Cyclization of alkyl halides

Table 5 Cobalt(I)-catalyzed radical 5-exo cyclizations of alkyl halides...

The tributyltin hydride-mediated carbon-carbon bond formation via radical addition and cyclization of alkyl halides with alkenes has often been a choice for construction of various organic molecules [1], However, the requirement for high-temperature initiators or photo initiation and the difficulties associated with purification of the products from tributyltin halides tend to limit the widespread use of these methods, despite the efforts to make the methods easier [Ic, 2], Recently, nickel-mediated radical additions and cyclizations have been introduced as promising alternatives to the tributyltin hydride methods. These are the nickel powder-acetic acid method for cyclization of haloamides to y-lactams, y -lactams and in-dolones, the borohydride exchange resin-nickel boride method for radical addition, nickel-catalyzed electroreductive cyclization and nickel-catalyzed Kharasch addition of polyhalo compounds. 

Last year, Alexanian and Bloome described a palladium-catalyzed carbonylative Heck-type cyclization of alkyl halides [32]. The treatment of a range of primary and secondary alkyl iodides in the presence of a palladium catalyst under CO pressure yielded a variety of synthetically versatile enone products. This novel palladium-catalyzed Heck-type cyclization is a rare example where inactivated alkyl halides with -hydrogens are involved. Various substituted alkenes were well tolerated, and mono- as well as bicyclic carbocycles are easily accessed (Scheme 7.13). 

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