Structure of alkynes

Introduction 392 9-2 Nomenclature of Alkynes 393 9-3 Physical Properties of Alkynes 394 9-4 Commercial Importance of Alkynes 395 9-5 Electronic Structure of Alkynes 396 9-6 Acidity of Alkynes Formation of Acetylide Ions 397 9-7 Synthesis of Alkynes from Acetylides 399 9-8 Synthesis of Alkynes by Elimination Reactions 403 Summary Syntheses of Alkynes 404 9-9 Addition Reactions of Alkynes 405... 

The electronic structure of alkynes is related to that of alkenes, and the photochemistry of the two classes of compound reflects this similarity. Because the photochemistry of alkenes has received greater attention and has already been described in systematic form - it is not unexpected that the present account should point out the ways in which alkyne photochemistry parallels, or is markedly different from, that of alkenes. There is a considerable difference, however, in the range of compounds which has been studied in each class. Reports of photochemical reactions of alkynes very often refer to mono- or disubstituted acetylenes in which the substituents are alkyl, aryl or alkoxycarbonyl. There have been studies on diyne and enyne systems, but as yet there has emerged nothing in alkyne chemistry to match the wealth of photochemistry reported for dienes and polyenes. This reflects in part the greater tendency of the compounds containing the C=C bond to undergo photopolymerization rather than any other reaction on irradiation. Within this limitation there is a wide variety of reactions open to the excited states of alkynes, and quite a number of the processes have synthetic application or potential. 

Figure 12.3 X-ray crystal structure of alkynes derived from cinchona alkaloids. Torsion angles defining twist sense, cf. Scheme 12.19.

Going from a phosphaalkene to a phosphaalkyne, we increase the. r-contribution in the carbon phosphorus multiple bond, and would therefore expect a further down-field shift of the phosphorus resonance. However, a glance at the situation in carbon carbon multiple bond systems in particular, alkenes and alkynes, tells us that C-NMR spectra of these molecules show the carbon resonance of alkynes upheld from that of alkenes. This is usually explained by anisotropic effects associated with the linear rod-shaped structure of alkynes versus the bend structure of alkenes. As the geometries of phosphaalkenes andphosphaalkynes are analogous to alkenes and alkynes, respectively, we can assume that the explanation given for the appearance of the carbon resonance in alkynes upheld from that for alkenes in C-NMR spectra is also applicable for the respective unsaturated phosphoms compounds. 

Abstract Bimetallic catalysts are capable of activating alkynes to undergo a diverse array of reactions. The unique electronic structure of alkynes enables them to coordinate to two metals in a variety of different arrangements. A number of well-characterised bimetallic complexes have been discovered that utilise the versatile coordination modes of alkynes to enhance the rate of a bimetallic catalysed process. Yet, for many other bimetallic catalyst systems, which have achieved incredible improvements to a reactions rate and selectivity, the mechanism of alkyne activation remains unknown. This chapter summarises the many different approaches that bimetallic catalysts may be utilised to achieve cooperative activation of the alkyne triple bond. 

Figure 6 Optimized structures of alkyne and vinylidene complexes. The geometries are given in Table 17.

The crystal structures of alkyne insertion products into Pd-G bonds of planar chiral cyclopalladated Schiff base complexes were published in 1998 by Mak etal A similar study including electrochemical studies was published in the same year. ... 

Carbon-13 NMR spectroscopy also is useful in deducing the structure of alkynes. For example, the triple-bonded carbons in alkyl-substituted alkynes resonate in the range of 6 = 65-95 ppm, quite separate from the chemical shifts of analogous alkane (5 = 5 5 ppm) and alkene (S = 100-150 ppm) carbon atoms (Table 10-6). 

Detailed study of the outcome of alkynes hydrophosphorylation reaction have revealed rather complicated picture with several by-products depending on the structure of alkyne, H-phosphonate, catalytic system and reaction conditions (Scheme 8.13) [76]. 

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