Utilization of the fascinating properties of β-oxodithioesters as building blocks in heterocyclic frameworks was started a few decades ago. Its similarity and dissimilarity with β-ketoesters in terms of its synthesis or as a substrate towards synthesis makes it an interesting and emerging field of synthetic organic chemistry. Tuning of catalysts and counter substrates towards selective utilization of its multiple reacting sites was summarized in the following discussion. A diverse range of monocyclic and polycyclic heterocycles with several different heteroatoms is plotted in an easy and reader friendly manner.

β-Oxodithioesters: a new frontier for diverse heterocyclic architectures

This chapter covers the general reactivity of pyridines and their benzo derivatives at the ring atoms of the heterocycle. Topics include reactions at nitrogen and ring carbons. At nitrogen, salt formation, halogenation, N-oxidation, acylation, and carbene additions are covered. Reactions at carbon proceed through electrophilic, nucleophilic, and free radical attack. These types of bond-forming transformations include halogenation, nitration, sulfonation, diazo coupling, oxidation, alkylation, arylation, acylation, addition of heteroatom nucleophiles, and metal-catalyzed couplings. The topics of the last section are thermal and photochemical reactions and those involving cyclic transition states. These include ring-expansion and ring-opening reactions of quinolyl and isoquinolyl nitrenes, thermolysis of pyridylbenzotriazole, Diels–Alder reactions of 2-pyridones, [4+4] photocycloaddition of pyridines and 2-pyridones, and photolysis of pyridinium salts to afford bicyclic aziridines. The chapter organization generally follows that used in Chapter 5.02 of CHEC-II(1996) except that metal-catalyzed cross-coupling reactions have been added. This chapter covers the literature of the period 1996–2006.

Pyridines and their Benzo Derivatives: Reactivity at the Ring

Detailed kinetic and computational investigation of the enantio- and diastereoselective allylation of aldehydes 1 with allyltrichlorosilanes 5, employing the pyridine N-oxides METHOX (9) and QUINOX (10) as chiral organocatalysts, indicate that the reaction can proceed through a dissociative (cationic) or associative (neutral) mechanism: METHOX apparently favors a pentacoordinate cationic transition state, while the less sterically demanding QUINOX is likely to operate via a hexacoordinate neutral complex. In both pathways, only one molecule of the catalyst is involved in the rate- and selectivity-determining step, which is supported by both experimental and computational data.

Mechanistic dichotomy in the asymmetric allylation of aldehydes with allyltrichlorosilanes catalyzed by chiral pyridine N-oxides.

The azomethine ylides p-R3C5H4N+CH−COC6H4R2-p (3 a: R3=H, R2=H, X=Br; 3 b: R3=H, R2=Me, X=I; 3 c: R3=H, R2=OMe, X=I; 3 d: R3=H, R2=F, X=I; 3 e: R3=Me, R2=Me, X=Br) react with the nitrile functionality of the closo-decaborate clusters [Bun4N][B10H9(NCR1)] (1 a: R1=Me; 1 b: R1=Et; 1 c: R1=Ph) in a CH3NO2 solution under mild conditions (20–25 °C, 2 min) to afford selectively products of the nucleophilic addition (ca. quantitative yields based on NMR analysis in [D6]DMSO, 71–87 % yield of isolated products). These products are the borylated enamino ketones as the salts bearing exclusively a tetrabutylammonium cation [Bun4N][B10H9{NCR1=C(N+C5H4R3-p)COC6H4R2-p}] (4 a–h,k–n) or the mixed salts [Bun4N]1-x[p-R3C5H4N+CH2COC6H4R2-p]x[B10H9{NCR1=C(N+C5H4R3-p)COC6H4R2-p}] (4 i, 4 j, and 4 o). This reaction represents the first example of the selective nucleophilic addition of pyridinium azomethine ylides to the nitrile group and the first example of new CC bond formation in the reaction of nitrilium derivatives of boron clusters with any nucleophiles so far tested. Compounds 4 a–h,k–n were characterized by ICP-MS, high resolution ESI-MS, molar conductivity, and IR, 1H, 13C{1H}, and 11B{1H} NMR spectroscopy. The structures of 4 a and 4 b were determined by a single-crystal X-ray analysis. Theoretical calculations at the DFT level allowed the interpretation of the observed reaction selectivity, which is driven mostly by thermodynamic rather than kinetic factors, and steric repulsion between the boron cluster and the azomethine ylide molecule plays a crucial role. The activation of the nitrile group upon binding to the boron cluster is explained in terms of the electrostatic arguments. The mechanisms of the nucleophilic addition and the hypothetical cycloaddition between the azomethine ylide and nitriles were investigated in detail.

E. Gálvez

Papers

Synthesis and structure of dithioester stabilised pyridinium ylides