Current Organic Chemistry - Volume 8, Issue 7, 2004

Recent applications of theoretical and physical approaches to the elucidation of acyl-transfer mechanisms have led to further clarifications of structures of intermediates and transition states involved in the stepwise reaction paths. Gas-phase acyl-transfer reactions, both experimental and theoretical, are surveyed, in which factors conductive to the stepwise path are discussed. Aminolysis reactions of carbonyl and thiocarbonyl esters in solution are reviewed with special emphasis of transition state variation with substituents in the nucleophile, acyl and leaving groups applying Marcus theory, Bell-Evans-Palanyi principle, principle of microscopic reversibility, DFTFukui function, reactivity-selectivity principle, natural bond orbital analysis and More O'Ferrall-Jencks diagram etc. The use of cross-interaction constants as a mechanistic tool is stressed.

Predictions of excitation energies of π-conjugated oligomers and band gaps of conducting organic polymers at various theoretical levels are reviewed and compared to experimental data in gas, liquid, and solid phases. High level ab initio calculations reproduce the experimental excitation energies of polyenes in the gas phase very well. The assignment of the peaks to adiabatic and vertical excitations is, however, not certain. Solvent effects lower and disorder increases the excitation energies. In the solid state, bands of molecular crystals split and shift compared to individual molecules. Therefore, theoretical data should not agree completely with experiment, unless all medium effects are accounted for. Band structure calculations usually neglect most of the medium effects but can produce trends, helping with the analysis of substituent effects. Properties of oligomers with increasing size do not necessarily depend linearly on inverse chain length. Even for defect free oligomers in the gas phase a saturation limit seems to exist. Theoretical investigations are particularly useful for studying underlying causes of changes in properties upon chemical modification. Understanding these factors can lead to structure property relationships that are needed for“tailoring” new systems for specific applications.

The choice of layered phosphates and phosphonates of tetravalent metals, mainly Zr(IV), plays a key role in the success of liquid phase heterogeneous catalysis for the preparation of useful intermediates in organic synthesis. The preparation, the chemical and thermal stability, and the structural aspects of this emerging class of inorganic and inorgano-organic materials will be reviewed with the aim to clarify their potential in heterogeneous liquid phase catalysis. The possibility of modifying their surface area and porosity and the nature of active catalytic centres by means of ion exchange, intercalation and exfoliation reactions will be also presented. Furthermore, this review will discuss the numerous efforts dedicated to the development of new synthetic methods using acidic or basic Zr(IV) phosphates and phosphonates as valuable alternative solid catalysts compared to other classes of heterogeneous catalysts. The reported methods clearly demonstrated the importance of this type of catalysis, especially for the following features: easy handling, mild reaction conditions, solvent free conditions (in many cases), simple work-up and the recyclable nature of the catalysts. Herein, we present a review of these various types of solid catalysts and their characteristics and applications in organic synthesis.

The review focuses on our results concerning the synthesis of functionalized and simple furyl hydroperoxides and their employment as oxygen donors in metal-catalysed asymmetric oxidations. The first part describes the synthetic approaches (chemical, photochemical, enzymatic) to furyl hydroperoxides as racemic and enantiopure compounds. The second part covers their employment in Sharpless modified procedure for the epoxidation of allylic alcohols, oxidation of prochiral sulfides and kinetic resolution of racemic sulfoxides. Furyl hydroperoxides have been used as oxidants using catalytic Ti(Oi-Pr)4 / (R)-BINOL system for the asymmetric sulfoxidation and when using enantiomerically enriched BINOL, non linear positive effects ((+)-NLE) have been observed. Finally, the first results as stereoselective oxidants in absence of chiral ligands have been reported in the epoxidation of allylic alcohols. Discussion includes highlighting of efficiency and performance of these oxidants as alternative reagents to commercial t-butyl hydroperoxide (TBHP) and cumyl hydroperoxide (CHP).

The prototypical anionic phospho-Fries rearrangement involves the conversion of an aryl phosphate ester [ArOP(=O)(OR)2] to an ortho-hydroxyarylphosphonate [o-OH-Ar-P(=O)(OR)2]. This review looks in detail at this 1,3-(O→C) migration, from its accidental discovery about 20 years ago to applications in the current literature. From a synthetic perspective, the rearrangement gives rise to a phenol with an ortho C-P bond. Prior to discussion of the details of the reaction, we briefly review the utility of these arylphosphonate compounds. The transposition of the phosphoryl moiety is initiated by formation of an aryl anion adjacent to the phosphate substituent. The phosphate group itself directs metallation to the ortho-position (i.e., via metal-hydrogen exchange), although direct metal-halogen exchange at the ortho position is a more efficient and reliable approach to anion formation. Methods for the synthesis of suitable precursor substrates, approaches to anion generation, and mechanistic aspects of the rearrangement are summarized. Factors which influence the outcome of the rearrangement include: reaction temperature, existing stereochemical elements, the nature of the substituents at phosphorus (phosphites and phosphoramidates have also been employed), existing substituents on the aromatic ring, and the propensity for further reactions. One of the most important issues in the case of unsymmetrical substrates is the regiochemical outcome of the reaction. The limited information available on this, as well as the compatibility of the reaction with other functionalities are discussed. We conclude with miscellaneous examples in relatively complex molecules, illustrating the potential for the reaction to be utilized more widely in synthetic applications.

In the field of conducting polymers, both poly(pyrrole) and poly(thiophene) have been investigated extensively and are used currently in a wide variety of applications including microelectronics, electrode materials, sensors and optoelectronics. Amongst these polymers, 3- and 3,4- substituted poly(pyrroles) and poly(thiophenes) have received significant attention in recent years as demonstrated by the increase in the number of patents and publications that describe their use. This review covers the development in the synthesis of 3- and 3,4- substituted poly(pyrroles) and poly(thiophenes) over the last 30 years, their polymerisation in addition to describing the material properties and applications of the resulting polymers. In particular, this review focuses upon the variety of methodologies employed for the synthesis of 3- and 3,4-substituted pyrroles and thiophenes as well as upon the broad range of functional groups that can be attached to the heterocyclic ring system in order to tailor the properties of the resulting polymers.