Alkyne Reductions

Abstract

15 IntroductionAlkynes are highly versatile building blocks for organic chemists due to the ease with which they can be modifi ed. The selective semi -hydrogenation of alkynes is an important process for the fi ne chemical industry, manufacturing of insect sex pheromones and vitamins [1] , and for industrial polymerization processes, where removal of alkynes from alkene feedstocks is crucial [2] . The hydrometallation of alkynes is perhaps the most straightforward method for the production of vinylmetal intermediates. Vinylmetal reagents are important organic intermediates that can function in oxidation and reduction reactions as nucleophiles or in Pdcatalyzed cross -coupling reactions.This chapter covers three methods for alkyne reductions: hydrogenation [3, 4] , hydroboration [5] and hydrosilylation [6] . For the hydroalumination and hydromagnesiation of alkynes, the reader is directed to the relevant sections in Chapter 18 . Their reduction with dissolved metals is discussed in Chapter 17 . Also of interest, but not discussed here, are the hydrometallation reactions including hydrostannation [7] , hydrogermylation [8] and hydroselenation [9] . HydrogenationThe addition of molecular hydrogen (H 2 ) to carbon -carbon multiple bonds has been studied extensively and is a technique frequently used by chemists [10] . The partial reduction of alkynes gives alkenes, whereas total reduction gives alkanes. Total reduction can be simply achieved by catalytic hydrogenation. Alkanes are easily obtained with palladium [11] , Raney nickel [12] or Adams ' platinum oxide [13] under low pressures of hydrogen. A number of alkynes have been fully reduced with the complexes [RhCl(PPh 3 ) 3 ] and [Ru(OAc)(PPh 3 ) 3 ] [14] . The electronrich titanium complexes [Cp 2 Ti(PhC ≡ CPh)(PMe 3 )], [(MeCp) 2 Ti(PhC ≡ CPh)(PMe 3 )]

An Amine-Assisted Ionic Monohydride Mechanism Enables Selective Alkynecis-Semihydrogenation with Ethanol: From Elementary Steps to Catalysis

“…Catalytic cis -semihydrogenation of internal alkynes is one of the most important approaches for synthesis of Z -alkenes. − A number of heterogeneous and homogeneous catalysts have been developed for direct hydrogenation of alkynes with H 2 − or transfer hydrogenation (TH) with HCOOH, − NH 3 BH 3 , − hydrosilanes, , or alcohols − as hydrogen donors. Although effective for reduction of low-functionality alkynes, the chemo-, regio-, and stereoselectivity of the state-of-the-art catalysts, in many cases, are insufficient to produce pure Z -alkenes in polyfunctionalized bioactive molecules. , Moreover, in order to avoid Z – E alkene stereoisomerization and overreduction (two common side reactions), most catalytic systems require careful handling of the reaction conditions, which may pose a challenge for industrial application . Thus, despite tremendous progress in this field, there is still room to develop more selective, general, and practical catalysts for alkyne semihydrogenation. , …”

Are Alkyne Reductions Chemo-, Regio-, and Stereoselective Enough To Provide Pure (Z)-Olefins in Polyfunctionalized Bioactive Molecules?

“…Many biologically active molecules incorporate carbon–carbon double bond(s) with well-defined configurations ( E or Z ), such as β-carotene, polyene antifungal drugs, , crocacine, but also polyunsaturated fatty acids (PUFAs), pheromones, − and cruentaren . In the last few decades, several methods have been developed to stereochemically control the desired configurations: (i) creation of carbon–carbon double bonds (e.g., Wittig, , Horner–Emmons–Wadsworth, Julia–Kocienski, Peterson, Takai olefination, olefin metathesis); , (ii) cross-coupling reactions; (iii) reduction of alkynes; − and (iv) elimination of halides…”

Copper-Catalyzed Formal Transfer Hydrogenation/Deuteration of Aryl Alkynes

“…The reduction of an alkyne to an alkane is a fundamental reaction in organic chemistry commonly accomplished using a heterogeneous catalyst and hydrogen gas . Transfer hydrogenation represents an alternative approach to reduce an alkyne, obviating the use of flammable hydrogen gas .…”