Alkynes synthetic application

Pyrano[3,4-b]indol-3-ones are the most useful equivalents of the indol-2,3-quinodimethane synthon which are currently available for synthetic application. These compounds can be synthesized readily from indole-3-acetic acids and carboxylic anhydrides[5,6]. On heating with electrophilic alkenes or alkynes, adducts are formed which undergo decarboxylation to 1,2-dihydro-carbazoles or carbazoles, respectively. 

The most common synthetic application of mercury-catalyzed addition to alkynes is the conversion of alkynes to ketones. This reaction is carried out under aqueous acidic conditions, where the addition intermediate undergoes protonation to regenerate Hg. ... 

Partial reduction of alkynes to Z-alkenes is an important synthetic application of selective hydrogenation catalysts. The transformation can be carried out under heterogeneous or homogeneous conditions. Among heterogeneous catalysts, the one that... 

As discussed in Section 6.2, nitro compounds are good precursors of nitrile oxides, which are important dipoles in cycloadditions. The 1,3-dipolar cycloaddition of nitrile oxides with alkenes or alkynes provides a straightforward access to 2-isoxazolines or isoxazoles, respectively. A number of ring-cleaving procedures are applicable, such that various types of compounds may be obtained from the primary adducts (Scheme 8.18). There are many reports on synthetic applications of this reaction. The methods for generation of nitrile oxides and their reactions are discussed in Section 6.2. Recent synthetic applications and asymmetric synthesis using 1,3-dipolar cycloaddition of nitrile oxides are summarized in this section. 

Various transition metal complexes, in particular of late transition metals, were reported to be effective catalysts for such double bond isomerization. Because organic synthesis is the focus of this volume, this section will cover the transition metal-catalyzed isomerization of alkenes, which has the significant synthetic and industrial utilities. This chapter will also include the synthetic application, asymmetric reactions,4-6 and isomerization of alkynes, in particular, that of propargylic alcohols. 

Abstract The basic principles of the oxidative carbonylation reaction together with its synthetic applications are reviewed. In the first section, an overview of oxidative carbonylation is presented, and the general mechanisms followed by different substrates (alkenes, dienes, allenes, alkynes, ketones, ketenes, aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, phenols, amines) leading to a variety of carbonyl compounds are discussed. The second section is focused on processes catalyzed by Pdl2-based systems, and on their ability to promote different kind of oxidative carbonylations under mild conditions to afford important carbonyl derivatives with high selectivity and efficiency. In particular, the recent developments towards the one-step synthesis of new heterocyclic derivatives are described. 

Zhang, W. Moore, J. S. Alkyne metathesis Catalysts and synthetic applications. Adv. Synth. Catal. 2007,349, 93-120. 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

Trialkylboranes generally show a low reactivity toward addition to alkenes and alkynes. Only trial-lylboranes react with alkynes under mild conditions. This reaction is often complicated by further intramolecular carboboration reactions and has found only limited synthetic application. 

Catalytic Metatheses of Alkenes and Alkynes, and Their Synthetic Applications... 

Reports on ruthenium catalytic activity focus more on mechanistic consideration of the prototypical phenylacetylene dimerization than in establishing its synthetic applicability. It is not unusual that changing the alkyne substituents results in reversed selectivity (i.e. R = Ph or SiMe3 gave ( )- or (Z)- isomers, respectively) [27]. Competitive alkyne cyclotrimerization (R = COOMe) [27] or butatriene formation (R= CH2Ph, Bu) [10, 21] have occasionally been reported as possible drawbacks in enyne synthesis. The operating mechanism restricts the reaction to terminal alkynes. 

Compared with cycloisomerization, enyne metathesis as a bond reorganization of an alkene and an alkyne to produce a 1,3-diene is less studied. A recent review by Diver and Giessert highlights some recent advances in synthetic applications, and mechanistic features [60]. 

The exploitation of intramolecular Diels-Alder reactions has included synthetic applications in the thienopyrimidine series as well. Thus, the 1,2,4-triazine with a tethered alkyne (276) participates in an intramolecular cyclization, with concomitant loss of RCN, to produce the thienopyrimidine (277) (Equation (97)) <87JOC4287>. A similar reaction occurs with oxygen analogues. 

A series of books 7) and reviews 8) written on the utility of these reactions highlights the synthetic applicability of this field. However, there was no attempt to systematize the otherwise vast literature available for selective hydroboration, e.g., hydroboration of double bond in the presence of triple bond and vice versa, hydroboration of one of the double bonds of dienes, monohydroboration of alkynes, and selective hydroboration of carbon-carbon multiple bonds in the presence of many other functional groups. These topics are the aim of this review. Included here also is a brief account of the developing field of haloboration. 

Continuing expansion of the chemistry of polynuclear cobalt-alkyne complexes is expected, especially in the areas of reactivity and synthetic applications, in the coming years. 

The insertion of olefins and hetero-alkenes and -alkynes (ketones, nitriles) into the M—C bonds of metallacyclopropenes, as present in metal alkyne and particularly benzyne complexes, leads to five-membered metallacycles.193,194 This reaction has widespread synthetic applications 195... 

The coordination of olefins to gold species is well known, despite the preference for alkynes, allenes, and carbonyl groups. This is reflected in the fact that synthetic applications based on olefin activation by gold have been developed to a lesser extent than aUcyne or carbonyl activation. 

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