Cascade reactions arene termination

Domino or cascade reactions provide valuable approaches, especially to various carbo- and heterocyclic systems with three, four, or even more annelated rings. The Heck reaction has successfully been employed in various inter-inter-, intra-inter-, inter-intra-, as well as all-intramolecular reaction cascades, hi this section, the termination of these processes by alkenes, arenes, and related ir-bond systems such as alkynes and allenes will be described. A cascade Heck reaction is considered to consist of an oxidative addition of a heteroatom-carbon bond to palladium (starter), carbopalladation of a nonaromatic carbon-carbon double or triple bond without immediate dehydropalladation (relay), one, two, or more fimher car-bopalladation(s) of a carbon-carbon double or triple bond, and eventually ensuing dehydropalladation. Crucial for a cascade reaction of this kind to occur is the blockage or retardation of a dehydropalladation at one of the intermediate stages by using 1,1-disubsti-tuted alkenes and appropriately substimted cycloalkenes, bicycloalkenes, or alkynes as relays since they give kinetically stable alkyl- or alkenylpalladium intermediates, respectively. 

Chiral amine catalysts have also been used in cascade reactions mediated by SOMO catalysis [143] and Lewis base catalysis [144]. MacMillan s group developed a powerful cascade reaction moderated by SOMO catalysis. The radical cation, generated from an enamine in condensation of imidazolidinone catalyst 208 with aldehyde 207 and subsequent oxidation by Cu oxidant, was expected to engage in a series of 6-endo-trig radical cyclizations terminated by a suitable arene to give a cyclohexadi-enyl radical. After a second oxidation, rearomatization, and liberation of the catalyst, the requisite 209 would be generated (Scheme 1.90). 

Similar palladium-catalyzed cascade arylations also occur with acyclic alkenes, including a,p-unsaturated sulfones (Equation 19.153), sulfonamides, phosphine oxides, and phosphonate esters. In contrast, typical conjugated olefins, such as a, 3-unsaturated esters and enones almost exclusively react to form products from Heck reactions. Direct arylations have also been conducted with disubstituted alkynes containing a terminal arene and a large group, such as an aryl or ferf-butyl group (Equation 19.154). ... 

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