A tribute to Professor Keith Smith: commemorative issue in honor of Professor K. Smith on the occasion of his 65th anniversary

Keith received an MSc in 1969 and a PhD in 1971 for his work in this area and was awarded a Royal Society European Exchange Fellowship to take up a postdoctoral research position with Professor Albert Eschenmoser at the ETH in Zürich, Switzerland, where he contributed to a project on chlorophyll derivatives. While Keith was in Switzerland, Andrew Pelter moved to Swansea University to take up a position as Professor of Organic Chemistry and a lectureship became available there. Keith was appointed to the position from October 1972 and rejoined his ollaboration with Pelter on organoboron chemistry. Among other reactions, they introduced methods for syntheses of unsymmetrical conjugated diynes from the corresponding dialkyldialkynylborates, of secondary and tertiary alcohols from reactions of trialkylboranes with bis(phenylthio)alkyllithiums8 or 2-lithio-2- alkylbenzodithioles, of N-substituted sulfonamides, and of dialkylbromoboranes

Regioselective chlorination of phenols in the presence of tetrahydrothiopyran derivatives

Four six-membered cyclic sulfides, namely tetrahydrothiopyran, 3-methyltetrahydrothiopyran, 4-methyltetrahydrothiopyran and 4,4-dimethyltetrahyrdrothiopyran have been used as moderators in chlorination reactions of various phenols with sulfuryl chloride in the presence of aluminum or ferric chloride. On chlorination of phenol, ortho-cresol and meta-cresol the para/ortho chlorination ratios and yields of the para-chloro isomers are higher than when no cyclic sulfide is used for all of the cyclic sulfides, but chlorination of meta-xylenol is less consistent, with some cyclic sulfides producing higher p/o ratios and others producing lower ratios than reactions having no sulfide present

Regioselective electrophilic aromatic substitution reactions over reusable zeolites

Zeolite catalysts can play an important role in the development of greener organic syntheses through their abilities to act as heterogeneous catalysts, support reagents, entrain by-products, avoid aqueous work-ups and enhance product selectivities via shape-selectivity. We have shown that zeolites under modest conditions can have advantages in para-regioselective nitration, alkylation, acylation, methanesulfonylation and halogenation of aromatic compounds. They can also assist in the electrophilic rearrangement of aryloxiranes. The zeolites can be easily removed from reaction mixtures by filtration and regenerated by heating and can then be reused several times to give the same selectivity

Development of efficient and selective processes for the synthesis of commercially important chlorinated phenols

para-Selective processes for the chlorination of phenols using sulphuryl chloride in the presence of various sulphur-containing catalysts have been successfully developed. Several chlorinated phenols, especially those derived by para-chlorination of phenol, ortho-cresol, meta-cresol, and meta-xylenol, are of significant commercial importance, but chlorination reactions of such phenols are not always as regioselective as would be desirable. We, therefore, undertook the challenge of developing suitable catalysts that might promote greater regioselectivity under conditions that might still be applicable for the commercial manufacture of products on a large scale. In this review, we chart our progress in this endeavour from early studies involving inorganic solids as potential catalysts, through the use of simple dialkyl sulphides, which were effective but unsuitable for commercial application, and through a variety of other types of sulphur compounds, to the eventual identification of particular poly(alkylene sulphide)s as very useful catalysts. When used in conjunction with a Lewis acid such as aluminium or ferric chloride as an activator, and with sulphuryl chloride as the reagent, quantitative yields of chlorophenols can be obtained with very high regioselectivity in the presence of tiny amounts of the polymeric sulphides, usually in solvent-free conditions (unless the phenol starting material is solid at temperatures even above about 50 °C). Notably, poly(alkylene sulphide)s containing longer spacer groups are particularly para-selective in the chlorination of m-cresol and m-xylenol, while, ones with shorter spacers are particularly para-selective in the chlorination of phenol, 2-chlorophenol, and o-cresol. Such chlorination processes result in some of the highest para/ortho ratios reported for the chlorination of phenols

Use of zeolites for greener and more para-selective electrophilic aromatic substitution reactions

Electrophilic aromatic substitution is one of the most important reactions in synthetic organic chemistry. Such reactions are used for the synthesis of important intermediates that can be used as precursors for the production of pharmaceutical, agrochemical and industrial products. However, many commercial processes to produce such materials still rely on technology that was developed many years ago. Such processes commonly lead to mixtures of regioisomers and in recent years several new approaches have been developed to gain control over the regiochemistry of the reactions. Zeolites can act as heterogeneous catalysts, support reagents, entrain by-products, enhance product para-selectivities via shape-selectivity and avoid aqueous work-ups. For example, zeolites can have advantages in para-regioselective nitration, halogenation, alkylation, acylation and methanesulfonylation reactions under modest conditions. Moreover, usually they can be easily removed from reaction mixtures by simple filtration and regenerated by heating and can then be reused several times to give almost the same yield and selectivity as fresh samples. This review surveys the use of zeolites as para-selective catalysts for a whole range of aromatic substitution reactions, based largely on our own work in the area, but set in the wider context of other related work

Synthesis of new thiophene derivatives and their use as photostabilizers for rigid poly(vinyl chloride)

Five new thiophenes, namely, N-[(3-bromo-2-methylthiophen-5-yl)methylene]-4-methoxyaniline (4a), N-[(3-bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethoxyaniline (4b), N-[(3-bromo-2-methylthiophen-5-yl)methylene]-3,4-dimethylaniline (4c), 3-[(3-bromo-2-methylthiophen-5-yl)methyleneamino]-2-methylquinazolin-4(3H)-one (4d), and 3-[(3-bromo-2-methylthiophen-5-yl)methyleneamino]-2-isopropylquinazolin-4(3H)-one (4e), have been synthesized. All of these materials brought about a reduction in the level of photodegradation of poly(vinyl chloride) (PVC) films containing the synthesized thiophenes (0.5%; by weight). The results obtained showed that the extent of photostabilization of PVC in the presence of an additive was in the order 4e > 4d > 4b > 4a > 4c. For the most favorable additive (4e), the rate of appearance of infrared absorption bands of degradation products was reduced by around two-thirds, while the quantum yield of chain scission was calculated to be reduced by a factor of more than one thousand. It is suggested that the additives may help stabilize PVC by direct absorption of UV radiation and dissipation of the energy as heat or that electrostatic attraction between the additives and PVC may assist transfer of energy from excited state PVC to the additive, from where it can be dissipated

Variations in site of lithiation of N-[(2-(4-Methoxyphenyl)ethyl)]pivalamide; Use in ring substitution

Lithiation of N-[(2-(4-methoxyphenyl)ethyl)]pivalamide at -20 to 0 oC with three mole equivalents of n-BuLi in anhydrous THF, followed by reactions with various electrophiles, gives high yields of products involving ring substitution ortho- to the pivaloylaminoethyl group, which was unexpected in view of earlier results reported with t-BuLi