Carbenes molybdenum complex

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

The molybdenum complex 1, a typical high-valent Schrock-type carbene, efficiently catalyzes the self-metathesis of styrene. On the other hand, the cationic iron complex 3 does not induce metathesis but stoichiometrically cyclopropanates styrene. The tungsten complex 2, again a Fischer-type carbene complex, mediates... 

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).

Two possible catalytic systems for olefin mclathesis are the commercially available carbene complexes 61 and 62. Molybdenum complex 62. developed by Schrock,25 was the first to be introduced. Nevertheless, it has the disadvantage of being both unstable and difficult to access. The newer Gruhbs catalyst 61 is now preferred because it is stable and also easy to synthesi/e. Both catalysts are also appropriate for starting materials containing heteroatoms (see alst) Chapter 16). 

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-... 

Olefin metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis, and the reaction has been generally accepted to proceed through a series of metallacyclobutanes and carbene complexe intermediates [40-43]. For this type of reaction, the most widely used catalysts include an alkoxyl imido molybdenum complex (Schrock catalyst) [44] and a benzylidene ruthenium complex (Grubbs catalyst) [43]. The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

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],... 

In addition to the compound classes mentioned so far, cyclopropenylidene complexes have also been reported. The first mononuclear carbene complex not stabilized by a heteroatom in the (X-position was the chromium complex 27, which proved to be unexpectedly stable. This complex was obtained in 20% yield by treatment of 3,3-dichloro-l, 2-diphenylcyclopropene with disodium pentacarbonylchromate at — 20 °C. However, isolation of the complex is easier when it is prepared from 3-ethoxy-l,2-diphenylcyclopropenium tetrafluoroborate. The corresponding molybdenum complex can also be obtained in this way. 

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