Alkyl bridges

The bimetallic mechanism is illustrated in Fig. 7.13b the bimetallic active center is the distinguishing feature of this mechanism. The precise distribution of halides and alkyls is not spelled out because of the exchanges described by reaction (7.Q). An alkyl bridge is assumed based on observations of other organometallic compounds. The pi coordination of the olefin with the titanium is followed by insertion of the monomer into the bridge to propagate the reaction. 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

AhMcfi Alkyl-bridged organometallics involving 3c-2e, bonds (pp. 259, etc.)... 

A particularly interesting case was the oxidative spiro dimerization of a,CO-bis (tocopheryl)alkanes (36), which basically present two a-tocopherol units linked at C-5a by an alkyl bridge.59 The reaction of other a,0)-bis(hydroxyphenyl)-alkanes, such as 37—40, proceeded similarly (Fig. 6.30). 

Electrogenerated monovalent Co complexes of the well-known open chain N202 Schiff base ligands salen (8), salphen (9), and their substituted derivatives undergo oxidative additions with alkyl halides. Reactions of the complex with substrates within the series RBr (R = Pr, Bu, t-Bu) proceed at different rates. The reaction occurs by an inner-sphere alkyl-bridged electron transfer, with a Co1- R+- X-transition state, which is sensitive to distortions of the complex in different configurations.124... 

Lippard characterized the structural effect of variation in the alkyl bridge length in tropocoro-nand complexes of zinc (80). Six complexes of ligands with bridge lengths from n = m = 3 to n = m = 6 were structurally characterized.702 The complexes had distorted tetrahedral-square planar geometries, with the exception of the m = n = 3 adduct which was five-coordinate with an additional pyridine ligand. 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Another alkyl-bridged PHOX (18, Fig. 29.6) was recently synthesized [17], and used to hydrogenate a series of substituted methylstilbenes in 75-95% ee, and /1-melhylcinnamic esters in 80-99% ee. The hydrogenation results suggest that the selectivity of these catalysts is mainly derived from the substitution at the stereogenic center on the oxazoline ring, with the other stereocenter having a relatively minor effect on the ee-value. 

Generalizations for cation- versus alkyl-bridged compound formation 271... 

For a long time, mechanisms for the exchange have been suggested, including the most likely one involving alkyl-bridged transition state 1 (equation 1). ... 

Direct reaction of fulvenes with metal atoms appears to afford a useful synthetic route to alkyl-bridged metallocenes (122), e.g.,... 

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