Electrophiles, metals additions initiated

Additions Initiated by Metals and Metal Ions as Electrophiles. 432... 

Complexes of the type CpCo(PR3)2 are alkylated at the metal with small alkyl halides to give CpCo(PR3)2R (Scheme 25). Bulky halides produce ring-substituted hydrido cations instead, explained by attack of the electrophile from the exo site followed by ring-to-metal proton transfer. This reaction could be electrophilic addition (see Electrophilic Reaction), 5ei, or more probably radical addition initiated by electron transfer similar to the RX reaction of cobaltocene (Section 7.1). Since the oxidation potential of CpCo(P(alkyl)3)2 is more negative than that of cobaltocene, this latter mechanism is very plausible. 

The cyclopropanation is initiated by the interaction of the electrophilic metal-carbene species with the jr-system of the olefin (Scheme 4). Two different mechanisms have been proposed for the formation of the cyclopropane ring a concerted pathway (a) or a two-step process via a metallacyclobutane (b). The first pathway (a) resembles the mode of addition of free carbenes to (C=C) double bonds [33] and has been proposed for reactions of metal carbenoids by various authors [7,11]. The principal bonding interaction in this case initially develops between the electrophilic carbenoid C-atom and the Ti-system of the C-C double... 

The stereochemistry of the second step of an addition initiated by a nonbridging electrophile like a proton will be controlled by which surface of the intermediate cation 5.57 is more easily attacked by the nucleophile. The addition of hydrogen chloride to an alkene is not stereospecifically anti, because the chloride does not necessarily attack the cation either specifically anti or syn to the proton,478 in contrast to addition initiated by bridging electrophiles like bromine, or metallic electrophiles like the mercuric ion, described below. The stereochemistry will depend instead on ion pairing or on the substituents in the cation 5.57, and how they influence the conformation at the time the nucleophile attacks. 

There are two main classes of molecules (substrates) that can perform oxidative additions to metal centers non-electrophilic and electrophilic. Oxidative addition reactions with either class of substrates are favored by metal complexes that are more electron rich. Common non-electrophilic substrates are H2, Si-H bonds, P-H bonds, S-H bonds, B-H bonds, N-H bonds, S-S bonds, C-H bonds, alkenes, and alkynes. An important criterion for these non-electrophillic substrates is that they require a sterically accessible open coordination site on the metal (16e configuration or lower) onto which they need to pre-coordinate before initiating the oxidative addition to the metal center. For these substrates, both ligand atoms typically become cisoidally coordinated to the metal center after the oxidative addition as anionic (T-donors (subsequent ligand rearrangements, of course can occur). H2 is the most important and common for catalysis and a well-studied reaction is shown in Equation (5). 

RCH=CHCOPh undergo a 1,4-addition of ArB(OEt)2, catalysed by (247b) in the absence of Mg + at 120 °C, affording the products with <98% ee Other reactions involving boron derivatives, which in fact act (partly) as nucleophiles, have been discussed above, in the section on Additions Initiated by Metals and Metal Ions as Electrophiles. 

Oxidative addition of alkyl halides can also occur in certain cases by an outer-sphere electron transfer mechanism involving a coordinatively saturated metal center and an alkyl halide. This pathway is shown in Scheme 7.7. Oxidative addition by this initial outer-sphere electron transfer pathway tends to occur instead of an 5, 2 pathway when the electrophile is particularly susceptible to electron transfer, when the electrophile possesses some steric hindrance, when the electrophile possesses a weak C-X bond, and when the metal lacks an available coordination site. Because of the lack of a coordination site at the metal, the initial electron transfer occurs without prior coordination of the electrophile to the metal. This initial step parallels the electron transfer and subsequent radical chemistry that occurs when some carbanions are treated with alkyl halides. ... 

The rate of oxidation/reduction of radicals is strongly dependent on radical structure. Transition metal reductants (e.g. TiMt) show selectivity for electrophilic radicals (e.g. those derived by tail addition to acrylic monomers or alkyl vinyl ketones - Scheme 3.89) >7y while oxidants (CuM, Fe,M) show selectivity for nucleophilic radicals (e.g. those derived from addition to S - Scheme 3,90).18 A consequence of this specificity is that the various products from the reaction of an initiating radical with monomers will not all be trapped with equal efficiency and complex mixtures can arise. 

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