Radical-based arylation

Another useful approach to styrenes via sp -sp -coupling reactions, which is beyond the scope of this section, is Meerwein-type arylations in which aryldiazonium salts undergo usually copper(I)- or palladium-catalyzed couplings with electron deficient alkenes, indicating a radical-based mechanism.The method is so useful for the synthesis of stilbenes from unsubstituted styrenes 92 or p>silylstyrenes. 93... 

Direct arylations of arenes are, however, not restricted to palladium-catalyzed transformations, but were also accomplished with, inter alia, iridium complexes. Thus, the direct coupling of various aryl iodides with an excess of benzene in the presence of [Cp IrHCl]2 afforded the corresponding biaryl products, but usually in moderate yields only (Scheme 9.30) [69]. The reaction is believed to proceed via a radical-based mechanism with initial base-mediated reduction of iridium(III) followed by electron transfer from iridium(II) to the aryl iodide. Rather high catalyst loadings were required and the phenylation of toluene (90) under these reaction conditions provided a mixture of regioisomers 91, 92, and 93 in an overall low yield (Scheme 9.30) [69]. 

Finally, a direct arylation of thiophene (106) was accomplished with sulfonyl chloride 107 at a relatively high reaction temperature, which most likely proceeded via a radical-based mechanism (Scheme 9.38) [51]. 

The reactivity studies have been extended to other substituted aryls such as tetralin, indane and cumene, that have been oxidized with TONs up to 6,100 and a remarkable 48-53% alcohol selectivity, providing an interesting alternative to other radical-based oxygenation systems (Table 21.4). 

The field of alkaloid synthesis via tandem cyclizations has favored the application of (TMS)3SiH over other radical-based reagents, due to its very low toxicity and chemoselectivity. For example, cyclization of the iodo aryl azide 12 mediated by (TMS)3 SiH under standard experimental conditions, produced the N-Si(TMS)3 protected alkaloid 13, which after washing with dilute acid afforded the amine 14 in an overall 83% yield from 12 (eq 40). The formation of the labile N-Si(TMS)3 bond was thought to arise from the reaction of the product amine 14 with the by-product (TMS)3SiI. The skeletons of ( )-horsflline, ( )-aspidospermidine and ( )-vindoline have been achieved by this route. ... 

Schemes aimed at the construction of elaborate two-dimensional -PAHs with boron at their center represent the cutting edge in synthetic methodology toward well-defined graphene-like materials with atomically precise 5-doping (Section 10.4.2.2). Yamaguchi and coworkers showed that oxidative (Friedel-Crafts/Scholl-type reactions) or radical-based cyclizations of triarylboranes with reactive pendant functionalities could be used to fuse the 5-aryl groups into planar arrays. The seminal synthesis of a planarized dibo-raanthracene (Scheme 10.7) is representative of the oxidative/acid-catalyzed approach [24]. Installation of pendant 2,6-di(2-propenyl)phenyl groups via...

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