"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
68 Reducing Agents (TMS) 2 SiOSiMe 3 (TMS) 3 Si (TMS) 3 SiO N N O Si(TMS)3 + + + (TMS) 3 SiH (TMS) 3 SiH N O N (TMS) 2 SiOSiMe 3 (TMS) 2 Si(H)OSiMe 3 N H + N O Si(TMS)3 (TMS) 3 SiO + + (TMS) 3 Si (TMS) 3 Si Scheme 4.4 Reaction mechanism for the reduction of nitroxide to corresponding amines analogous tin adduct (Scheme 4.5) [80]. Indeed, the reactions of N-nitroamines with (TMS) 3SiH under normal radical conditions afford the corresponding Nnitrosoamines in good yield (Reaction 4.44) [81]. For the fate of (TMS) 3SiO: radical refer to Scheme 4.4. Z N NO 2 Z = CH 2 or O O R N O Si(TMS) 3 (TMS) 3 SiH AIBN, 80 �C RNO Z N NO R + ONOSi(TMS) 3 + OSi(TMS) 3 60 - 62% Scheme 4.5 Possible mechanistic paths for the fragmentation of nitroxide (4.44)
Tris(trimethylsilyl)silane 69 Tris(trimethylsilyl)silane reacts with phosphine sulfides and phosphine selenides under free radical conditions to give the corresponding phosphines or, after treatment with BH3–THF, the corresponding phosphine–borane complex in good to excellent yields (Reaction 4.45) [82]. Stereochemical studies on P-chiral phosphine sulphides showed that these reductions proceed with retention of configuration. An example is given in Reaction (4.46). R R' P R'' X (TMS) 3SiH AIBN, 80 �C X = S or Se OMe Ph P Me S R R' P R'' a) (TMS) 3 SiH AIBN, 80 �C b) BH 3 –THF, r.t. BH 3 –THF R R' P R'' OMe Ph P Me H3B BH 3 (4.45) (4.46) The photochemical reduction of 1-methylquinolinium ions by (TMS) 3SiH proceeds regioselectively to afford the corresponding 1,4-dihydroquinones in a water–acetonitrile solvent system (Reaction 4.47) [83]. Mechanistic studies demonstrated that the reactions are initiated by photoinduced electron transfer from the silane to the singlet excited states of 1-methylquinolinium ions to give the silane radical cation–quinolinyl radical pairs, followed by hydrogen transfer in the cage to yield 1,4-dihydroquinones and silicenium ion. Silyl cations are quenched by water. 4.3.5 APPENDIX N Me (TMS) 3SiH hν H H N Me (4.47) It is worth pointing out that the replacements of a variety of functional groups by a hydrogen described so far are not only efficient and straightforward processes but the work-up is rather simple: in most cases, the reaction mixtures are concentrated by evaporation of the solvent and then flash-chromatographed to isolate the products. Furthermore, it has been shown that (TMS) 3SiH and its silylated by-products are not toxic [66]; this is very important for the pharmaceutical application of the silane reagent, since the biological assays on the final compounds are not affected by any remaining silylated materials. An increase in the choice of (TMS) 3SiH as a reducing agent it is
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68 Reduc<strong>in</strong>g Agents<br />
(TMS) 2 SiOSiMe 3<br />
(TMS) 3 Si<br />
(TMS) 3 SiO<br />
N<br />
N<br />
O<br />
Si(TMS)3<br />
+<br />
+<br />
+<br />
(TMS) 3 SiH<br />
(TMS) 3 SiH<br />
N<br />
O<br />
N<br />
(TMS) 2 SiOSiMe 3<br />
(TMS) 2 Si(H)OSiMe 3<br />
N<br />
H<br />
+<br />
N<br />
O<br />
Si(TMS)3<br />
(TMS) 3 SiO<br />
+<br />
+<br />
(TMS) 3 Si<br />
(TMS) 3 Si<br />
Scheme 4.4 Reaction mechanism for the reduction <strong>of</strong> nitroxide to correspond<strong>in</strong>g<br />
am<strong>in</strong>es<br />
analogous t<strong>in</strong> adduct (Scheme 4.5) [80]. <strong>In</strong>deed, the reactions <strong>of</strong> N-nitroam<strong>in</strong>es<br />
with (TMS) 3SiH under normal radical conditions afford the correspond<strong>in</strong>g Nnitrosoam<strong>in</strong>es<br />
<strong>in</strong> good yield (Reaction 4.44) [81]. For the fate <strong>of</strong> (TMS) 3SiO:<br />
radical refer to Scheme 4.4.<br />
Z<br />
N NO 2<br />
Z = CH 2 or O<br />
O<br />
R N O Si(TMS) 3<br />
(TMS) 3 SiH<br />
AIBN, 80 �C<br />
RNO<br />
Z N NO<br />
R + ONOSi(TMS) 3<br />
+<br />
OSi(TMS) 3<br />
60 - 62%<br />
Scheme 4.5 Possible mechanistic paths for the fragmentation <strong>of</strong> nitroxide<br />
(4.44)
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