Abstract

Light is a powerful activator for many standard and non-standardorganic transformations. This review focuses on the photochemical activationof organic reactions that can also occur thermally, in particular isomerization,acylation, hydrolysis, ionization, enolization and aromatic substitutions.Asymmetric reactions with polarized light will also be briefly discussed.

Abstract

High pressure activation is presented as a potent synthetic strategydepending on the magnitude of the activation volume. Coupling of highpressure with ancillary activation methods such as catalysis widens themethodology. The application fields are reviewed: Diels-Alder reactions, 1,3-dipolar cycloadditions, [2+2] cycloadditions, Michael and related reactions,Baylis-Hillman additions etc... Many valuable substances can be synthesizedin this way. Some recent developments are outlined, directed towardgeneration of pressure by water freezing, supramolecular chemistry,inhibition of steric hindrance.

Abstract

In the present chapter an overview about various ultrasound assistedreactions are given. Based on fundamental physical principlesmodern trends in various reaction types are discussed. Especially typicalaspects for scaling up ultrasound-assisted reactions are presented.

Abstract

Microwave irradiation has been successfully applied in organicchemistry. Spectacular accelerations, higher yields under milder reactionconditions and higher product purities have all been reported. Indeed, anumber of authors have described success in reactions that do not occur byconventional heating and even modifications of selectivity (chemo-, regio andstereoselectivity). An overview of the application of microwaveirradiation in organic synthesis and the combination with other syntheticmethodologies is included. The effect of microwave irradiation in organicsynthesis is a combination of thermal effects, arising from the heating rate,superheating or “hot spots”, the selective absorption of radiation by polarsubstances and the existence of non-thermal effects of the highly polarizingradiation – the “specific microwave effect”. Such phenomena are not usuallyaccessible by classical heating.

Abstract

The Lewis acids derived from Ga, In, Sb and Bi were slowlyreplacing “traditional” Lewis acids. The reaction conditions using such Lewisacids are usually mild and compatible with much functionality. In addition,these Lewis acids frequently show unique chemistry, which foster nicheapplications and prompt further research into their chemistry.

Abstract

Perchlorates have been frequently employed to promote a variety ofsynthetic transformations under ambient conditions. For example, theLiClO4-diethyl ether solvent medium has proven to be a powerful substitutefor external high pressure for Diels-Alder reactions and other organicreactions. In this chapter are discussed several such organic transformationswhere perchlorates have played highly significant role in promoting theirrates and yields.

Abstract

The immobilization of homogeneous catalysts on supports is usedto facilitate the catalyst separation from reaction mixtures and their recycling.In addition to this practical merit, some reports have demonstrated that theimmobilization of a catalyst can also positively influence catalyticperformance (increase in catalytic activity, stability and selectivity). In thischapter, the positive effects of immobilization on catalytic efficiency arediscussed in terms of site-isolation, steric constraints, cooperation effect andimprinting effects, etc., with the aim to understand as to how such effects leadto positive changes in catalytic properties.

Abstract

Catalysis plays a vital role in chemical transformation. Thediscovery of the Wilkinson catalyst, Rh(Ph3P)3Cl, for hydrogenationstimulated a lot of attempts to develop the enantioselective version of thesereactions by using optically active transition metal complexes [1]. In thebeginning, only very low enantioselectivity was obtained in thehydrogenation of 2-phenylacrylic acid and 2-phenyl-1-propene with certainchiral tertiary phosphine rhodium complexes as catalysts [2]. However, thesituation was dramatically changed by the invention of well-designed Rhcomplexes. This chemistry later became the standard method for the synthesisof optically active amino acids. In particular, the discovery of Ru-BINAPcomplexes in the mid 80’s significantly expanded the scope of asymmetrichydrogenation [3]. High rates of reduction of C=X functional groups wereattainable only by the coordination of structurally well-designed catalystsand suitable reaction conditions. The use of appropriate chiral diphosphineligands, particularly the BINAP class compounds, and chiral diaminesresulted in rapid and productive asymmetric hydrogenation of a range ofaromatic and heteroaromatic ketones and gave consistently highenantioselectivity. Certain amino and alkoxy ketones were also used as thesubstrates. In fact, this asymmetric transformation tolerated manysubstituents including F, Cl, Br, I, CF3, OMe, OBn, COOCH(Me)2, NO2, NH2,and NRCOR as well as various electron-rich and -deficient aromatics. Inaddition, cyclic and acyclic a,b-unsaturated ketones were converted to chiralallyl alcohols of high enantiomeric purity. Particularly noteworthy was thatthe hydrogenation of configurationally labile ketones allowed for thedynamic kinetic discrimination of diastereomers, epimers and enantiomers.This important method showed promise in the synthesis of a wide variety ofchiral alcohols from achiral and chiral carbonyl compounds. Its versatilitywas further manifested by the asymmetric synthesis of many biologicallyactive chiral compounds. These scientifically interesting and industriallyattractive processes have been recently applied to the synthesis of variousnatural and unnatural chiral compounds in a practical manner.

Abstract

Supramolecular catalysis by cyclodextrins involving non-covalentinter molecular forces and host-guest complexation has been shown to resultin high selectivities in a variety of organic reactions by the cyclodextrinmolecular recognition ability and effective use of their hydroxyl groups andderivatives. Some of the applications of cyclodextrins discussed are: enzymemodels, enzyme catalysed reactions, regioselective ring opening reactions,photo chemical reactions, oxidations, enantioselective reactions etc. Recentdevelopments in the modification of cyclodextrins to attain good geometriccontrol for accelerating the reactions has been highlighted.

Abstract

Review of organic reactions in which solvophobic effects could beimplied in the activation process is presented with a particular emphasis onreactions performed in water.

Abstract

Reactive intermediates such as carbocations, carbanions, radicalsand radical ions can be electrochemically generated from variouselectroactive species. Those intermediates may react chemically (C) orelectrochemically (E) according to EC, ECE mechanisms. Anodic oxidationsproduce acidic or electrophilic species, which can react with nucleophiles or(and) eliminate protons or electrophiles. Cathodic reductions afford basic ornucleophilic species, which can react with protons or electrophiles or (and)eliminate nucleophiles. In this way, functional group conversion,substitution reactions, addition reactions, cleavage reactions and couplingreactions can be selectively performed by using direct electrolyses.Activation by transition-metal catalysts is required when the organicsubstrate is not electroactive or leads to non desired reactions. The metalcatalysedelectrosynthesis proceeds by a double activation: i) chemicalactivation of the organic substrate by the electrogenerated active form of atransition metal catalyst that generates an organometallic species more easilyreduced than the organic substrate, ii) followed by activation by electrontransfer of the organometallic species formed in the previous chemicalactivation step. This double chemical and electrochemical activation causesnew reactions to proceed, which involve either the classical organic reactivespecies, produced in any electrochemical steps (carbanions) ororganometallic complexes (anionic or neutral) as the basis of new reactivity.

Abstract

Biocatalytic reactions using enzymes or whole microbial cells areincreasingly employed for organic synthesis, taking advantage of their highversatility and exquisite selectivities (chemo-, regio- and stereoselectivity).Resolution of racemates and its refinements, desymmetrisation of meso- andprochiral compounds by oxidoreduction, some C-C bond forming reactionsand other miscellaneous reactions will be outlined, illustrating the mostcharacteristic aspects of environmentally friendly and yield-effectiveproduction of commodity chemicals, stereospecific building blocks orintermediates, and competitive step-replacement strategies for asymmetricsyntheses.

Abstract

In this chapter, we describe some of the general strategiesimplemented by the experimenter to challenge the immune system for theproduction of antibodies that act as catalysts. Some representative strategiesinclude the use of haptens that are transition state analogues, the “bait andswitch” strategy and the concept of “reactive immunisation”. These protocolshave allowed the preparation of catalytic antibodies for various synthetictransformations including reactions for which there is no enzymatic catalystavailable. In addition to its value for synthesis, this area of research providesan opportunity to better understand the rules of enzyme catalysis.