Radical boron-tethered

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 boron-tethered (C-B-O) intramolecular Diels-Alder (IMDA) approach has been used to prepare cyclic alkenyl boronic esters 140 (Scheme 19). Thus, reaction of 2equiv of the dienyl alcohol 138 with 137 in THF, in the presence of molecular sieves, provides the corresponding IMDA precursors 139. The IMDS reaction was then accomplished at 190 °C in a toluene solution, with 5 mol% of 2,6-di-fer7-butyl-4-methylphenol as a free radical inhibitor. Transformation of the carbon-boron bond in 140, using standard organoborane reactions, can then afford a variety of functionalized cyclohexene derivatives <1999JA450>. 

In a related process, boron-tethered radical cyclizations are a useful alternative to the widely used silicon versions. In some cases, rearranged products were produced as a result of an intramolecular S i reaction of a carbon-centered radical at boron (Scheme 9.13) [31]. 

DAB= Me02C- N=N— COgMe Scheme9.13 Boron-tethered radical cyclizations. 

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