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VI. Compounds with Carbon–Hydrogen Bonds

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    A. 2-Propanol

    Few compounds in which a carbon–hydrogen bond must serve as the hydrogen-atom source are reactive enough to function as hydrogen-atom transfers in radical reactions of carbohydrates. The reason for this is that when less reactive donors are used, other reactions become competitive. Even com­pounds with quite reactive C–H bonds are poor hydrogen-atom transfers when compared to tri-n-butyltin hydride or tris(trimethylsilyl)silane. One com­pound that does have the necessary reac­tivity, but just barely, is 2‑propanol. When reaction of the xanthate 16 is conducted with 2‑propanol as the solvent, hydrogen-atom abstraction is in spirited com­petition with xanthate-di­thio­car­bonate rearrangement (eq 15).59 This competition exists because hydrogen-atom abstraction by the carbo­hydrate radical R· is slow enough that addition of R· to another molecule of the xanthate 16 has a comparable rate (Scheme 5). The adduct radical formed by this addition fragments to give the dithio­car­bon­ate 18 and a carbo­hydrate radical (R·).

    A(15).png

    As5.png

    B. Cyclohexane

    The xanthate 19 reacts to form the corresponding deoxy sugar in 85% yield (eq 16).60 In this reaction cyclohexane functions as the hydrogen-atom transfer. Since cyclohexane is not a noticeably better hydrogen-atom transfer than 2-propanol, it is initially surprising that no dithiocarbonate is formed from 19 even though (as described in the previous section) dithiocarbonate formation is significant in reaction of the xanthate 16 (eq 15). The structural difference between the starting materials (16 and 19) in these two reactions accounts for their difference in reactivity. Un­like 16, the xanthate 19 has a sulfur atom directly attached to the carbohydrate portion of the molecule. This means that when the carbo­hydrate radical R· adds to 19, the options available to the adduct radical 20 are either regen­er­at­ing the starting materials or expelling an unsta­bil­ized, primary radical (Scheme 6). Not surprisingly, no dithiocarbonate from primary radical expulsion is observed; therefore, the only operative pathway for the radical 20 is reforming of R· and the xan­thate 19. Each regeneration of R· creates a new oppor­tun­ity for it to abstract a hydrogen atom. With these multiple opportun­ities even a marginally effective hydrogen-atom transfer event­ually is able to react with R· to produce the hydrogen-abstraction product RH.

    A(16).png

    As6.png

    Even though the yield is good, the reaction shown in eq 16 ­is not an attractive option for deoxy sugar synthesis because it requires reaction of the carbohydrate to replace a C–O bond with a C–S bond before conducting the Barton-McCombie reaction. The additional steps neces­sary for this conver­sion add to the effort required for deoxygen­ation.

    C. Silylated Cyclohexadienes

    Silylated cyclohexadienes, such as 21, are effective hydrogen-atom transfers in Barton-McCombie reactions (eq 17).61 Compound 21 has the advantage of being a solid material that can be easily stored and handled. Although this compound (21) is an order of magnitude less reactive than (Me3Si)3SiH (3), it is able to support chain reactions. The propagation steps in a proposed mech­an­ism for replacement of an O-phenoxythiocarbonyl group with a hydrogen atom sup­plied by 21 are given in Scheme 7.

    A(17).png

    As7.png

    This page titled VI. Compounds with Carbon–Hydrogen Bonds is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.