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Photohydrodimerization

Some carbonylated compounds (ketones, aldehydes) can be excited by irradiation due to a chromophore and subsequently be reduced by adding a hydrogen atom from an appropriate hydrogen donor (an alcohol, hydrocarbon or amine). In a second step, two intermediate ketyl radicals can combine and form a 1,2-ethanediol. This photoinduced two-reaction-process including reduction and dimerization is called photohydrodimerization. The general reaction scheme is given in Figure 1. The products of the photohydrodimerization belong to the substance class of pinacols and are of remarkable interest on an industrial scale as precursors for pharmaceuticals or fertilizing agents. After the discovery of CHIAMICIAN and SILBER,[1] who reached good yields for the photohydrodimerization of benzophenone to benzopinacol, only some additional research effort was needed to bring pinacolizations to large scale industry.[26] The product of main interest was 3,4-diphenyl-1,2,5,6-tetrahydrobenzoic acid whose derivatives allow the active regulation of plant growth. The initial step in the synthesis route is the photohydrodimerization of acetophenone.

  photohydrodimerization

Figure 1: Reaction scheme of the photohydrodimerization with activation, hydrogen abstraction and dimerization.

The kinetics of the reaction are primarily effected by the hydrogen abstraction rate constant (second row in figure 1), that changes with the nature and energy level of the excited ketone, the homolytic dissociation energy of the H–B-bond of the hydrogen donor, steric and polar effects, the solvent, excited state quenching factors and their interactions. In either case, if the lowest energy excited state occurs as singlet or triplet state of the (n, π) electronic configuration at the carbonyl group, the ketone is most reactive. In triplet state, the reactivity is even comparable to alkoxy radicals, explaining the high driving force to dimerization. This differs from the singlet state where basically a fast intersystem crossing limits a faster reaction rate.

Considering amines as hydrogen donors, their ionization potential plays an important role. A lower ionization potential leads to a higher reactivity, thus tertiary amines are the most reactive ones. Hereby, the reaction mechanism may change from hydrogen transfer corresponding to a high ionization potential to an electron transfer mechanism when the ionization potential is rather low. If both the ionization energy and the bond dissociation energy of the hydrogen donor are high, hydrogen abstraction will not occur in substantial amounts (e.g. acetonitrile in water). Alternatively, other photochemical reactions like the Norrish Type I reaction can become more favorable, especially at temperatures above room temperature. Another possible reaction pathway is the reduction of the carbonyl group to the corresponding alcohol, that occurs when the ketone is sterically hindered. After abstraction of the first hydrogen atom, a second hydrogen atom is abstracted or immediate disproportion to the alcohol happens. Regarding the total quantum yield Φp of photohydrodimerizations following the scheme from figure 1, three key step quantum yields can be defined:

  • the intersystem crossing efficiency, i.e. the quantum yield for the formation of the triplet state ϕISC,
  • the radical formation efficiency from the excited state of the ketone, thus the hydrogen transfer efficiency ϕh influencing the rate constant of hydrogen absorption kh and
  • the dimerization efficiency forming pinacols out of the ketyl radicals ϕr. This results in a total quantum yield of

dimerizationeq1

kd sums up all parallel occurring deactivating rate constants of first or pseudo-first order for the triplet state of the ketone. It is important to note that these equations are only valid for reactions proceeding in the absence of oxygen as it quenches the triplet state, forcing ϕh to become almost negligible. Hence, the reaction is typically carried out under inert gas conditions and the concentration of the hydrogen donor is raised by using it as undiluted solvent if possible. That way, ϕh approaches unity letting the efficiency of hydrogen absorption reach almost 100%.

 

  1. CIAMICIAN; P. SILBER: Ber., 1900, (33), 2911.
  2. TURRO, N. J.; LEERMAKERS, P. A.; VESLEY, G. F.: Cyclohexylidenecyclohexane, Organic Syntheses, 1967, 47, 34, DOI: 15227/orgsyn.047.0034.
  3. C. SCAIANO: J. Photochem, 1973, (8). THE CHEMICAL SOCIETY (Ed.): Photochemistry, Specialists Periodical Reports, 1970, London.
  4. P. J. WAGNER: Topics in Curretlt Chemistry, vol. 1, 1976.
  5. N.J. TURRO: Modem Molecular Photochemistry, 1978, Menlo Park.

 



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