Biacetyl hydrocarbons

Backstrom and Sandros<54-55) found that the phosphorescence of biacetyl in benzene solution at room temperature was quenched at a diffusion-controlled rate by aromatic hydrocarbons when the triplet energy of the hydrocarbon was sufficiently below that of biacetyl. 

This reaction was first described as a new synthesis for mixed benzoins. A solution of the aryl glyoxal in the aromatic hydrocarbon is stirred at 0° for 3-20 hours with aluminum chloride. Carbon disulfide may be used as a solvent if necessary. The yields vary from 35% to 90%. The reaction has been extended to the preparation of a-hydroxy ketones of the types RCOCHOHAr and CHjCOCOHlCHjlAr by substituting r-butylglyoxal and biacetyl, respectively, for the aryl glyoxal. 

Considerable data are available for triplet yields of benzene in dilute solutions of different solvents (see Table 13). In the main, two techniques have been used sensitized phosphorescence of biacetyl, sensitized cis-trans isomerization of butene-2, octene-2, and stllbene. All yield comparable results. In saturated hydrocarbon solvents at room temperature, the triplet yield for CgHg is found to be about 0.24 0.01. There is a solvent dependence of this quantity, the yield dropping to 0.15 in ethanol, 0.13 in methanol, and 0.09 in acetonitrile (91). In determining the effect of environment on the rate constant controlling intersystem crossing, values for emission lifetimes in the various systems are needed. These are, as mentioned previously, often unreliable. Cundall and Pereira (91) have reported... 

The dimerization processes are, not surprisingly, influenced by geometrical effects and for example, while the tetrachloro compound (280) behaves analogously to the hydrocarbon (278) in both direct and biacetyl sensitized irradiations, only the 9,10-,9, 10 -dimer is formed from the tetrachloro isomer (281) under both sets of conditions. Bouas-Laurent and co-workers recently reported that with a -Si(Me)2 unit connecting the two anthracenes at the... 

In order to increase the quantum yield of cis -> trans isomerization, it is proposed to use aromatic hydrocarbons such as anthracene as a carrier of the quantum chain process. (Z,E)-(cis-18) and (E,E)-l,4-di(3,5-di-tert-butylstyryl) benzene (trans-18) underwent mutual isomerization on biacetyl sensitization with a quantum chain process to afford a 4>c >t value of 1.2-1.3 in a cis isomer concentration of 1 X 10 M [96]. The quantum chain process proceeds through the energy transfer from the trans triplet (H ) to the ground state cis isomer ( c). On addition of anthracene (AN) as a quencher or a carrier of the excited state, the quantum... 

Hexafluorobiacetyl. In the gas phase, hexafluorobiacetyl behaved 169> in the same manner as biacetyl, affording carbon monoxide and hexa-fluoroethane in stoichiometric ratio. However, in the presence of hydrocarbon vapor or in hydrocarbon solution, the disappearance of dione was not accompanied by formation of cleavage products. For example, photolysis (A >3000 A, 65°) of hexafluorobiacetyl in 150 molar excess of 2,3-dimethylbutane until 84% of starting material had disappeared resulted in only 0.5% of carbon monoxide, an equivalent amount of trifluoromethane, and a complex mixture of other products. On the basis of carbonyl and hydroxyl absorption in the infrared, it was assumed that the excited dione abstracted hydrogen from hydrocarbons to give products of photoreduction, photoaddition, etc. Occurrence of H-abstraction in the gas phase is unique. 

The hydrogen donors for which dimers of the corresponding radical have been reported are hydrocarbons. Thus biacetyl 20 79> and camphorquinone 15> reactions with cyclohexene afforded, in part, bicyclohexenyl. Dimers have also been reported in reactions of alkylbenzenes 15,20,101, I29,i33)( diphenylmethane 148>, and thioxanthene 148> (64). 

Reactions with Saturated Hydrocarbons. The limited number of examples of this type indicate that H-abstraction can occur with saturated hydrocarbons. The gas phase reaction of hexafluorobiacetyl with 2,3-dimethylbutane was mentioned earlier H-abstraction also occurred i69) jn solution but products were not identified. Reaction of biacetyl in cyclohexane afforded 20> 10% of 1,2-adduct 96, 36% of diketopinacol 63, 24% of acetylcyclohexane, and 7% of the /1-diketone 91. With tetra-methyltetralindione 60) (73), 1,2-adduct 94 (R = cyclohexyl) accounted... 

Rebbert, R. E., and P. Ausloos Quenching of the Triplet State of Acetone and Biacetyl by Various Unsaturated Hydrocarbons. J. Am. Chem. Soc. 87, 5569... 

Since Kharasch reported radical-mediated carboxylation of saturated hydrocarbons with phosgene as a radical trap in the 1940s [39], no successful radical acylation and carboxylation reactions have appeared. In intermolecular radical acetylations, biacetyl was used as a radical trap (Scheme 18) [44]. The addition of an alkyl radical to the carbonyl carbon of biacetyl gives the methyl ketone along with an acetyl radical. 

Acetyladamantanes. Acetyl groups are introduced into the bridgehead positions of adamantane when the hydrocarbon and biacetyl are submitted to oxidation conditions [02-Co(OAc)2 in HOAc, 60°]. 

Figure 15.25 Rate constants for the triplet energy transfer from biacetyl, throngh a hemicarcerand cage, to aromatic hydrocarbons in solntion. The fnll line represents the Golden Rnle calcnlations and the other lines the ISM calculations differing in the parameters shown in the plot.

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