Advances in Organic Synthesis

Catalysis has played a prominent role in recent decades allowing chemists to develop novel and efficient reactions in almost every class of chemical transformation. With the tuning of the catalysts’ steric and electronic properties, sophisticated reactions have been discovered, sometimes featuring several individual steps and resulting in one-pot formation of complex chemical structures with high atom-economy. With the increasing recognition of the importance of green and sustainable chemistry, the concept of step-economy has gained traction and one-pot multistep reactions have been developed. Current research in this area now focuses on the use of multiple catalysts within the same reactor to convert simple and available substrates into complex and valuable products. In this chapter, we review a selection of examples of catalytic tandem reactions triggered by the introduction of a carbonyl function either formed by oxidation of alcohols, hydroformylation, isomerisation or carbonylation. In particular, we emphasize nanocatalysis, the use of metal nanoparticles as catalysts. The in situ formation of reactive carbonyl electrophiles opens a wide array of possible subsequent reactions as illustrated in the following pages.

Knölker type iron complexes, used directly or generated in situ, are suitable catalysts for (de)hydrogenation reactions enabling the synthesis of carbonyl derivatives, amines and alcohols of industrial and academic interest. Enantioselective hydrogenation of prochiral C=X double bonds (X: O, N) was achieved with up to 94% e.e. using iron-complexes bearing chiral ligands or Knölker’ complex in combination with a chiral additive. Knölker type complexes were also competent catalysts for sequential reactions based on temporary dehydrogenative alcohol activation including dynamic kinetic resolution of racemic secondary alcohols (99% e.e.) and borrowing hydrogen methodology. The latter method was applied to synthesize N-alkyl substituted amines, α-alkylated ketones and oxygen- or nitrogen-heterocycles.

Recent advances in organic synthesis based on superelectrophilic activation of alkynes, alkenes, allenes, and their trifluoromethyl substituted derivatives under the action of Brønsted superacids (CF3SO3H, FSO3H, HF), strong Lewis acids (AlCl3, AlBr3), conjugate Brønsted-Lewis superacids (CF3SO3H–SbF5, FSO3H–SbF5, HF–SbF5, HBr–AlBr3, HCl–AlCl3) or superacidic H+-zeolites are considered. These reactions proceed through an intermediate formation of carbocationic and other cationic species and lead to the formation of various functionalized compounds, carbocycles, heterocycles under mild conditions and in high yields of target products. In many cases, cationic reaction intermediates are detected and thoroughly studied by means of NMR in superacids (CF3SO3H, FSO3H), that help to prove reaction mechanisms.

This chapter provides a recent description of chemical synthesis, encapsulation of chitosan microspheres and the incorporation of microorganisms involving technological advances to bioremediation of inorganic and organic environmental pollutants as well as their antimicrobial activity. In addition, the potential of chitosan and their derivatives in the bioremediation of water polluted by oil spills, heavy metals, dyes, phenols and pesticides is explored.

Hypervalent iodine(III) reagents, including iodosobenzene diacetate, iodosobenzene bis(trifluoroacetate), iodosylbenzene, iodobenzene dichloride, hydroxy(tosyloxy) iodobenzene (Fig. 1), have been vastly used in various oxidative transformations in organic synthesis due to their low toxicity, commercial availability, ease of handling and environmental benignity. A striking feature of these hypervalent iodine(III)- mediated approaches is that these transformations may not need the participation of any transition metals. Thus, the development of transition-metal-free method mediated by hypervalent iodine reagent for oxidative coupling reactions is a topic of great interest as such transformations represent an attractive alternative to the traditional transitionmetal- catalyzed oxidative reaction. Most strikingly, the hypervalent iodine has been utilized for the synthesis of a variety of biological interesting N-containing heterocyclic compounds including indoles, oxazoles, oxindoles, azirines, diazepin-2-ones, carbazolones, quinolin-2-ones, 1,4-benzodiazepines, spirooxindoles, etc under mild reaction conditions (Fig. 2). In this chapter, we highlight all the representative methodologies that have applied the common hypervalent iodine(III) reagents as oxidant in synthesizing the biologically interesting N-containing heterocyclic compounds through intramolecular oxidative C-C, C-N, C-O, N-N bond forming reactions as well as cascade reactions in some cases. The presentation is organized according to the ring sizes and the ring patterns of the intermediate (spiro vs. fused) formed during the intramolecular oxidative cyclization step. However, the application of hypervalent iodine(III) reagents together with the transition metals for the synthesis of N-containing heterocyclic compounds will not be discussed in this chapter.

The reactions involving the carbon-carbon bond formation are the most significant and fundamental reactions in synthetic organic chemistry. Amongst such various types of reactions, Morita Baylis-Hillman (MBH) reaction has become one of the most constructive and popular routes for the production of functionalized molecules with vast utility and prospective. This reaction involves coupling of the activated alkenes with a variety of aldehydes under catalytic influence of tertiary bicyclic amines, phosphines or ionic liquids. The present significance of this reaction is credited to the fact that it leads to one-pot atom-economical construction of carbon-carbon bonds providing multifunctional molecules. Recently, ionic liquids are most extensively used for the synthesis of adducts of this reaction. This review is presented by the purpose of summarizing the various advances in ionic liquids and their use in this reaction.