Electrophilic complex

This is a further example of a carbonyl-electrophile complex, and equivalent to the conjugate acid, so that the subsequent nucleophilic addition reaction parallels that in hemiacetal formation. Loss of the leaving group occurs first in an SNl-like process with the cation stabilized by the neighbouring oxygen an SN2-like process would be inhibited sterically. It is also possible to rationalize why base catalysis does not work. Base would simply remove a proton from the hydroxyl to initiate hemiacetal decomposition back to the aldehyde - what is needed is to transform the hydroxyl into a leaving group (see Section 6.1.4), hence the requirement for protonation. 

Reactions of silylenes with lone-pair donors such as alcohols and ethers are also quite rapid (ca 10% as fast as addition to rr-bonds). For such reactions, the silylene-donor complexes have been spectroscopically detected (Section V.A). Even when rearrangement to stable products is facile, as in the case of alcohols, the second, nucleophilic, step is believed to have a higher barrier than the electrophilic complex-forming step. 

The mechanism of the Friedel-Crafts acylation reaction, formulated below for reactions using acid chlorides, probably involves the acylium ion (2) as the reactive electrophilic species, although an electrophilic complex (1) between the acid chloride and aluminium chloride may also be involved. 

If this reaction is carried out in the presence of alcohols, homogeneous catalysis of silane alcoholysis occurs, as will be discussed below. Heterolysis of Et3SiH in the highly electrophilic complex cis-Re(CO)4(PR3)(ri2 -HSiEt3)] [A] (R = Ph, Cy) occurs (Scheme 8) (85) much... 

Electrophilic metal carbene complexes such as (CO)5W=C(Ph)OMe generally exhibit poor activity as catalysts for metathesis polymerisation, and higher temperatures are required to bring about the polymerisation of high-strained cycloolefins such as norbornene or cyclobutene [84,85], However, their activity can be enhanced by the addition of a Lewis acid such as TiCL into the polymerisation system [86]. Electrophilic complexes such as (CO)5W=CPh2 also generally exhibit poor activity but they are more active than those mentioned above and enable the polymerisation of various cycloolefins [87,88],... 

Figure 6.4 Schematic presentation of metal-carbon bonding in (a) a transition metal carbene electrophilic complex and (b) a transition metal alkylidene nucleophilic complex...

Lewis acids have long been known to influence free radical polymerizations [117]. They have been particularly important in copolymerizations of hydrocarbon olefins with electron-poor monomers such as acrylates or acrylonitriles. In this way strictly alternating copolymers can be synthesized from monomer pairs which in the absence of Lewis acids would give more random copolymers. The Lewis acid complexes with the electron pair of the acceptor group of the acrylate or acrylonitrile to form the more electrophilic complexed monomer, which then copolymerizes in alternating fashion with the electron-rich hydrocarbon olefin. 

The mildly electrophilic complex MeRe03 condenses with 2 equiv of benzenedithiol to give MeRe(0)(S2C6H4)2 (28), a rare alkyl metal dithiolene. 

Interestingly, when an acetate was present at the propargylic position (126), a completely different behavior was observed, giving birth to a mixture of bicyclic products 127 and 128. 7r-Electrophilic complexation (130), which can be in equilibrium with zwitterionic complex 129, triggers a 1,2-0-... 

Alkynes can be selectively dimerized, cyclotrimerized, or polymerized with a large variety of transition metal and lanthanide catalysts nickel also catalyzes the cyclote-tramerization of HC=CH to cyclooctatetraene. Very electrophilic complexes such as Cp 2LnR and Group 4 compounds,137 as well as 18-electron species such as Cp RuH3(L) and Ru(Tp)Cl(PPh3)2, catalyze the linear dimerization of terminal alkynes 138... 

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