Molybdenum complexes vinylidenes

Tables I and II summarize the structural studies of mononuclear and binuclear vinylidene complexes, and Table III those of propadienylidene complexes which had been reported to mid-1982. As can be seen, the C=C bond lengths range from 1.29 to 1.38 A, and the M-C bond (1.7-2.0 A) is considerably shorter than those found in alkyl or simple carbene complexes. Both observations are consistent with the theoretical picture outlined above, and in particular, the short M-C bonds confirm the efficient transfer of electron density to the n orbitals. In mononuclear complexes, the M—C=C system ranges from strictly linear to appreciably bent, e.g., 167° in MoCl[C=C(CN)2][P(OMe3)2]2(fj-C5H5) these variations have been attributed to electronic rather than steric factors. In the molybdenum complex cited, the vinylidene ligand bends towards the cyclopentadienyl ring (111).

Experimental Procedure 3.1.4. Preparation of a Molybdenum Vinylidene Complex from a Carbyne Complex Tetrabutylanunoniuih Cyano(ethoxycarbonyl) vinylidene (dicarbonyl) hydro-tris(3,5-dimethyl-1 -pyrazblyl)borato molybdenum [526] [37] pp 151 and 188... 

Murakami et al. reported a ring-closing metathesis reaction of allenynes using Schrock s molybdenum alkylidene complex [37]. Treatment of allenynes ISl with a catalytic amount of the complex 15 2 in toluene at rt gave cyclopentene derivatives 1 S3 in good yield. Two possible reaction mechanisms were proposed, one through a vinylidene complex 154 and the other through a carbene complex, but based on several mechanistic studies, they favored the vinylidene complex pathway, which is shown here (Scheme 5.42). 

Deprotonation of some molybdenum-carbyne complexes affords vinylidene derivatives suitable reagents are aryldiazonium salts, trifluoroiodo-methane 34), or n-butyllithium (55) ... 

Protonation of vinylidene and acetylide ligands was also found to be useful for the synthesis of high-oxidation state molybdenum alkylidyne complexes. Green and co-workers demonstrated protonation of the vinylidene complex 20 as shown in Eq. (21) (64). Selegue and co-workers... 

Equations 13.10-13.12 show three examples of the synthesis of vinylidene complexes by reactions of metal-acetylide complexes with acid or base. The molybdenum(II) acetylide complex in Equation 13.10 reacts with acid to protonate the p-carbon and generate a cationic vinylidene complex. In this case, the vinylidene complex is thermodynamically unstable. Warming to 0 °C leads to rearrangement of this species to the tautomeric alkyne complex. In contrast, the more electron-rich molybdenum-acetylide complex in Equation 13.11 containing three phosphite donors generates a vinylidene complex upon addition of a proton from alumina to the 3-carbon of the acetylide. The vinylidene form of the complex is apparently more stable than the alkyne complex in this case. 

When trans-halo-tetracarbonylcarbyne complexes of molybdenum and tungsten, X(CO)4M=CR, are treated with sodium cyclopentadienyl, not only the displacement of the halide, but an additional elimination of two carbonyl ligands is observed, affording dicarbonyl(rj -cyclopenta-dienyl)carbyne complexes [2,3]. A different synthetic approach converts vinylidene ligands into carbyne ligands to yield comparable bisdimethylphosphite substituted complexes of molybdenum [4]. 

Another class of transition metal complexes studied by our group consists of alkyne and vinylidene complexes of tungsten and molybdenum in high oxidation states. Alkyne complexes are interesting from a theoretical point of view because they may be considered to be either side-on coordinated ligand complexes or metallacyclopropenes. They are also important as possible intermediates in the cyclooligomerization and polymerization of alkynes. o " Scheme 3 shows the two types of alkyne complex that were studied. 

R. Stegmann, A. Neuhaus, and G. Frenking,/. Am. Chem. Soc., 115,11930 (1993). Theoretical Studies of Organometallic Compounds. V. Alkyne and Vinylidene Complexes of Molybdenum and Tungsten in High Oxidation States. 

CYCLOAROMATIZATION VIA CHROMIUM-, MOLYBDENUM-, AND TUNGSTEN-VINYLIDENE COMPLEXES... 

In 2002, Lo et al. found the catalytic formation of substimted furans 48 from the corresponding epoxyalkynes 47 via ruthenium-vinylidene complexes as key reactive intermediates (Scheme 21.21) [30]. This transformation is considered to be the ruthenium version of molybdenum-catalyzed reaction of Scheme 21.4. Varela-Fernandez et al. reported the ruthenium-catalyzed cycloaromatization of terminal alkynes bearing either a hydroxy or an amino group (49 and 51) into the corresponding benzofurans and indoles (50 and 52) (Scheme 21.22) [31]. At the same time, Nair et al. reported the same cycloaromatization by using a bifunctional ruthenium complex as a catalyst [32]. In both cases, ruthenium-vinylidene complexes were supposed to work as key reactive intermediates. 

Isolable ruthenium vinylidene and carbene complexes are involved in the coupling of alkynes with allylic alcohols. Some of these transformations were previously known from model reactions. The system aUows the synthesis of a large range of enones (Scheme 32). While most coupling reactions, particularly those applied to organic synthesis, remain faithful to palladium, molybdenum carbonyl complexes, too, have found uses. 

---