Molybdenum complexes carbene

Methylenation with 56a is typically performed by adding the carbonyl compounds to a solution of the reagent prepared in situ (Table 4.17). This methylenation is of interest in that the reaction is extremely sensitive to steric hindrance. The molybdenum compound 56a methylenates aldehydes such as heptanal and ben-zaldehyde. Although 56a methylenates acetophenone, diethyl ketone and henzo-phenone are not transformed to the corresponding terminal olefins. Thus, a molecule containing both ketone and aldehyde fimctions is selectively methylenated only at the aldehyde carbonyl (entry 3). 

Similar treatments of Cl3(0)Mo, (EtO)3MoCl2, and (EtO)2MoCl3 with methyl-lithium afford the corresponding molybdenum-methylidenes 56b, 56c, and S6d, which also methylenate aldehydes [122]. As regards the structure of 56b, later investigations indicated it to be the dimeric l,3-dioxo-l,3-dimolybdenum(V) cydo-butane complex, similar to S6a [124]. The reactivity order of these reagents for 

It is noteworthy that aldehydes and ketones can be methylenated in aqueous or ethanohc media with the molybdenum compounds S6a and S6b [125]. The yields of the olefins obtained by the reactions with 56b carried out in EtOH are virtually the same as those obtained using anhydrous and alcohol-free THE as the solvent. The reagent 56a also methylenates aldehydes in aqueous or ethanohc solvents, but the yields of the olefins are lower. These methods seem to be advantageous for the olefination of hydrophilic substrates. 

Slow addition of three and four equivalents of MeLi to M0CI5 produces Cl2(Me)Mo=CH2 (56e) and Cl(Me)2Mo=CH2 (56f), respectively. These carbene complexes react with aromatic aldehydes to afford alkenes in good yields [127]. 

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Z,Z)-l,4-Dialkoxy-l,3-dienes can be readily prepared from propargyl ethers and molybdenum carbene complexes (equation 185)307. High stereoselectivity in this reaction may be due to the formation of stable vinyl hydride complex with the enol ether. 

The metal-catalysed olefin metathesis (equation 122) when applied to dienes results in ring-closure and expulsion of an olefin (equation 123). Thus the molybdenum carbene complex 241 promotes the decomposition of the 1,6-heptadiene derivative 242 to a mixture of the cyclopentene 243 and ethylene (equation 124)122. An analogous reaction of the alcohol 244 gives 245 (equation 125), and 4-benzyloxy-l,7-decadiene (246) affords the cyclohexene 247 and 1-butene (equation 126). These transformations, which occur in benzene at room temperature, proceed in excellent yields122. 

A remarkable reaction, discovered by McGuire and Hegedus in 1982 [292], is the photochemical conversion of heteroatom-substituted chromium and molybdenum carbene complexes into intermediates with ketene-like character (Figure 2.21). This reaction has been reviewed by Hegedus [203]. 

Experimental Procedure 3.1.1. Preparation of a Molybdenum Carbene Complex by Nucleophilic Abstraction (2,6-Piisopropylphenylimido)bis[ 1,1 -bis(trifluoro-methyl)ethoxy](2-methyl-2-phenyH-propylidene)molybdenum [37,416,417]... 

Kinetic resolution of chiral dienes has been realized by RCM with enantiomeri-cally pure molybdenum carbene complexes [888-890]. High enantiomeric excesses of recovered diene (> 99% ee) and cycloalkene (93% ee) could be obtained. 

Since the discovery of ruthenium and molybdenum carbene complexes that efficiently catalyze olefin metathesis under mild reaction conditions and that are compatible with a broad range of functional groups, olefin metathesis has increasingly been used for the preparation of alkenes on insoluble supports. In particular, the ruthenium complexes Cl2(PCy3)2Ru=CHR, developed by Grubbs, show sufficient catalytic activity even in the presence of air and water [781] and are well suited for solid-phase synthesis. 

Ring-closing metathesis is well suited for the preparation of five- or six-membered heterocycles, and has also been successfully used to prepare tetrahydropyridines on insoluble supports (Entries 1 and 2, Table 15.23). Because metathesis catalysts (ruthenium or molybdenum carbene complexes) are electrophilic, reactions should be conducted with acylated amines to avoid poisoning of the catalyst. 

The ROMP of 136 may be used as the first stage in the preparation of polyacetylene molecules with mesogenic (liquid-crystalline) functional groups at the chain ends the ROMP of 136 is initiated by a molybdenum carbene complex and the living ends terminated by reaction with a substituted benzaldehyde bearing a mesogenic group, followed... 

Most of the work has been done with RUCI3, OSCI3 or IrCl3 as catalysts at 50-80 °C in water, aqueous emulsion, an aromatic solvent, or mixtures of an alcohol and water. Tungsten or molybdenum carbene complexes in toluene are effective at 20 °C with monomers that do not contain hydroxyl groups. Thus 8W (R = Me) gives polymers of very high... 

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-... 

Several catalysts for ring-closing metathesis are now known. Prominent examples are shown in Scheme 1 the molybdenum carbene complex 8, introduced by Schrock et al. [4], methyltri-oxorhenium 9, discovered by Herrmann et al. [5] and the ruthenium carbene complex 10, developed by Grubbs et al. [6]... 

Silver carbene complexes are the most commonly used carbene transfer complexes [83], Other carbene transfer agents include lithium adducts [56], potassium complexes [53], molybdenum carbene complexes [83,84] or chromium carbene complexes [85],... 

Like the corresponding chromium complex shown in Scheme 7-3, the molybdenum-carbene complex (j/ -C5H4Me)Fe jy -C5H3[l-(OEt)C=Mo(CO)5](2-Me) undergoes a single one-electron oxidation in DME solution. The redox potential ( ° = -I- 0.73 V) is nearly coincident with that of the chromium complex (see Table 7-4) [36]. 

Like the chromium- and molybdenum-carbene complexes of the type illustrated in Scheme 7-3, the corresponding octahedral tungsten complexes exhibit only a one-electron oxidation, which is thought to be centered on both the ferrocenyl and the tungsten fragments. The redox potentials are summarized in Table 7-10. 

Fischer chromium and molybdenum carbene complexes also react with unactivated 1,3-dienes, to give mono adducts of butyl(methoxy) carbene in high yields. The best results were achieved when ca, 25% excess of carbene precursor was used. The high regio-, chemo- and stereoselectivity of carbene addition were observed in these reactions. 

Intermolecular addition of molybdenum carbene complexes 35 (M = Mo) to dienylacetylene 34 leads to the generation of divinylcyclopropanes, which rearrange to bicyclo[5.3.0]deca-l,5-dienes 36. ... 

Hegedus, L.S., Schultze, L.M., Toro, J., and Yijun, C, Photolytic reaction of chromium and molybdenum carbene complexes with imines. Synthesis of cepham, oxapenam, and oxacepham derivatives. Tetrahedron, 41, 5833, 1985. 

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