Molybdenum complexes double bonds

Salient structural features of CpL2Mo(rj2-CRCR2) complexes include (1) the orientation of the two Cg substituents roughly orthogonal to the MCaCj3 plane (this is opposite to the location of Cp substituents of rj -vinyl ligands which lie in the MC C plane), (2) short M—Ca distances (1.94-1.96 A) appropriate for a molybdenum-carbon double bond (174),... 

Very recently, synthesis and structure of molybdenum and tungsten complexes of the relatively unhindered disilene Si2Me4 were reported. The x-ray structure of 84 shows a metallacyclosilane structure with W — Si = 2.606(2) A and Si —Si = 2.260(3) A. The W — Si bond length is within the range of various estimates of the Si and W covalent radii and the Si —Si distance falls midway between the expected values for a single (2.35 A) and a double bond (2.14 A) (Fig. 13). 

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

Asymmetric Synthesis Using a Chiral Molybdenum Catalyst In olefin metathesis, a double bond is cleaved and a double bond is formed. Thus, a chiral carbon center is not constructed in the reaction. To realize the asymmetric induction by ring-closing metathesis, there are two procedures a kinetic resolution and desym-metrization of symmetric prochiral triene. Various molybdenum complexes are synthesized in order to explore the viabihty of these approaches (Figure 6.2). 

The same ligands used successfully to synthesize stable Cjq complexes of molybdenum, tungsten and chromium have been used to synthesize the corresponding C7Q complexes. Some examples are M(CO)3(dppb)( ti C q) [42] (with M = Mo, Cr, W), Mo(CO)3(dppe)(Ti C7o) [50], W(CO)3(dppf)(Ti C7o) [41] or Mo(CO)-(phen)dbm(r C7Q) [47, 48] (abbreviations see [52]). As far as could be proven via X-ray spectroscopy the addition takes place at the poles of C q at the 1,2-double bond. The same coordination site was also found for the brown-black Pd complex (p2c2o)Pd(PPh3)2 [53]. 

The most active d metal peroxo complexes toward nucleophilic substrates, like amines, phosphines, thioethers, double bonds etc., are molybdenum, tungsten and rhenium derivatives vanadium and titanium catalysis is also important, in particular when... 

The molybdenum-hydroperoxide complex (Step 3) reacts with the olefin in the rate-determining step to give the epoxide, alcohol, and molybdenum catalyst. This mechanism explains the first-order kinetic dependence on olefin, hydroperoxide, and catalyst, the enhanced reaction rate with increasing substitution of electron-donating groups around the double bond, and the stereochemistry of the reaction. 

It seems likely that the active site for dinitrogen binding involves the molybdenum atom. It has been established by EXAFS that the coordination sphere consists of several sulfur atoms at distances of about 235 pm. An Mo=0 double bond, so common in complexes of Mo(IV) and Mo(Vl), is not present. There are other heavy atoms, perhaps iron, nearby (—270 pm). The ultimate source of reductive capacity is pyruvate, and the electrons are transferred via ferredoxin (see page 911) to nitro-... 

Some simple oximes have been shown to act as N—O bidentates by side-on coordination of the oxime groups alone. The reaction of acetone with a hydroxylamido complex of molybdenum results in this bonding form for 2-propanone oxime.64,243 The N—O and C—N bond lengths indicate single and double bond character respectively. The reaction was related to the prebiotic formation of HCN from formaldehyde and hydroxylamido-Mo chelates. A similar form of bonding for this ligand was concluded with some fluorotungstate(IV) complexes.65... 

Stereochemically, the [(f 5 1-C5H4C2H4)Mo(CO)(> 4-diene)] complexes 85a-85c are closely related to the 72-diene complexes 84a-84c, with the free C=C double bond in the former coordinated to the molybdenum instead of a CO ligand. Two rotamers can be expected, with C-l and C-4 of the dienes pointing toward the five-membered ring (o) or away from it (n). 

In organic chemistry one surely thinks at once of the construction of cyclopropane derivatives from olefins and carbenes. Indeed, it has been shown that this also is possible with our complexes and with C=C double bonds that are electron-poor and arc either polarized or easily polarizable (77-81). As an example of this, I would like to cite the reaction of penta-carbonyl[methoxy (phenyl) carbene]chromium (0), -molybdenum (0), or -tungsten(0) with ethyl vinyl ether (79). One obtains the corresponding cyclopropane derivatives in this case, however, only when one removes... 

Scheme 9 that both the molybdenum and iron complexes can catalyze the allylic amination of nonfunctionalized alkenes with an ene-like transposition of the double bond, but also that the yield of the allyl amine formed, 113, is moderate to high. It is generally found that higher substituted alkenes tend to give the best yields, and un-symmetrical alkenes (trisubstituted) react with virtually complete regioselectivity, as only one isomer is detected. The byproducts are primarily azoxybenzene and aniline, which arise from condensation of nitrosobenzene with phenyl hydroxylamine and reduction of phenyl hydroxylamine, respectively. 

The retarding effect of alcohols on the rate of epoxidation manifests itself in the observed autoretardation by the alcohol coproduct.428,434 446,447 The extent of autoretardation is related to the ratio of the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes. This ratio will vary with the metal. In metal-catalyzed epoxidations with fe/T-butyl hydroperoxide, autoretardation by tert-butyl alcohol increased in the order W < Mo < Ti < V the rates of Mo- and W-catalyzed epoxidations were only slightly affected. Severe autoretardation by the alcohol coproduct was also observed in vanadium-catalyzed epoxidations.428 434 446 447 The formation of strong catalyst-alcohol complexes explains the better catalytic properties of vanadium compared to molybdenum for the epoxidation of allylic alcohols.429 430 452 On the other hand, molybdenum-catalyzed epoxidations of simple olefins proceed approximately 102 times faster than those catalyzed by vanadium.434 447 Thus, the facile vanadium-catalyzed epoxidation of allyl alcohol with tert-butyl hydroperoxide may involve transfer of an oxygen from coordinated hydroperoxide to the double bond of allyl alcohol which is coordinated to the same metal atom,430 namely,... 

In the early 1980s, Schrock prepared a series of tungsten- and molybdenum-based carbyne complexes, and demonstrated that they are viable catalysts for performing stoichiometric and catalytic alkyne metathesis [7]. With the defined carbyne complexes, he laid the foundation for the mechanistic understanding of alkyne metathesis, and was the first to demonstrate that vinyl-substituted carbyne complexes are stable [8] and that alkyne metathesis could be performed in the presence of C=C double bonds. 

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