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II. Ease of Reaction between a Carbon-Centered Radical and a Multiple Bond

  • Page ID
    24619
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    Once structural requirements have been met, successful radical cycli­zation depends on re­ac­tion rates. The basic question is “Will ring formation occur before competing reactions inter­vene?” The answer to this question depends upon the nature of the radical center and multiple bond and on the separation between these two. The ability of a radical to add to a multiple bond to form a new ring will be addressed first; then, the effect of the separ­ation between the radical center and the multiple bond will be considered.

    A beginning point for discussing reactivity between a radical center and a multiple bond during internal addition is to recall some of the findings in Chapter 18 about addi­tion reactions that are not internal. Such reactions take place rapidly when a radical is nucleo­philic (as are most carbon-centered radicals) and a multiple bond is electron-deficient. This descrip­tion fits the reac­tion shown in eq 1.1,2 If a multiple bond is not electron-deficient, radical addition normally is too slow to compete with hydrogen-atom abstraction; how­ever, minimizing or elimin­ating effec­tive hydrogen-atom transfers from a reaction mixture can enable addition to occur even when the multiple bond ­is not electron-deficient. An example of this type of reaction is shown in eq 2, where Bu3SnH is not present in the reaction mixture even though Bu3Sn· is there and acts as the chain-carrying radi­cal.3,4

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    Addition of a radical to a multiple bond is potentially much faster when the reaction is intramolecular. If a radical center and a multiple bond in a molecule are positioned so that they frequently come within bonding distance, the rate of internal addition increases to the point that even for a multiple bond that is not electron-deficient, cyclization competes effectively with hydrogen-atom abstraction. In the reaction shown in eq 3, internal addition to a double bond that is not electron-deficient takes place even in the presence of Bu3SnH.5

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