Boron oxidative/radical cyclizations

The radical addition to 1-alkenyl or 1-alkynylboronic esters or amides took place extremely smoothly because the boron atom stabilizes the resulting ct-radical intermediates. Bu3SnH and PhSH predominated the trans-addition products 287 in the addition to 1-alkynylboronic amides at 90 °G, whereas Bu3SnH and Ph2Ph produced the m-addition products 288 at 0°C (>98% Equation (82)).455 Intramolecular addition to 1-alkenylboronic esters has been demonstrated in boron-tethered radical cyclization that provided 1,3- or 1,4-alkanediols 290 via oxidative workup (Equation (83)).456 Inter- and intramolecular additions of alkyl radical457 and sulfonyl radical458 have also been studied. 

A Favorskii-type rearrangement occurs on boron trifluoride promoted lead tetraacetate oxidation of enamines274 (Scheme 119). Five-membered rings may also be constructed by a Michael addition-radical cyclization process275 (Scheme 120). 

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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