Solvent-Free Organic Syntheses

Ecological points of view must nowadays be taken into consideration in the development of new syntheses and apportioned due importance in assessing the viability of a new synthesis. A crucial factor here is the need to restrict the use of ecologically harmful organic solvents. Examples are presented which show that solvent-free reactions are not only of interest from an environmental viewpoint, but also offer considerable advantages in terms of yield, selectivity, and simplicity of the reaction procedures.     

Toward a Symphony of Reactivity: Cascades Involving Catalysis and Sigmatropic Rearrangements

Catalysis and synthesis are intimately linked in modern organic chemistry. The synthesis of complex molecules is an ever evolving area of science. In many regards, the inherent beauty associated with a synthetic sequence can be linked to a certain combination of the creativity with which a sequence is designed and the overall efficiency with which the ultimate process is performed. In synthesis, as in other endeavors, beauty is very much in the eyes of the beholder. It is with this in mind that we will attempt to review an area of synthesis that has fascinated us and that we find extraordinarily beautiful, namely the combination of catalysis and sigmatropic rearrangements in consecutive and cascade sequences.

1 Sometimes the assessment of beauty is nearly unanimous. The first four notes of Beethoven’s 5th Symphony (Symphony No.5 in C minor, Op.67) represent perhaps the most well‐known and popular motif in classical music. The orchestral score is shown in the background of the cover graphic. Accessed March 20, 2013 from http://imslp.org/wiki/Symphony_No.5,_Op.67_%28Beethoven,_Ludwig_van%29 .

     

Copper(II)‐Catalyzed Nitroaldol (Henry) Reactions: Recent Developments

Self‐assembled copper(II) complexes are described as effective catalysts for nitroaldol (Henry) reactions on water. The protocol involves a heterogeneous process and the catalysts can be recovered and recycled without loss of activity. Further, C2‐symmetric N,N′‐substituted chiral copper(II) salan complexes are found to be more effective catalysts than chiral copper(II) salen complexes for reactions in homogeneous catalysis, with high enantioselectivities. The reactions involve bifunctional catalysis, bearing the properties of a Brønsted base, as well as a Lewis acid, to effect the reaction in the absence of external additives.     

ChemInform Abstract: Model Studies of Palladium-Catalyzed Coupling Reactions for the “South- North” Attachment of Avermectins.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Asymmetric organocatalytic domino reactions

The current status of organic synthesis is hampered by costly protecting-group strategies and lengthy purification procedures after each synthetic step. To circumvent these problems, the synthetic potential of multicomponent domino reactions has been utilized for the efficient and stereoselective construction of complex molecules from simple precursors in a single process. In particular, domino reactions mediated by organocatalysts are in a way biomimetic, as this principle is used very efficiently in the biosynthesis of complex natural products starting from simple precursors. In this Minireview, we discuss the current development of this fast-growing field.     

二氧化碳参与的有机化学反应*

由于对日趋严重的温室效应的担忧,对廉价二氧化碳的固定和转化成为一个非常有吸引力的研究领域。本文主要介绍了二氧化碳参与的有机化学反应,包括与环氧化合物、胺类化合物、不饱和碳氢化合物以及醇类和烃类等化合物的反应。文章分别对这几类反应的发展历程、研究现状进行了总结,重点介绍了各反应所用到的催化剂体系及有关反应机理,并展望了该领域的研究前景。     

Kinetic equation for reversible heterogeneous catalytic reactions

It is demonstrated that when operating far from equilibrium, despite the non-linearity of a complex reaction system, the overall rate of a heterogeneous catalytic reaction is expressed by a classical equation, where the equilibrium constant is determined by the ratio of constants of the forward and reverse reaction, which do not include the lateral interactions in the adsorbed layer.     

Late‐Stage Functionalization of Biologically Active Heterocycles Through Photoredox Catalysis

The direct C? H functionalization of heterocycles has become an increasingly valuable tool in modern drug discovery. However, the introduction of small alkyl groups, such as methyl, by this method has not been realized in the context of complex molecule synthesis since existing methods rely on the use of strong oxidants and elevated temperatures to generate the requisite radical species. Herein, we report the use of stable organic peroxides activated by visible‐light photoredox catalysis to achieve the direct methyl‐, ethyl‐, and cyclopropylation of a variety of biologically active heterocycles. The simple protocol, mild reaction conditions, and unique tolerability of this method make it an important tool for drug discovery.     

Radical-chain functionalisation at C-H centres using an O-oxiranylcarbinyl O-silyl ketene acetal

A readily prepared O-oxiranylcarbinyl O-silyl ketene acetal has been used to bring about functionalisation at various types of saturated C-H group, through the intermediacy of an oxiranylcarbinyl radical that undergoes rapid ring opening to give a highly reactive allyloxyl radical.     

Uniform Heterogeneous Catalysts: The Role of Solid-State Chemistry in their Development and Design

Heterogeneous catalysts are generally assumed to be multiphasic and multicomponent; many of them are, and this is one of the resons why disentangling the factors that govern their mode of action is so difficult. But there is a large class of heterogeneous catalysts where the solid is monophasic and where the activity may be envisaged as being dispersed in a spatially uniform fashion throughout its bulk. This is true both of zeolites and many other microporous catalysts on the one hand, and of certain mixed metal oxides, where the non-stoiohiometry is inextricably mingled with the catalysis, on the other. By recognizing this broad classification numerous operational advantages follow: the performance of existing catalysts and the design of those yet to be prepared can be placed on a rational footing; moreover, the myriad techniques of solid-state chemistry and physics, often regarded as inapplicable to the subtle and special problems of surface chemistry, are seen to be of direct relevance as probes for the structure and properties of proven uniform heterogeneous catalysts as well as for the synthesis and development of new ones. This review, which draws analogies with and lessons from the chemistry of enzyme catalysts, focuses largely on the catalytic conversions of hydrocarbons over zeolites, clays, microporous AlPO₄ and a wide range of metal oxides.     

Titanocene-catalyzed cascade cyclization of epoxypolyprenes: straightforward synthesis of terpenoids by free-radical chemistry

The titanocene-catalyzed cascade cyclization of epoxypolyenes, which are easily prepared from commercially available polyprenoids, has proven to be a useful procedure for the synthesis of C(10), C(15), C(20), and C(30) terpenoids, including monocyclic, bicyclic, and tricyclic natural products. Both theoretical and experimental evidence suggests that this cyclization takes place in a nonconcerted fashion via discrete carbon-centered radicals. Nevertheless, the termination step of the process seems to be subjected to a kind of water-dependent control, which is unusual in free-radical chemistry. The catalytic cycle is based on the use of the novel combination Me(3)SiCl/2,4,6-collidine to regenerate the titanocene catalyst. In practice this procedure has several advantages: it takes place at room temperature under mild conditions compatible with different functional groups, uses inexpensive reagents, and its end step can easily be controlled to give exocyclic double bonds by simply excluding water from the medium.     

Enantioselective palladium-catalyzed total synthesis of vitamin e by employing a domino Wacker-Heck reaction

An enantioselective total synthesis of vitamin E in which a novel palladium-catalyzed domino reaction was employed as the key step is described. This reaction allows the formation of the chiral chroman framework and the concurrent introduction of part of the side chain of vitamin E. The sequence comprises an enantioselective Wacker cyclization and a subsequent Heck reaction. Accordingly, reaction of alkenylphenol 12 with methyl vinyl ketone (13) in the presence of catalytic amounts of Pd(OTFA)(2) (TFA = trifluoroacetate), the enantiopure ligand (S,S)-Bn-BOXAX (8 b; Bn = benzyl, BOXAX = 2,2'-bis(oxazolyl)-1,1'-binaphthyl, and p-benzoquinone (9) as an oxidant gives access to chiral chroman 10 with an enantioselectivity of 97 % ee in 84 % yield. Chroman 10 is then converted into 24 by an aldol condensation reaction with (3R)-3,7-dimethyloctanal (11). Subsequent 1,2-addition of methyllithium, elimination of water, and hydrogenation yields vitamin E.     

Stereo‐ and Chemodivergent NHC‐Promoted Functionalisation of Arylalkylketenes with Chloral

Stereo‐ and chemodivergent enantioselective reaction pathways are observed upon treatment of alkylarylketenes and trichloroacetaldehyde (chloral) with N‐heterocyclic carbenes, giving selectively either β‐lactones (up to 88:12 dr, up to 94 % ee) or α‐chloroesters (up to 94 % ee). Either 2‐arylsubstitution or an α‐branched iPr alkyl substituent within the ketene favours the chlorination pathway, allowing chloral to be used as an electrophilic chlorinating reagent in asymmetric catalysis.