Study of the mechanism of hydride transfer with the participation of triphenylmethane derivatives by the method of chemically induced dynamic nuclear polarization

Ionic and Organometallic‐Catalyzed Organosilane Reductions

“…Since the organosilane reduction of benzyl alcohols to the corresponding toluene derivatives is known, it is not surprising that the reduction of an aryl aldehyde to a toluene is possible. This transformation has been carried out with Et 3 SiH/TFA, 69,351,352 (EtO) 3 SiH, and Et 3 SiH with various catalysts, 353 Et 3 SiH/(C 6 F 5 ) 3 B, 281 PMHS/Pd/C, 316 and PMHS/ (C 6 F 5 ) 3 B. 354 The last combination also reduces alkyl aldehydes to the corresponding alkanes (Eq.…”

“…Basic aqueous workup converts the silicon reaction products derived from the organosilicon hydride into the corresponding silanols and disiloxanes, which may be removed from the desired reduction products by simple distillation. 69 Benzaldehyde itself forms no toluene; only dibenzyl ether and benzyl trifluoroacetate are formed. Triethylsilane (2.2 equivalents) causes the transformation of p-anisaldehyde into p-methylanisole in 76% yield after only 30 minutes.…”

“…Use of a three-fold excess of dimethylphenylsilane in place of the triethylsilane results in a slight improvement in yield to 83% after 45 minutes. 69 Similar treatment of a trifluoroacetic acid solution of p-tolualdehyde with triethylsilane gives only a 20% yield of p-xylene after 11 hours reaction time followed by basic workup. Use of 2.5 equivalents of dimethylphenylsilane enhances the yield to 52% after only 15 minutes.…”

“…For example, a trifluoroacetic acid solution of 2,4,6-trimethylbenzaldehyde forms isodurene in 98% yield within 15 minutes when 2.2 equivalents of triethylsilane are added to the reaction mixture at room temperature. 69 In contrast, when trifluoroacetic acid is added to a stirred solution of triethylsilane and 2,4,6-trimethylbenzaldehyde, isodurene is formed in only 70% yield after basic aqueous workup. Minor side products under these reaction conditions are mesitylene (formed via acid-catalyzed decarbonylation of the aldehyde) and the Friedel-Crafts product 2,4,6,2 ,3 ,4 ,6 -heptamethyldiphenylmethane (Eq.…”

“…65 Trityl and tropylium (cycloheptatrienyl) cation salts having complex metalhalide anions such as SbX 6 − , AsF 6 − , PF 6 − , FeCl 4 − , and BF 4 − undergo reduction with trialkylsilanes and aryldialkylsilanes at rates that are independent of the nature of the anion or of ring strain in the organosilicon hydride, are kinetically first order with respect to both cation and organosilicon hydride, and that display primary deuterium kinetic isotope effects of k H /k D = 1.41-1.49 in dichloromethane. 67,68 It is argued that these reactions proceed by way of a threestep mechanism involving a rate-determining single-electron transfer step 69 to create a charge-transfer complex between the carbocation and the organosilicon hydride followed by a faster transfer of hydride to the carbon center and the creation of a silicenium ion intermediate that is then rapidly captured by the counterion present (Eq. 5).…”