Lewis acid-mediated radical complexes

Lewis acids 436 metal complex-mediated radical polymerization 484-6 molecular weight distributions 251,453-4, 458-60,490-1.499-501 molecular weight conversion dependence 452-3,455... 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

In one of the earliest reports on enantioselective radical reactions, chiral Lewis acid mediated conjugate addition followed by enantioselective H-atom transfer a to a carbonyl was reported by Sato and co-workers (Scheme 3) [22], The single point binding chiral aluminum complex presumably coordinates to the carbonyl oxygen of the lactone as shown in 10. The strong Lewis acidity of the aluminum complex activates the substrate 7 to nucleophilic conjugate addition, which is followed by an enantioselective H-atom transfer from BuaSnH in a chiral environment provided by BINOL ligand in 8. Only 28% ee was observed for product 9. 

The formation of a quaternary carbon center by the radical-mediated allylation of an a-iodolactone was examined for substrate 341 by Murakata, Jono, and Hos-hino [71]. Lewis acids for this reaction were prepared from a bis-sulfonamide and tri-methylaluminum in dichloromethane. Other aluminum compounds were employed in the preparation of the catalyst but all resulted in similar or lower asymmetric induction. The Lewis acid was complexed with the lactone and then the allylation procedure in Sch. 44 was performed. It was found that superior asymmetric induction could be achieved if the Lewis acid was prepared from the ligand with two equivalents of trimethylaluminum. It was also interesting that some turnover could be achieved, as indicated by the data obtained from use of 50 mol % catalyst. 

Metal-mediated and metal-catalyzed reactions require a transition metal. However, certain transition-metal complexes (e.g., TiCU, FeCl3) act only as Lewis acids in organic reactions, and others (TiCl3, Sml2) act as one-electron reducing agents like Na and Li reactions promoted by these compounds are classified in polar acidic, pericyclic, or free-radical classes. 

The cyclopropane ring, due to its ring strain, can be considered as a functional group comparable to the double bond with the synthetic potential to generate functionalized three carbon chains via ring opening. Besides thermal-, photochemical-, oxidative-, reductive-, radical-, nucleophile- and Lewis acid or electrophile-mediated activation, the conversion of cyclopropanes mediated by transition metals plays an important role in synthetic uses of small-ring compounds. In most of these cases, prior to conversion, a complexation of the cyclopropane system by the transition metal is necessary. 

The first catalytic asymmetric radical-mediated allylation was reported in late 1997 by Hoshino and coworkers, who studied the allylation of an a-iodolac-tone substrate, Eq. (19) using trimethylaluminum as Lewis acid and a silylated binaphthol as the chiral catalyst, with triethylborane as radical initiator [62]. Use of one equiv. of diethyl ether was crucial for high enantioselectivity, providing an ee up to 91% in the presence of one equiv. of catalyst, with only a 27% ee in the absence of ether, and poorer ee s when other ethers were employed. In the catalytic version, the ee s dropped off vs. the stoichiometric reaction, with an ee of 81% with 0.5 equiv., and 80% with 0.2 equiv., and 72% with 0.1% catalyst. As in the above example, the presumed chiral intermediate involves complexation of the lactone radical with the Lewis acid-binaphthol complex, with the diethyl ether perhaps as a ligand on the aluminum. 

In free radical reduction mediated by chiral hydrostannanes, there is a remarkable enhancement of enantioselectivity by Lewis acids.When using a hydrosilane in the ketone reduction, Rh complexes of 87, 88, and 89 serve well as chiral catalysts. 

In general, the compounds of the Group 4 metals, such as halides and alkoxides, are well known as Lewis acids to catalyze two-electron electrophilic reactions, and their metallocenes coupled with alkylation and/or reduction agents were effective catalysts for the coordination polymerization of olefins. For the transition metal-catalyzed radical polymerization, their alkoxides, such as Ti(Oi-Pr)4, have also been employed as an additive for a better control of the products. Contrary to the common belief that the Group 4 metals rarely undergo a one-electron redox reaction under mild conditions, there have been some reports on the controlled radical polymerization catalyzed or mediated by titanium complexes, although the conflict in the mechanism between the (reverse) ATRP and OMRP is also the case with the Group 4 metal complexes. 

Not only the highly Lewis acidic early transition metal-based polymerization catalysts suffer from poisoning by coordination of functional groups. Even in late transition metal-based complexes, the possible o-coordination in certain functional groups has a negative impact on polymerization reactions. The prominent example here is the still ongoing search for active acrylonitrile (AN) copolymerization catalysts. This reaction can serve as an ideal example to illustrate the challenges in late transition metal-catalyzed insertion polymerizations with polar functionalized comonomers. The metal-mediated copolymerization of AN has numerous appearances in literature however, in most cases, the reaction mechanism seems to be of ionic or radical nature. 

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