"Front Matter". In: Organosilanes in Radical Chemistry - Index of
"Front Matter". In: Organosilanes in Radical Chemistry - Index of "Front Matter". In: Organosilanes in Radical Chemistry - Index of
52 Reducing Agents On the other hand, (Me3Si) 3Si: radicals produced by thermal Reaction (4.6) gave only combination products [18]. Under the same conditions, Me3SiSi(H)R 1 R 2 , where R 1 and R 2 are methyl and/or phenyl groups resulted in less than 1 % yields of the coupling products. t-BuOOBu-t 2 (Me3Si) 3SiH (Me3Si) 3Si Si(SiMe3) 3 130�C 92% The kinetics data on the reactions of silyl radicals with carbon-centred radicals are also available. The rate constant for the cross-combination of CH3: with Me3Si: was measured to be 6:6 10 10 M 1 s 1 in the gas phase [19]. Studies on the steady-state and the pulse radiolysis of Et3SiH in methanol showed that the cross-combination of Et3Si: with CH3O: and HOCH2: occurs with rate constants of 1:1 10 8 and 0:7 10 8 M 1 s 1 , respectively [20]. 4.2 RADICAL INITIATORS The most popular thermal initiator is 2 0 ,2 0 -azobisisobutyronitrile (AIBN), with a half-life (t 1=2) of1hat818C and 10 h at 65 8C in toluene [8,21]. Generally, 5– 10 mol% of initiator is added either all in one portion or by slow addition over a period of time. There are other azo compounds which can be chosen, depending on the reaction conditions. Indeed, the nature of the substituent play an important role as can be seen for 2 0 ,2 0 -azobis-(4-methoxy)-3,4-dimethyl-valeronitrile (AMVN), with a t 1=2 of 1 h at 56 8C and 10 h at 33 8C in toluene. There are also hydrophilic azo compounds, such as 2 0 ,2 0 -azobis-(2-methylpropionamidine) dihydrochloride (APPH), with a t 1=2 of 10 h at 56 8C in water. CN CN N N AIBN HN H 2 N N N APPH MeO NH NH 2 CN N AMVN 2 HCl CN N OMe Peroxides are used when the reaction requires a more reactive initiating species. Thermolysis of dibenzoyl peroxide [PhC(O)O w OC(O)Ph], with a t 1=2 of 1 h at 95 8C and 7 h at 70 8C, is the most familiar to synthetic chemists. It initially produces acyloxyl radicals that often decarboxylate prior to undergoing bimolecular reactions and affording the equally reactive phenyl radicals. (4.6)
Tris(trimethylsilyl)silane 53 Thermolysis of tert-butyl perbenzoate [PhC(O)OOBu-t] and di-tert-butyl peroxide (t-BuOOBu-t), whose t 1=2 are 1 h at 125 8C and 147 8C, respectively, has been used from time to time. Photochemically generated radicals in chain reactions are less familiar to synthetic chemists [8,21]. The above mentioned peroxides have been used in the presence of light to initiate radical chain reactions at room or lower temperatures. Azo compounds are also known to decompose photolytically to afford alkyl radicals. AIBN has rarely been used under such conditions. Triethylborane in the presence of very small amounts of oxygen is an excellent initiator for radical chain reactions. For a long time it has been known that trialkylboranes R3B react spontaneously with molecular oxygen to give alkyl radicals (Reaction 4.7), but only recently has this approach successfully been applied as the initiation [22]. The reactions can be run at temperatures as low as 78 8C, which allow for a better control of stereoselectivity (see below). R3B þ O2 !R2BOO: þ R: (4:7) A correct choice of the initiator is generally decided according to the experimental conditions. For example, a-cyanoalkyl radicals derived from AIBN are capable of abstracting a hydrogen atom from (TMS) 3SiH, but not from Ph2SiH2. Both alkoxyl and aryloxyl radicals are able to abstract a hydrogen atom efficiently even from Et3SiH, that has one of the strongest Si w H bond energies. It is advisable to add the initiators slowly during the course of the reaction, either in solution or portion-wise, with due regard for the half-life of their decomposition at the operating temperature of the reaction. The direct addition at once of the thermal initiator together with temperatures much higher than those of 1 h half-life are generally not the optimal conditions. At low temperatures, when the thermal initiation is obviously impractical, the choice of Et3B=O2 initiation is particular useful in combination with (TMS) 3SiH as reducing agent. Alternatively, photoinitiation using AIBN or dibenzoyl peroxide could be useful. 4.3 TRIS(TRIMETHYLSILYL)SILANE The synthetic application of free radical reactions has increased dramatically since the mid-1980s. Tributyltin hydride is the most commonly used reagent for the reduction of functional groups. However, it was later shown that organotin compounds are toxic and environmentally not tolerated, and they are difficult to remove from non-polar reaction products [1–7]. It becomes now more evident that it is inappropriate to propose these reagents for application in medicinal chemistry. Therefore, a great deal of research has been devoted to finding new reagents with favourable characteristics from toxicological
- Page 7 and 8: PREFACE A large number of papers de
- Page 9 and 10: ACKNOWLEDGEMENTS I thank Keith U. I
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- Page 19 and 20: 10 Formation and Structures of Sily
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- Page 25 and 26: 16 Formation and Structures of Sily
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- Page 39 and 40: 32 Hydrogen Donor Abilities of Sili
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52 Reduc<strong>in</strong>g Agents<br />
On the other hand, (Me3Si) 3Si: radicals produced by thermal Reaction (4.6)<br />
gave only comb<strong>in</strong>ation products [18]. Under the same conditions,<br />
Me3SiSi(H)R 1 R 2 , where R 1 and R 2 are methyl and/or phenyl groups resulted<br />
<strong>in</strong> less than 1 % yields <strong>of</strong> the coupl<strong>in</strong>g products.<br />
t-BuOOBu-t<br />
2 (Me3Si) 3SiH (Me3Si) 3Si Si(SiMe3) 3<br />
130�C<br />
92%<br />
The k<strong>in</strong>etics data on the reactions <strong>of</strong> silyl radicals with carbon-centred<br />
radicals are also available. The rate constant for the cross-comb<strong>in</strong>ation <strong>of</strong><br />
CH3: with Me3Si: was measured to be 6:6 10 10 M 1 s 1 <strong>in</strong> the gas phase<br />
[19]. Studies on the steady-state and the pulse radiolysis <strong>of</strong> Et3SiH <strong>in</strong> methanol<br />
showed that the cross-comb<strong>in</strong>ation <strong>of</strong> Et3Si: with CH3O: and HOCH2: occurs<br />
with rate constants <strong>of</strong> 1:1 10 8 and 0:7 10 8 M 1 s 1 , respectively [20].<br />
4.2 RADICAL INITIATORS<br />
The most popular thermal <strong>in</strong>itiator is 2 0 ,2 0 -azobisisobutyronitrile (AIBN), with<br />
a half-life (t 1=2) <strong>of</strong>1hat818C and 10 h at 65 8C <strong>in</strong> toluene [8,21]. Generally, 5–<br />
10 mol% <strong>of</strong> <strong>in</strong>itiator is added either all <strong>in</strong> one portion or by slow addition over a<br />
period <strong>of</strong> time. There are other azo compounds which can be chosen, depend<strong>in</strong>g<br />
on the reaction conditions. <strong>In</strong>deed, the nature <strong>of</strong> the substituent play an important<br />
role as can be seen for 2 0 ,2 0 -azobis-(4-methoxy)-3,4-dimethyl-valeronitrile<br />
(AMVN), with a t 1=2 <strong>of</strong> 1 h at 56 8C and 10 h at 33 8C <strong>in</strong> toluene. There are also<br />
hydrophilic azo compounds, such as 2 0 ,2 0 -azobis-(2-methylpropionamid<strong>in</strong>e)<br />
dihydrochloride (APPH), with a t 1=2 <strong>of</strong> 10 h at 56 8C <strong>in</strong> water.<br />
CN CN<br />
N N<br />
AIBN<br />
HN<br />
H 2 N<br />
N<br />
N<br />
APPH<br />
MeO<br />
NH<br />
NH 2<br />
CN<br />
N<br />
AMVN<br />
2 HCl<br />
CN<br />
N OMe<br />
Peroxides are used when the reaction requires a more reactive <strong>in</strong>itiat<strong>in</strong>g<br />
species. Thermolysis <strong>of</strong> dibenzoyl peroxide [PhC(O)O w OC(O)Ph], with a t 1=2<br />
<strong>of</strong> 1 h at 95 8C and 7 h at 70 8C, is the most familiar to synthetic chemists. It<br />
<strong>in</strong>itially produces acyloxyl radicals that <strong>of</strong>ten decarboxylate prior to undergo<strong>in</strong>g<br />
bimolecular reactions and afford<strong>in</strong>g the equally reactive phenyl radicals.<br />
(4.6)
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