Benzophenone ketyl radicals, rate

Irradiation of a system consisting of 2-methyl-1,4-dihydroxynaphthalene and benzophenone leads to formation of benzophenone ketyl radical and 2-methyl-naphthosemiquinone, and 1 a,7a-dihydro-1 a-methyl-1,1 -diphenyl-1 H-cyclopropa-[b]naphthalene-2,7-dione derivatives (4) in the presence of Mg(C104)2 give indenonaphthoquinones (5) (Scheme 1), by photoinduced intramolecular electron transfer in almost quantitative yield. In agreement with Rehm-Weller predictions, the rate constants for electron transfer between the triplet states of various substituted naphthoquinones and N,N-dimethylaniline (DMA) in acetonitrile solution were found to be diffusion controlled. " Using this information, it has been shown that the efficiencies of formation of naphthalene " and DMA are unity. 

A number of rate constants of primary alkyl radical reactions have been determined in this way reduction by sodium naphthalenate (2 X 10 sec" ) reaction with lithium benzophenone ketyl radical ... 

KINETICS OF OXIDATION OF BENZOPHENONE KETYL RADICALS ABSOLUTE REACTION RATES AND OSCILLATORY BEHAVIOUR... 

The greatest enhancement in the rate of reaction was observed when a catalytic amount of benzophenone was added to the reaction mixture. This modification had the added advantage that the reaction became self-indicating as a result of the characteristically dark blue sodium benzophenone ketyl radical. Dropwise addition of a solution of diphenyldiselenide in a minimal volume of THF appeared to result in virtually instantaneous formation of the cream-coloured sodium phenylselenide and the reaction was complete when the last mauve tinges had disappeared. 

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

We may be left to wonder how the rate of formation for the ketyl radical from benzophenone could get faster by adding 1,7-octadiene, or any other invisible substrate. The answer is simple, the rate is not faster, but rather, the rate constant for the signal growth is larger. In fact, the actual initial rate (see Eq. 10) is the same, as long as the same concentration of RO is generated initially in the presence or absence of XH. At any other time the rate will be lower in the presence of XH than in its absence, since XH will cause a decrease in RO concentration at aU times except at zero time. 

At 0.1M acceptor, for example, the rate of energy transfer is 19.2 times the rate of abstraction. However, this means that about 5% of the benzophenone triplet will still abstract from the solvent to form ketyl radicals. A compound which is itself a poor hydrogen abstractor may show a greatly enhanced quantum yield of photoreduction under these conditions. 

Marcus treatment does not exclude a radical pathway in lithium dialkyl-amide reduction of benzophenone. It does, however, seem to be excluded (Newcomb and Burchill 1984a,b) by observations on the reductions of benzophenone by N-lithio-N-butyl-5-methyl-l-hex-4-enamine in THF containing HMPA. Benzophenone is reduced to diphenylmethanol in good yield, and the amine yields a mixture of the acyclic imines no cyclic amines, expected from radical cyclization of a putative aminyl radical, were detected. An alternative scheme (17) shown for the lithium diethylamide reduction, accounts for rapid formation of diphenylmethoxide, and for formation of benzophenone ketyl under these conditions. Its key features are retention of the fast hydride transfer, presumably via the six-centre cyclic array, for the formation of diphenylmethoxide (Kowaski et al., 1978) and the slow deprotonation of lithium benzhydrolate to a dianion which disproportion-ates rapidly with benzophenone yielding the ketyl. The mechanism demands that rates for ketyl formation are twice that for deprotonation of the lithium diphenylmethoxide, and, within experimental uncertainty, this is the case. 

Direct evidence for the formation of radical ion intermediates in the benzophenone-amine system was also obtained by Peters et al. [153-156]. Picosecond laser-flash photolysis studies have indicated the formation of ketyl radical anions concomitant with the decay of the benzophenone triplet. For 1.0 M dime-thylaniline and diethylaniline the rate of electron transfer to the benzophenone triplet was 3.6 x 10 and 4.2 x 10 m s , respectively. On the basis of their studies Peters et al. proposed a mechanism in which a solvent-separated ion pair... 

Kinetic measurements of reactions of t-BuMgCl with 19 different substituted benzophenones led the authors to the conclusion that, in all of these reactions, the most probable common rate-limiting step is the electron transfer from the Grignard reagent to the ketone to form the ketyl radical anion and the r-butyl radical. This brought them to the statement that This would indicate that the radical path is not a side reaction, but rather constitutes the main, if not the only mechanism [44]. 

Two time-resolved CIDNP investigations of hydrogen transfer were reported, in which the rates of hydrogen exchange between carbonyl compounds (benzaldehyde and benzophenone [97a] benzoquinone [97b]) and their ketyl radicals were measured. The experiments also yielded the homogeneous recombination rate of the radicals. 

Rate constants for reaction of triplet ( , ) biacetyl with the same hydrogen donors are also included, being generally two or three orders of magnitude less. CIDNP studies on the photoreduction of pyruvic acid by propan-2-ol show the occurrence of abstraction [reaction (12)], followed by the transfer of a hydrogen atom from the polarized ketyl radical to ground-state pyruvic acid [reaction (13)],23 exactly as for the corresponding photoreduction of benzophenone. 

TABLE 24 Quenching Rate Constants, (A,) of Benzophenone (BP) and Benzoylpyridine (BPy) Phosphorescence by CD, Glucose, and Saccharose and Limit Qnantnm Yields of the Ketyl Radical of Benzophenone ([Pg.100]

Kinetic studies [85, 86] have led to the conclusion that the rate determining step in this type of reaction is a single electron transfer from the organomagne-sium species to benzophenone under the formation of the ketyl radical (see also further references in [85, 86]). 

---