Alkenes, Alkynes, and Polyenes

The photolysis of ethylene has been studied at 147, 163.4, and 184.9 nm.8 The relative quantum yields for reactions (7)—(9), expressed as percentages, are as follows  

The reactions of the energized vinyl radical, summarized by reactions (10) and (11), were also studied. Fluorescence from propylene and several other monoolefins has been observed with quantum yields of 10-e.8a 

The vacuum-u.v. photolysis of the difluoroethylenes matrices has been compared with vapour-phase studies.9 

1-ene and the 2,2-dimethylbuta-l,3-diyl biradical arising from the Hg(3Pj) photosensitization of the olefin,12 and the formation of carbene intermediates in the photochemistry of alkenes,13 have been discussed. 

In the vapour-phase photolysis of cyclopentene, photoionization occurs at an excitation wavelength of 123.6 nm with an efficiency of 0.16, and leads to the formation of cyclopentane through ion-molecule reactions.14 At 147 and 

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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. 

Alkene, alkyne, and polyene complexes D. M. P. Mingos, Bonding of Unsaturated Organic Molecules to Transition Metals, In Comprehensive Organometallic Chemistry, G. Wilkinson, F. G. A. Stone, and E. W. Abel, Eds., Pergamon Press Oxford, 1982, Vol. 3, pp. 47-67. 

At room temperature, sulfur dioxide reacts with alkenes, alkynes, and polyenes to generate polymeric materials,... 

Since then, the metathesis reaction has been extended to other types of alkenes, viz. substituted alkenes, dienes and polyenes, and to alkynes. Of special interest is the metathesis of cycloalkenes. This gives rise to a ring enlargement resulting in macrocyclic compounds and eventually poly-... 

Unsaturated organic molecules such as alkenes, alkynes, dienes, polyenes and arenes can also stabilize low oxidation states in metal complexes, being both o donors (filled bonding jt orbitals) and jt acceptors (empty antibonding jt orbitals). In these so-called Jt complexes, only jt orbitals are involved in the metal-to-ligand bonds. This latter type of complex is beyond the scope of this chapter and only a few examples will be given. 

This chapter deals with the photochemistry of alkenes, alkynes, dienes, polyenes, and related compounds through a choice of the literature published during the period January 2010 — December 2011. Furthermore, recently many researchers are developing the photochemistry of these compounds for energy conversion, e.g. through nanotechnology applications, such as molecular devices, chemomechanics, molecular switches, etc. This chapter also covers the nanotechnology aspects that are based upon the utilization of isomerization/electrocyclization/cycloaddition reactions of the title compounds. 

Carbon can also form multiple bonds with other carbon atoms. This results in unsaturated hydrocarbons such as olefins (alkenes), containing a carbon-carbon double bond, or acetylenes (alkynes), containing a carbon-carbon triple bond. Dienes and polyenes contain two or more unsaturated bonds. 

Oxidative coupling of organometallic precursors with alkenes, alkynes, conjugated and nonconjugated polyenes and polyalkynes remains the main method of synthesizing various classes of five-membered rings with other elements. However, some alternative methods do exist. 

TT-Bonded ligands can be closed shell, 2n electron donors (n = 1 — 3) such as alkenes, allenes, alkynes, arenes, and polyenes these have been termed even polyene ligands. They may also be open shell, odd polyene ligands such as allyl, pentadienyl, cyclopropenyl, and cyclopentadienyl, which are formally viewed as anionic hgands, donating 2n electrons (n = 1 - 3). 

Coverage in this chapter is restricted to the use of alkenes or alkynes as enophiles (equation 1 X = Y = C) and to the use of ene components in which a hydrogen is transferred. Coverage in Sections 1.2 and 1.3 is restricted to ene components in which all three heavy atoms are carbon (equation 1 Z = C). Thermal intramolecular ene reactions of enols (equation 1 Z = O) with unactivated alkenes are presented in Section 1.4. Metallo-ene reactions are covered in the following chapter. Use of carbonyl compounds as enophiles, which can be considered as a subset of the Prins reaction, is covered in depth in Volume 2, Chtqiter 2.1. Addition of enophiles to vinylsilanes and allylsilanes is covered in Volume 2, Chapter 2.2, while addition of enophiles to enol ethers is covered in Volume 2, Chapters 2.3-2.S. Addition of imines and iminium compounds to alkenes is presented in Volume 2, Part 4. Use of alkenes, aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.9. Coverage of singlet oxygen, azo, nitroso, S=N, S=0, Se=N or Se=0 enophiles are excluded since these reactions do not result in the formation of a carbon-carbon bond. 

The hydrocarbons we have examined thus far— including the alkanes, alkenes, and alkynes, as well as the conjugated dienes and polyenes of Chapter 16— have been aliphatic hydrocarbons. In Chapter 17, we continue our study of conjugated systems with aromatic hydrocarbons. 

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. 

Alkene metathesis catalysts are also capable of undergoing reaction with aUene and alkyne functionality, which can lead to the synthesis of polyene compounds. For example. Diver has explored the use of alkene/alkyne CM, which leads to 2-substituted 1,3-dienes (Scheme 2.11). Prunet has recently disclosed an elegant metathesis cascade sequence towards the synthesis of Taxol, involving two alkenes and one alkyne functional group (Scheme 2.12) the desired product was accompanied by small quantities of a side product that resulted from metathesis of the two alkenes. 

Polysubstituted alkenes, alkynes, polyenes, and enynes are present in many naturally occurring biologically active compounds such as terpenoids, pheromones, etc. They are also key intermediates in a number of transformations leading to natural products and have remained an active area of research for organic chemists. 

F. Sato developed titanium (Il)-based c/s-reduction of alkynes as shown in Scheme 5 [14], and the method was applied to the synthesis of pheromones by Kitching (Scheme 6) [15]. This titanium (Il)-based reaction gives pure (Z)-alkenes. Kitching summarized the contemporary methods for the synthesis of skipped polyynes and their reduction to skipped polyenes [15]. 

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