Benzopinacols

To the wa-propyl alcohol filtrate is added another i5o-g. portion of benzophenone and the solution is exposed to sunlight as in the first reduction. The benzopinacol which separates is filtered and dried. The yield in the second and subsequent runs is 142-143 g. (94-95 per cent of the calculated amount). This procedure can be repeated with the same filtrate until six or seven portions (900-1050 g.) of benzophenone have been reduced. 

If wa-propyl alcohol is not available, absolute ethyl alcohol can be used. With ethyl alcohol the reaction is slower and a yellow solution is obtained nevertheless, the crystals of benzopinacol are colorless. 

Benzopinacol has been prepared by the action of phenylmag-nesium bromide on benzil 1 or methyl benzilate. Usually it has been obtained by reduction of benzophenone, the reducing agents being zinc and sulfuric acid or acetic acid, aluminum amalgam, and magnesium and magnesium iodide. The present... 

To the acetic acid filtrate is added another loo-g. portion of benzopinacol and the reaction is carried out in the same way. The yield of benzopinacolone in the second and subsequent runs is 94-94.5 g. (98-99 per cent of the theoretical amount). 

The benzopinacol obtained by photochemical reduction of benzophenone (p. 8) may be used directly without purification. 

Benzopinacol [464-72-2] M 366.5, m 170-180 (depends on heating rate). Crystd from EtOH. 

Reductive dunenzation to form fluorinated benzopinacols proceeds m the partly fluormated case either with zinc or by photolysis but is not observed with perfluorobenzophenone [651 (equation 53). Trifluoroacetophenone is reduced electrochemically in dimethylformamide to a stable radical anion, which, m the presence ot lithium ion, rapidly dunerizes to pinacol in higher yield than that available by photoreduction [66] (equation 54)... 

Photochemical Biradical Formation Investigated in Micro Reactors Organic synthesis 87 [OS 87] Benzopinacol formation by two-radical combination... 

The pinacol formation reaction follows a radical mechanism. Benzopinacol, benzophenone and the mixed pinacol are formed jointly with many radical species [72, 74]. In the course of the reaction, first a high-energy excited state is generated with the aid of photons. Thereafter, this excited-state species reacts with a solvent molecule 2-propanol to give two respective radicals. The 2-propanol radical reacts with one molecule of benzophenone (in the ground state, without photon aid) to lengthen the radical chain. By combination of radicals, adducts are formed, including the desired product benzopinacol. Chain termination reactions quench the radicals by other paths. 

In order to further probe the mechanistic course of the above reaction, the possibility of a pinacolic intermediate was addressed. To test this hypothesis, the reaction of active uranium with benzopinacol was conducted. 

Benzopinacol contains two acidic hydrogens which could account fpr the greater yield of TPA. When benzopinacoRL was reacted with U, >73% of the deuterium was incorporated into the TPA. This clearly shows that the benzylic hydrogens of TPA may originate from a uranium hydride. 

In order to further examine the role of a pinacolic intermediate, a crossover experiment wa conducted. In the reaction of a lsl mixture of TBP and benzopinacol with U, a statistical distribution of all 6 coupled products was seen. This surprising result shows that the carbon-carbon bond of the pinacol is broken before the products are formed. 

Santos et al. [128,129] and Guerrero et al. [130] prepared segmented poly(EO) containing bistrialkylbenzopinacolate moieties to synthesize poly(St)-poly(EO) block copolymers. The St polymerization with the polymeric iniferter 27 was comparable to that initiated with small molecular benzopinacolates. 

The structure of free radical 85 produced by dissociation of a stannyl benzopinacolate, was investigated by ESR spectroscopy and confirmed by ESR simulation. Free radical 85 is a good source for stannyl free radicals, as shown in reaction 10262. 

Bis(tributylstannyl)benzopinacolate PTOC ester or Barton ester... 

The photochemical reduction of benzophenone to benzopinacol in the presence of ferrocene31 apparently relates to a high concentration of ketone so that sufficient unquenched benzophenone triplets are present and proceed to the dimer in the usual fashion. 

In both light and high energy irradiation induced reactions, phenyl-thiol, 2-mercaptomesitylene, and their sulfides inhibit nonchain processes,201202 e.g., the light-induced conversion of benzophenone in 2-propanol to benzopinacol and acetone.99 217 The reaction converts compounds into radicals by removal or addition of hydrogen atoms. The sulfur compounds in rapid hydrogen transfer processes convert the free radicals to stable molecules and may do so repeatedly.46 46... 

Mustafa et al.197 reported the formation of 9,9 -bithiaxanthene 5,5-dioxide (XVIIIb) with benzophenone with the reduction of the latter to benzopinacol. 

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