Molybdenum, silyl

Diene 265, substituted by a bulky silyl ether to prevent cycloaddition before the metathesis process, produced in the presence of catalyst C the undesired furanophane 266 with a (Z) double bond as the sole reaction product in high yield. The same compound was obtained with Schrock s molybdenum catalyst B, while first-generation catalyst A led even under very high dilution only to an isomeric mixture of dimerized products. The (Z)-configured furanophane 266 after desilylation did not, in accordance with earlier observations, produce any TADA product. On the other hand, dienone 267 furnished the desired macrocycle (E)-268, though as minor component in a 2 1 isomeric mixture with (Z)-268. Alcohol 269 derived from E-268 then underwent the projected TADA reaction selectively to produce cycloadduct 270 (70% conversion) in a reversible process after 3 days. The final Lewis acid-mediated conversion to 272 however did not occur, delivering anhydrochatancin 271 instead. 

Ketones and carboxylic esters can be a hydroxylated by treatment of their enolate forms (prepared by adding the ketone or ester to LDA) with a molybdenum peroxide reagent (MoOs-pyridine-HMPA) in THF-hexane at -70°C. The enolate forms of amides and estersand the enamine derivatives of ketones can similarly be converted to their a hydroxy derivatives by reaction with molecular oxygen. The M0O5 method can also be applied to certain nitriles. Ketones have also been Qc hydroxylated by treating the corresponding silyl enol ethers with /n-chloroperoxy-... 

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]. 

With the silyl-substituted difluoroallene 245 and molybdenum hexacarbonyl, the expected [2+ 2+ 1] cycloaddition is not observed. CO is not incorporated and the cyclobutene derivatives 246 can be isolated in good yields (Scheme 15.77) [148]. 

The mechanism for allyl alcohol isomerisation has been studied and the presence of alkoxy and oxo groups in the metal catalyst seems to be essential [5], Not only vanadium, but also rhenium and molybdenum analogues are catalyst for this reaction. The mechanism is depicted in Figure 5.8. The substituents at oxygen can be alkyl groups or silyl groups. 

Peroxo-/i-oxo-bis bisOrimcLhy]si]y])methyl tin (316) is an organotin analog of the FOZ of a symmetrically tetraalkylated olefin (278). It is obtained in low yield by oxidation of the corresponding silylated dialkyltin in the presence of a molybdenum catalyst. The structure was elucidated by XRD analysis and has some characteristic features (a) The 0—0 distance is unusually long (154.3 pm) as compared to that of similar transition metal... 

Other methods for a-hydroxy ketone synthesis are addition of O2 to an enolate followed by reduction of the a-hydroperoxy ketone using triethyl phosphite 9 the molybdenum peroxide-pyridine-HMPA oxidation of enolates 10 photooxygenation of enol ethers followed by triphenylphosphine reduction 11 the epoxidation of trimethyl silyl enol ethers by peracid 1 - the oxidation of trimethylsilyl enol ethers by osmium tetroxide in N-methylmorpholine N-... 

Ojima and co-workers first reported the RhCl(PPh2)3-catalyzed hydrosilylation of carbonyl-containing compounds to silyl ethers in 1972.164 Since that time, a number of transition metal complexes have been investigated for activity in the system, and transition metal catalysis is now a well-established route for the reduction of ketones and aldehydes.9 Some of the advances in this area include the development of manganese,165 molybdenum,166 and ruthenium167 complex catalysts, and work by the Buchwald and Cutler groups toward extension of the system to hydrosilylations of ester substrates.168... 

The molybdenum complex [(CF3)2MeCO]2Mo(NAr)(=CHCMe2Ph) has been observed to be a more efficient catalyst for cyclization of vinyl silyl ether dienes than the ruthenium complex Cl2(PCy3)2Ru(=CHPh), probably because this type of alkene is sterically more demanding (than allyl derivatives) and therefore requires a catalyst less sensitive to steric bulkiness near the reaction center. However, some examples of the RCM of substituted vinylsilanes catalyzed by ruthenium complexes have been reported [127, 131] (Eq. 74). For more examples see Ref. [127]. 

TRICARBONYL-(ii5-CYCLOPENTADIENYL)SILYL COMPLEXES OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN... 

The orange complex is air sensitive in the solid state. It is reasonably soluble and stable in THF but decomposes in nonpolar solvents, such as benzene, to give polymeric halogen-bridged products.9 It is paramagnetic with a magnetic moment of 3.63 BM, and its IR spectrum shows a very intense band at about 820 cm-1 due to coordinated THF. Its reactions with tertiary phosphines in THF give a series of molybdenum (III) tertiary phosphine complexes,9 and with trimethyl-silyl azide Mo(V) nitrido complexes are formed.8... 

This chapter is primarily concerned with keto-enol equilibrium and the chemistry of dissociated enols (enoxides) and lithium enolates. The acid-base aspects of the chemistry of other metal enolates (e.g. silyl enol ethers "", boron enol ethers" " " , aluminium , tin " , gallium , bismuth , zinc " ", rhodium , palladium " , manganese ", copper , nickel , magnesium " , titanium " , molybdenum , zirconium" " and ammonium" " enolates) have been reported elsewhere. 

Ethene, molybdenum complex, 26 102 rhenium complex, 26 110 Europium, tetrakis[T) -l, 3-bis(trimethyl-silyl)cyclopentadienyl]di-p.-chloro-di-, 27 171... 

OLuSiC,HH24, Lutetium, bis(T] -cyclopenta-dienyl)(tetrahydrofuran)[(trimethyl-silyl)methyl]-, 27 161 O, Oxide, gold complex, 26 326 ON, Nitrosyls, molybdenum and tungsten, 26 132, 133... 

Many metal silyl complexes, especially those with early transition metals, have weak metal-silicon bonds. The molybdocene silyl hydride complexes seem to have molybdenum-silicon bonds of sufficient strength to withstand nucleophilic attack by strong hydride reagents (Eq. 4)... 

We must however keep in mind that some of the above reactions may not be simple reactions at the silicon atom, since transition metal complexes show multicenter reactivity (metal atom, ligands) as exemplified in the chemistry of triphenylgermyl-carbene complexes of cobalt carbonyl (253). Thus, displacements of a silyl ligand may result from a multistep process and a thorough examination of these reactions has to be made. An example can be drawn from molybdenum-germanium chemistry (247). As shown in Scheme 59, germanium is displaced from complex 167 by HO with retention of configuration. Actually,... 

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