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III. Organosilanes

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  • Difficulties associated with use of tri-n-butyltin hydride have prompted chem­ists to search for alternative, hydrogen-atom sources, ones that avoid the pro­blems associated with organotin compounds. Most attention has focused on organo­silanes, compounds that do not have the toxicity associated with organo­tin rea­gents.4 Initially, the outlook was not pro­mising because simple organo­silanes are poor hydrogen-atom transfers when reacting with alkyl radicals and do not support chain reac­tions under nor­mal con­di­tions.4 Innovative ideas, however, have overcome these prob­lems.

    A. Tris(trimethylsilyl)silane (3)

    One approach to converting organosilanes into useful hydrogen-atom transfers begins with deter­mining which structural features in a typical silane might be changed to make it a more effective hydrogen-atom transfer. Since the bond dissociation energy (BDE) for the sili­con–hydrogen bond in triethyl­silane [90 kcal mol-1 (377 kJ mol-1)]36 is 16 kcal mol-1 (67 kJ mol-1) greater that the BDE for the tin–hydrogen bond in tri-n-butyltin hydride [74 kcal mol‑1 (310 kJ mol‑1)],4 lowering the BDE of the silicon–hydrogen bond by changing the organo­silane structure has the potential for im­proving its hydro­gen donating ability.36 Replacing the ethyl groups in tri­ethyl­silane with trimeth­yl­silyl groups gives tris(tri­methyl­silyl)­silane (3, Figure 1), a compound with an Si–H ­bond dis­sociation energy of only 79 kcal mol-1 (331 kJ mol‑1).5,6,36


    When tris(tri­methyl­silyl)­silane (3) was tested as a sub­stitute for tri-n-butyltin hy­dride, this silane was found to be an excellent, although less reac­tive, hydrogen-atom transfer.4,5 In some instances, notably the reaction of pri­mary thi­ono­carbonates, 3 was superior to Bu3SnH because reaction occurred at lower temperature and produced "cleaner" reaction mix­tures.37 In these reactions tri-n-butyltin hydride was less effective due to greater rever­sibility of Bu3Sn· addition to the thio­carbonyl group (Scheme 2). Because the S–Si bond [90 kcal mol-1 (377 kJ mol-1)]38 is stronger than the S–Sn bond [65 kcal mol-1 (272 kJ mol-1)],38 reversal of addition of (Me3Si)3Si· is less likely to occur. Reduced reversibility appears to be the factor responsible for better product yields.39,40 Once a silyl radical has added to an O-thiocarbonyl group, a difficult forward reaction (i.e., one producing a primary radical) can compete more effectively with reverse reaction to the starting materials. Limiting reversibility is even more effective when reaction is conducted under the low temper­ature conditions made possible by Et3B–O2 initiation.41 Although, as described in the following sections, other silanes can be used in radical reactions, none is as widely so as tris(tri­methyl­silyl)silane (3).


    B. Phenyl-Substituted and Related Silanes

    9,10-Dimethyl-9,10-dihy­dro-9,10-disila­anthra­cene (4)42 and 9,10-dihydro-9,10-disila­an­thra­cene (5)43 are silanes that can serve as hydrogen-atom transfers in radical reactions of carbohydrates (Figure 1). Although as indicated in Figure 1 these compounds are among the more effective, hydrogen-donating silanes; examples of their use are limited. One indication that they may not be generally useful is an investigation that found phenylthionocarbonates to be the only O-thiocarbonyl compounds for which 5 is an effective donor.43

    Tet­ra­phenyldisilane (6)44–46 and mono-, di-, and triphenylsilanes (7-9, respectively)47,48 all have been tested as hydrogen-atom transfers in radical reactions (Figure 1). Although conditions exist under which each of these com­pounds functions effectively, each has a significant drawback or special requirement or both. Reactions with the phenyl­silanes 7-9 all require nearly an equal molar amount (sometimes more) of initiator added in portions until reaction is complete.47,48 A compli­cation with diphenyl­silane (8) is that it can react with free hydroxyl groups to form silyl ethers (eq 8).47 Although tetra­phenyl­disilane (6) has the attractive characteristics of being a crystalline, air-sta­ble compound, its use requires a full equivalent of 6 be added to a reaction mixture for com­plete con­version to take place because only one of the two hydrogen atoms attached to silicon is available for abstraction.44–46


    C. Triethylsilane

    1. An Inefficient Hydrogen-atom transfer

    Radical reactions with triethyl­si­lane (10) as the hydrogen-atom transfer are similar to those in which phenyl-substituted silanes fill this role in that repeated addition of substantial amounts initiator is neces­sary. The low reactivity of 10 as a hydrogen-atom transfer for carbon-centered radicals can be partially compensated for by using triethylsilane (10) as the reac­tion solvent (eq 9).48 Another indication of the in­ability of the silicon–hydrogen bond in tr­iethyl­si­lane (10) to function effec­tively as a hydrogen-atom transfer is that a signif­icant portion of hydrogen-atom trans­fer to a car­bon-centered radical comes from the ethyl groups attached to sili­con.49,50 When this information is combined with the rate constant for hydrogen-atom abstraction from triethylsilane by a carbon-centered radical (kH = 3 x 103 M‑1s‑1 at 50 oC),51 it points to 10 not being able to support a chain reaction under normal condi­tions because hydrogen-atom abstraction is too slow.52


    Another disadvantage to reactions with triethylsilane (10) is that 2,2'-azo­bis(iso­butyro­nitrile) is not an ef­fect­ive initiator. Its failure apparently is due to the azo compound trapping the silyl radical to produce a hydroazyl radical that itself acts as a radical scavenger (Scheme 3).53 Per­oxides are more efficient initiators in such reactions even though benzoyl per­oxide also can trap tri­ethyl­silyl radicals.48 A final cautionary note in the use of triethylsilane (10) is that aromatic reaction sol­vents need to be avoided because triethylsilyl radicals react with aromatic com­pounds.2,53


    2. Effect of Polarity-Reversal Catalysis

    Triethylsilane (10) becomes a more effective hydrogen-atom transfer when reaction is conducted in the presence of a compound that functions as a polarity-reversal catalyst; t-dodecanethiol is one such compound.2 (Polarity-reversal catalysis is discussed more thoroughly in Chapter 7 of Volume I.) Briefly, this thiol acts as a catalyst by causing a polarity “mis­matched” reac­tion to be replaced with a pair of polarity “matched” reac­tions (Scheme 4).2,54 When this occurs, chain reac­tions involving hydrogen-atom abstraction from triethylsilane (10) can take place.


    Polarity-reversal catalysis has been proposed as an explanation for triethylsilane (10) being able to participate in the Barton-McCombie reaction even though it is a poor hydrogen-atom transfer (eq 10).2 According to this proposal, reduction of xanthates (and, presumably, other O-thiocarbonyl compounds) by silanes produces COS (eq 11), a compound that then reacts with the silane to give the corresponding silanethiol (eq 12).55,56 This thiol then func­tions as a polarity-reversal catalyst in the deoxygenation process.