Stannane-Mediated Radical Addition to Arenes. Generation of Cyclohexadienyl Radicals and Increased Propagation Efficiency in the Presence of Catalytic Benzeneselenol

The Mechanism of Bu3SnH‐Mediated Homolytic Aromatic Substitution

“…In the event, careful D NMR spectroscopy of the crude product showed no detectable signal for deuterium attached to an arene, a result that precludes the formation of a dihydro intermediate and suggests that homolytic aromatic substitution is not directly dependent on the use of Bu 3 SnH as radical carrier. This conclusion agrees with that of Crich and Hwang, [7] who found that intermediate p radicals in Bu 3 SnH-mediated cyclizations could be effectively intercepted by the efficient hydrogen-atom donor PhSeH (with the formation of a dihydro spiro-cyclized product) but not by Bu 3 SnH.…”

“…Whilst oxidation during workup may be plausible, dihydrotype systems, such as 2, are not oxidized rapidly in air. [7] The yields are also higher than 50 %, and this rules out disproportionation as the main mechanism.…”

The frequently overlooked importance of solvent in free radical syntheses

“…This prudence turned out to have been fully justified because subsequent synthetic work (largely involving intramolecular radical additions to aromatic rings with the consequent formation of cyclohexadienyl radicals, see, e.g., Scheme 13) revealed that cyclohexadienyl radicals were virtually incapable of abstracting the H-atom from tin hydrides. 51,52 The reason for this is that H-atom abstraction from r 3 SnH {D[r 3 , which is too small to keep a radical chain going. In contrast, H-atom abstractions from r 3 SnH by alkyl radicals are strongly exothermic and fast (k E 2-10 Â 10 6 M À1 s…”

“…46 However, with an azo compound, such as AIBN, as the initiator, it is the azo compound that serves this function. 46,51,52,56 As a consequence, relatively large quantities of the azo initiator are required and it ''should be regarded as an expendable reagent. '' 46 Turning now to the use of benzene as a solvent in tin hydride reductions, it has been reported 46 that the AIBN initiated reaction of 1-bromooctane with (n-Bu) 3 SnH for 40 min at 70 1C yields 80% octane in cyclohexane, but yields only 28% octane in benzene.…”

A free radical cyclization approach to indolo-benzodiazocine derivatives

“…No products from radical attack on the aromatic hydrocarbon solvents were seen. 12 The increased yield of cyclization products and the diminished formation of reduced, uncyclized by-products in high boiling point aromatic solvents suggests that the amide rotamer (B) required for cyclization after radical formation is increased in concentration in the equilibrium at higher temperatures relative to rotamer A. 5 Also, with radical attack on the heteroaromatic ring likely to be slow, 4 the increased temperature would be favourable for this reaction.…”