Cyclization, radicals allenes with alkenes

Most of the recent synthetic developments in the field of radical cyclization have involved the reactions of carbon-centered radicals with alkenes and alkynes. Other useful acceptors include allenes,31 dienes30 and vinyl epoxides.32 The same methods are used for cyclizations to these acceptors as for radical additions, and the preceding chapter should be consulted for specific details on an individual method (the organization of this section parallels that of Section 4.1.6). Selection of a particular method to conduct a proposed cyclization is based on a variety of criteria, including the availability of the requisite pre-... 

Reactions with other radical initiators, such as AIBN, were reported as well the desired dihydrobenzofurans could be produced selectively. Besides the cyclization of alkenes mentioned previously, intramolecular cyclization of allenes and allq nes was reported as well. In 1995, Grigg and coworkers developed a palladium-catalyzed cyclization-amination of allenes. They demonstrated that the effect of the base was obvious—different regioselectivities were observed by using K2CO3 and Ag2C03 as bases. The corresponding heterocyclic compounds were isolated in good yields (Scheme 2.42). 

The mechanistic pathway for the reaction is shown in Scheme 10.56, where the generated phenylseleno radical attacks the terminal alkyne, followed by ring-opening to afford the malonate radical substituted with a phenyl-selenoalkene. The malonate radical then adds to the alkene generating a mdical that cyclizes back onto the allene, which is subsequently followed by the loss of the phenylseleno radical to afford the cyclopentane derivative. 

Carbonyl couplings. Many variations of cross couplings are possible these include aldehydes with a-dicarbonyl compounds. Ketyl radicals derived from carbonyl compounds also add to alkenes such as acrylonitrile " or N-allyl moieties. Intramolecular cyclizations on terminal alkenes or allene species have also been exploited for synthetic purposes. 

Radical reactions ofallenes are largely unknown [57]. Recently, Ma and coworkers [58] developed the first example of a radical addition/cyclization reaction of allene-enes in an alkene-to-allene manner (Scheme 5.32). The reaction of the allene 146 and polyfluoro alkyl iodide 147 with zinc powder as a cheap, readily available, efficient, and mild initiator led to 148 in moderate to good yields (53-86%), which on treatment with TBAF (tetra-n-butylammonium fluoride) gave the allene 149. 

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