Recently (1) the rate data for the generalized nucleophilic displacement reaction were reviewed.

Hard and soft acids and bases

The interest in heterogeneous photocatalysis is intense and increasing, as shown by the number of publications on this theme which regularly appear in this journal, and the fact that over 2000 papers have been published on this topic since 1981. This article is an overview of the field of semiconductor photocatalysis : a brief examination of its roots, achievements and possible future. The semiconductor titanium dioxide (TiO 2 ) features predominantly in past and present work on semiconductor photocatalysis; as a result, in the most of the examples selected in this overview to illustrate various points the semiconductor is TiO 2 .

An overview of semiconductor photocatalysis

The use of visible light sensitization as a means to initiate organic reactions is attractive due to the lack of visible light absorbance by organic compounds, reducing side reactions often associated with photochemical reactions conducted with high energy UV light. This tutorial review provides a historical overview of visible light photoredox catalysis in organic synthesis along with recent examples which underscore its vast potential to initiate organic transformations.

Visible light photoredox catalysis: applications in organic synthesis

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

The mechanism of lipid peroxidation and the manner in which antioxidants function is reviewed. beta-Carotene is a purported anticancer agent, which is believed by some to have antioxidant action of a radical-trapping type. However, definitive experimental support for such action has been lacking. New experiments in vitro show that beta-carotene belongs to a previously unknown class of biological antioxidants. Specifically, it exhibits good radical-trapping antioxidant behavior only at partial pressures of oxygen significantly less than 150 torr, the pressure of oxygen in normal air. Such low oxygen partial pressures are found in most tissues under physiological conditions. At higher oxygen pressures, beta-carotene loses its antioxidant activity and shows an autocatalytic, prooxidant effect, particularly at relatively high concentrations. Similar oxygen-pressure-dependent behavior may be shown by other compounds containing many conjugated double bonds.

Beta-carotene: an unusual type of lipid antioxidant

Deuterium-isotope Effects in the Autoxidation of Aralkyl Hydrocarbons. Mechanism of the Interaction of PEroxy Radicals1

Solvent Effects in the Reactions of Free Radicals and Atoms. II. Effects of Solvents on the Position of Attack of Chlorine Atoms upon 2,3-Dimethylbutane, Isobutane and 2-Deuterio-2-methylpropane1

“Ed Janzen, Tom Strom and I are indeed flattered to have one of our publications included in the SCI most-cited paper list. Frankly, at the time this diffuse paper was written in 1963, the editor and I had some reservations about its suitability. We had in hand some rather nice examples of the occurrence of electron transfer (ET) in organic systems. However, I felt that these hard-core examples of ET between diamagnetic substrates did not place in proper perspective the scope or implications of this process. Therefore, I tried to gather together various scraps from the older literature, various predictions about when ET was likely to occur, and a variety of illustrations from our own work to form a sort of summary paper as our initial full publication. Our basic premise was that one electron oxidation or reduction of an unsaturated organic system did not require classical agents such as the alkali or other metals, radical anions, transition metal ions, or electrode surfaces. Instead, diamag netic carbanions (≡ C: ) or nitranions (> N: ) could spontaneously ET to an unsaturated system (π) to yield a free radical ( ≡C or >N. ) and the radical anion derived from the unsaturate (π .— ). while carbocations (≡C ) or dications could accept an electron from easily oxidized molecules. One variation in the overall scheme was to increase the electron affinity of the acceptor by photo-excitation. In many ways we simply extended basic concepts of inorganic and electroanalytical chemistry to the organic sphere. However, I feel we did perform a service for subsequent workers in this area by providing a key reference or benchmark from which they could develop their own discussions. I think the paper added another dimension to the organic chemist’s outlook. At the time this paper appeared many chemists thought of reactions of organic anions or cations only as being electrophilic or nucleophilic in nature. Electrons were presumed to move in pairs, and perhaps it was a small but important step forward to demonstrate that electrons do not always move as pairs. In fact, I think the editor in accepting this paper missed this point completely but accepted the paper because he felt it had something to do with dihydropyridine chemistry, a subject of considerable interest but one which involves mainly electron pair chemistry. “The circumstance which led me to the somewhat presumptuous attempt to summarize a diffuse area of chemistry in a research article can be traced to our application of new techniques and instrumentation. In the early 1960’s my group was one of the first to apply electron spin resonance (ESR) spectroscopy to the study of organic reactions We soon recognized that some of the standard reactions of organic chemistry required a new mechanistic formulation involving ET. One such process described in a later paper in this series was the base-catalyzed coupling of nitrosobenzene and phenylhydroxylamine to yield azoxybenzene. Our appreciation of ET led us to look for new examples of this process possessing utility. Thus, we subsequently discovered the ET chain mechanism for the substitution reaction between carbanions and a -substituted nitro alkanes. This process, dubbed the S RN 1 reaction, was later shown by others to be an important general replacement reaction in a variety of aromatic systems.” This paper documents by electron spin resonance (ESR) techniques the widespread occurrence of electron transfer between monoand di-anlons, wherein the negative charge is centered on carbon or nitrogen, and a wide variety of unsaturated organic compounds, particularly those containing nitro, nitroso, azo or carbonyl functional groups, or easily reducible saturated compounds such as peroxides, disulfides, and alkyl or aryl halldes. Electron transfer was also observed using a variety of organo metalllc reagents as the donors and various polynuclearor hetero-aromatic compounds as the electron acceptors. [The SCI indicates that this paper was cited 330 times in the period 1964-1977.]

Glen A. Russell

Papers

Deuterium-isotope Effects in the Autoxidation of Aralkyl Hydrocarbons. Mechanism of the Interaction of PEroxy Radicals1

Solvent Effects in the Reactions of Free Radicals and Atoms. II. Effects of Solvents on the Position of Attack of Chlorine Atoms upon 2,3-Dimethylbutane, Isobutane and 2-Deuterio-2-methylpropane1

Electron-Transfer Processes. I. The Scope of the Reaction between Carbanions or Nitranions and Unsaturated Electron Acceptors

Coupling Reactions of the 2-Nitro-2-propyl Anion1

Directive Effects in the Attack of Phenyl Radicals on Carbon-Hydrogen Bonds