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
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.