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Alkyl-substituted metallacycles

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). [Pg.171]

Besides the reversible processes just mentioned, yS-SiRs ehmination from a silyl substituted metal alkyl or metallacycle is a well-documented process [185,186]. This reaction accounts for an easy loss of a silyl group that can be used to generate a M-S1R3 moiety in catalytic processes such as dehydrogenative silation reactions (see below). This reaction, and the analogous yS-SnRs elimination, may also be involved in the loss of regioselectivity found for some C-C coupling reactions of vinyl silanes or vinyl tin derivatives (cine substitution in Hiyama and Stille... [Pg.354]

The procedures given by Eqs. (16)—(18) may be used either to alkylate silicon halides or tin halides in very high yields (90-100%), or to substitute the alkyl functions in the metallacycles by halogen atoms. The two products in these reactions can be easily separated from one another and compounds 57-60 are isolated in high yields (63,69). [Pg.285]

Metallacycles have also been prepared from two molecules of alkenes or alkynes or from palladated o-alkyl- or aryl- substituted aryls by C-H activation. Metallacycles usually have five- or six-membered ring structures, and four-membered metallacycles have been shown to be intermediates in metathesis (Chapter 6). [Pg.185]

Yamazaki s complex (Structure 5) contains two alkyne molecules linked together to form a five-membered metallacycle. Arene-solvated cobalt atoms, obtained by reacting cobalt vapor and arenes, have been used by Italian workers to promote the conversion of a,w-dialkynes and nitriles giving alkynyl-substituted pyridines [20]. -Tolueneiron(0) complexes have also been utilized for the co-cyclotrimerization of acetylene and alkyl cyanides or benzonitrile giving a-substituted pyridine derivatives. However, the catalytic transformation to the industrially important 2-vinylpyridine fails in this case acrylonitrile cannot be co-cyclotrimerized with acetylene at the iron catalyst [17]. [Pg.1254]

Ind-amido titanium complexes with o -alkenyl functions in position 2 of the indenyl ring have been synthesized and characterized. After activation with MAO, these complexes were used as homogeneous and heterogeneous catalysts for the homopolymerization of ethylene and propylene and the co-polymerization of ethylene and 1,7-octadiene.406 A series of alkyl-, u -alkenyl-, and u -phenylalkyl-substituted Cp- and Ind-amido dichloro titanium complexes have been synthesized and characterized. The cj-phenylalkyl-substituted complexes react with LiBu to give metallacycles via a CH activation reaction on the ortho-position of the phenyl group (Scheme 305).741 742 After activation with MAO, these complexes catalyze ethylene polymerizations. The substituents on the aromatic system influence the polymerization activity of the catalysts and the properties of the polyethylene. The u -alkenyl-substituted catalysts show self-immobilization in ethylene polymerization. [Pg.454]

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... [Pg.244]


See other pages where Alkyl-substituted metallacycles is mentioned: [Pg.376]    [Pg.374]    [Pg.376]    [Pg.375]    [Pg.373]    [Pg.406]    [Pg.67]    [Pg.114]    [Pg.906]    [Pg.500]    [Pg.1905]    [Pg.5]    [Pg.296]    [Pg.182]    [Pg.1490]    [Pg.430]    [Pg.172]    [Pg.1490]    [Pg.151]    [Pg.92]    [Pg.115]    [Pg.179]   
See also in sourсe #XX -- [ Pg.265 ]




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2-Substituted alkyl 3-

Alkyl substitute

Alkyls metallacyclic

Metallacycles

Substitution alkylation

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