Molybdenum-methylidenes

The most abundant molybdenum methylidene species (47), which is the most stable methyhdene supported on the most-stable (110) surface, is associated with high-energy barriers in the metathesis reaction with ethene, thereby indicating that the most abundant alkylidene species are not the active catalysts. In contrast, the alkylidene species on the less-stable (100) surface (44 and 45) are predicted to be, in general, more active because the surface constraints around this site significantly destabilized the formation of the metallacyclobutane intermediate. 

Several molybdenum-methylidene complexes 56 are employed as olefination reagents (Scheme 4.53). Some of them differ from the conventional carbonyl olefination reagents in their acidic character and chemoselectivity. Generally, molybdenum carbene complexes are thermally unstable and attempts to isolate them have hitherto been unsuccessful. Therefore, the olefinating reagents are generated in solution in the presence of the carbonyl compounds such that they react immediately. 

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... 

Tab. 4.17. Olefination of carbonyl compounds with the molybdenum-methylidene complex 56a.

Scheme 4.55. Chemoselective carbonyl olefination with the molybdenum-methylidene 56b.

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Grubbs-type initiators are well-defined ruthenium aUcylidenes. Compared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based Grubbs catalysts is somewhat different. In terms of polymer structure, ROMP of norbom-2-enes and norbomadienes using ruthenium-based systems generally results in the formation of polymers that, in most cases, predominantly contain frany-vinylene units. Polymerizations initiated by Grubbs-type initiators are best terminated by the use of ethyl vinyl ether, yielding methylidene-terminated polymers. 

Mo(NAr)(CHCH=CMe2)(Me2Pyr)(OHMT) is relevant to the Z-selective homocoupling of 1,3-dienes by molybdenum and tungsten MAP complexes. The formation of the desired M=CHR complex can be complicated by the reformation of M=CHR or the metathesis of R CH=CH2 (when added in large excess) to give R CH=CHR and ethylene, which lead to the formation of methylidenes and unsubstituted metaUacyclobutane complexes. 

The modeling of molybdenum-based classical catalysts supported on alumina was improved by the use of calculations with periodic boundary conditions [10, 49, 52], which better represent the alumina surface [67]. It became possible to describe the relative stabilities of the different surface sites, including the effect of temperature and water pressure. The more stable (110) and (100) alumina surfaces were considered, and the investigation focused on the structure of the potential initial molybdenum-oxide monomeric and dimeric species, as well as the corresponding methylidene species and their reactivity with ethene [10,49]. 

In mechanistic studies, the molybdacyclobutane of a MAP catalyst was found to break up to ethylene/methylidene intermediates 4500 times faster than the corresponding tungstacycle (at 40°C) [19]. Syn and anti proton exchange were also found to be significantly faster (up to lOOx s) for molybdacycles than for tungsta-cycles. Methylidene rotation about the M=C bond was determined to be comparatively slower for molybdenum complexes (<0.2 s ) than for tungsten complexes (3.6-230 s ). Schrock and coworkers proposed that many of these properties contribute to the superior efficiency of the tungsten MAP catalysts relative to their molybdenum counterparts. 

Bis iiitrosyl(i -pentamethylcyclopentadienyl)[(trimethylsilyl)methyl][(trimethylsilyl)-methylidene]molybdenum(II) (Dilithium)tris(tetrahydrofuran) (12) [Mo(CH2TMS)2Cp (NO)l (200 mg, 0.46 mmol) and LiHMDS (90 mg, 0.46 mmol) were intimately mixed and cooled to -100 °C in a small flask. THF was slowly poured dovm the sides of the flask and allowed to freeze onto the solid mixture. Over the course of 4 h, a color change from purple to red occurred the final mixture was talcen to dryness in vacuo. The remaining red solid was extracted into pentane (2 mL), and the extracts were filtered through Celite. Slow evaporation of the pentane filtrate resulted in the deposition of pale red crystals, which were recrystallized (pentane) to obtain pale yellow crystals of 12 yield 143 mg (62%). 

Carbonyl Olefination Using Zirconium, Tantalum, Niobium, Molybdenum 1191 Tab. 4.18. Methylidenation of aldehydes with molybdenum metallacycles 57. 

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