Bridges unsaturated

The scope of this review is a detailed survey of reactions proceeding through vinyl cations and an attempt of a systematic definition of the properties of these intermediates with reference to those of saturated carbonium ions. Although attention will be particularly devoted to linear cations, bridged unsaturated species will be considered as alternative structures of vinyl cations rather than as a distinct type of reactive intermediates. The 77--complex terminology (Dewar, 1949) widely abused in the past decades to indicate especially cyclic cations and recently reassessed by Banthorpe (1970) will be generally avoided. The most recent Btudies not covered by published reviews on the subject (Rappoport, 1969 Richey and Richey, 1970 Richey, 1970 Hanack, 1970) are discussed in greater detail than others and data are collected in pertinent Tables. 

Reactions of bridged unsaturated cations have not been investigated in detail and are currently interpreted in terms of ring opening to generate a back-shielded vinyl cation which may react with available electron pairs or as a direct nucleophilic attack at one of the carbon atoms of the cyclic cation to give the final products. 

Ketene equivalents have found widespread use as partners in Diels-Alder reactions for the construction of cyclic, fused, and bridged unsaturated ketones. However, ketene equivalents based on simple vinyl sulfoxides are poor dienophiles and show low levels of diastereocontrol. Recently, Aggarwal and colleagues reported that (l/ ,3/ )-2-methylene-l,3-dithiolane 1,3-dioxide (187) was a highly reactive and highly selective chiral ketene equivalent.116 Diels-Alder cycloaddition of this... 

Bridges that will be surveyed include saturated hydrocarbon bridges, unsaturated conjugated hydrocarbon bridges, and DNA. This review is not intended to be exhaustive, because many extensive reviews have already been published in this area [6-14]. 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

Certain state highway authorities are studyiag the use of ftber-reiaforced polymers, typically thermosets such as epoxy or unsaturated polyester, for bridge constmction. On an even more futuristic scale, fiber optics that employ polymeric jacketing and, ia some cases, optically active polymeric cores, may someday be employed ia place of wines for home security systems, climate control, etc (10,91). 

The more facile migration of aryl and other unsaturated groups involves bridged intermediates formed by an addition process. In the case of aryl migration, the intermediate is a cyclohexadienyl radical ... 

The oxidation of certain steroidal <5,e-unsaturated amines with either N-chlorosuccinimide or lead tetraacetate yields bridged aziridines, see structure (117)... 

The reactions of enamines with a,/3-unsaturated acid chlorides have provided bridged bicyclic diketones 312,313). 

The reactivity of the unsaturated system of oxepin gives rise to a variety of cycloaddition reactions. A number of 3,6-bridged oxepins have been reacted with peracids to give 2,3-ep-oxyoxepins i,129 154.213.218 At higher temperature, the 2,3,6,7-diepoxy derivatives 2 are formed.164... 

Grafting through techniques involving polymerization of a monomer in the presence of a polymer backbone carrying randomly distributed unsaturations have also been attempted 140). The major drawback is the possibility of bridge formation between backbone molecules. 

The reaction of the coordinatively unsaturated ruthenium amidinates with [Cp RuCl]4 tetramer or [CpRufMeCNlsJPFg provides access to novel amidinate-bridged dinuclear ruthenium complexes (Scheme 146), which in turn can be transformed into cationic complexes or hydride derivatives. In these complexes, a bridging amidinate ligand perpendicular to the metal-metal axis effectively stabilizes the highly reactive cationic diruthenium species. 

Matsumoto et al. reported some reactions of diruthenium complexes containing a bridging disulfide ligand with unsaturated compounds such as olefins and ketones [135]. These diruthenium complexes show unique reactivities towards double-bond systems since the S-S bond has some double bond character from the contribution of the canonical structure B in Scheme 36. 

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