

Scheme 56 Reaction of niobium and molybdenum disulfido complexes with alkyne...

The symmetrical dienyne 58a was converted to a fused bicyclo [4.3.0] ring in 95% yield [17] (Eq. 27). With substrate 58c containing an unsymmetrical diene tether, two different products, 59c and 59c, were obtained in a ratio of 1 to 1 (Eq. 28). The reaction course in the formation of the different bicyclic rings is shown in Scheme 8. This dienyne metathesis is also catalyzed by tungsten or molybdenum complex 62 or 63 (Fig. 1), and a dienyne bearing terminal alkyne 58b could be cyclized to give 59b in 97% yield. 

As part of a study of the reactions of metallacyclic y-ketovinyl complexes of molybdenum and tungsten with acetylenes, directed toward the synthesis of complexed -/-lactones, Stone has reported92 the isolation of several vinyl-ketene complexes. When complex 72 was heated with 2-butyne, one molecule of the alkyne was incorporated into the complex with concomitant carbonylation. X-ray analysis of the product (73) has shown unequivocally that the C-l to C-4 vinylketene fragment is bonded in a planar, rj4-configu-ration. In contrast to the thermal reaction, ultraviolet irradiation of 72 or 74 in the presence of 2-butyne affords the complexes 75 and 76, respectively, where the lone carbonyl remaining after alkyne insertion had been replaced by a third molecule of the alkyne. 

The bulky thiolate ligand 2,4,6-triisopropylbenzenethiolate (tipt) forms the stable complex [Mo(tipt)4], which gives monoadducts with alkynes, nitriles, CO and other small ligands and, unlike [Mo(SBu )4], the molybdenum does not appear to be reduced in these reactions.182... 

Electrocyclization of E is expected to give cyclohexadienone complex F a related molybdenum complex 6 (Scheme 4), in which two carbonyl ligands have been replaced by alkyne ligands, has been isolated from the reaction of a vinyl molybdenum carbene complex with 3-... 

Syntheses of simple d4 alkyne monomers of molybdenum and tungsten with no chelating or cyclopentadienyl ligands in the coordination sphere were first reported in 1982 (44). During the past several years a variety of M(CO)(RC=CR)L2X2 complexes has been prepared, but reaction chemistry of the bound alkyne in these complexes has not yet been developed. 

As stated in the Introduction, Templeton7 published a comprehensive review entitled Four Electron Alkyne Ligands in Molybdenum(II) and Tungsten(II) Complexes, including halocarbonyl complexes, which dealt with the literature up to 1987. Hence, this section of the review deals with the literature on halocarbonyl alkyne and alkene complexes published from 1987 to mid-1995. The vast majority of papers that have appeared during this period are concerned with the reactions of [MI2(CO)3(NCMe)2] (M = Mo, W) and their derivatives with alkynes, which begin this section of the review. 

PhS, p-tolS) initially yields the bis(nitrile) complexes [MI2(CO) (NCMe)2(i72-RC2R )]. which dimerize to yield the iodo-bridged complexes [ M(/t-I)I(CO)(NCMe)(T)2-RC2R ) 2].245 The reactions of the diphenylacet-ylene molybdenum complex with donor ligands and alkynes are shown in Scheme 6. 

Thus far, the reactions of the diiodo-complexes [MI2(CO)3(NCMe)2] with alkynes have been described. In this section the reactions of other seven-coordinate molybdenum(II) and tungsten(II) halocarbonyls with alkynes are described. 

Thiiranes can be prepared directly from alkenes using specialized reagents. Thiourea with a tin catalyst gives the thiirane, for example. " Interestingly, internal alkynes were converted to 1,2-dichorothiiranes by reaction with S2CI2 (sulfur monochloride).It is noted that epoxides are converted to thiiranes with ammonium thiocyanate and a cerium complex. " A trans-thiiration reaction occurs with a molybdenum catalyst, in which an alkene reacts with styrene thiirane to give the new thiirane. 

This is a general reaction for alkynes (acetylene, phenylacetylene, diphenylacetylene, cyclodecyne, etc.) that occurs at RT, producing 75% yields of products. The two cobalt atoms and two carbons form a tetrahedron. Analogous iron complexes are prepared in low yield by reaction of diphenylacetylene with Fe3(CO)i2 . Similar molybdenum complexes are prepared by addition of alkynes to molybdenum multiple bonds ... 

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