Polyenes addition reactions

These remarks represent only the barest outline of at least two aspects of PVC degradation which have been the focus of attention for several years and remain incompletely understood namely the mechanism involved and the related problem of the involvement of HC1. Several excellent reviews give more comprehensive summaries of the earlier work (10, 11, 12). More recent work has made it clear that under appropriate conditions the presence of HC1 can affect the initiation, propagation and termination steps as well as influencing the distribution of polyene sequence lengths. In addition it can undergo photochemical addition reactions with the polyenes, i.e. the reverse of the dehydrochlorination process, as well as forming colored polyene/HCl complexes. These various possibilities will be considered in turn. 

Despite the enormous importance of dienes as monomers in the polymer field, the use of radical addition reactions to dienes for synthetic purposes has been rather limited. This is in contrast to the significant advances radical based synthetic methodology has witnessed in recent years. The major problems with the synthetic use of radical addition reactions to polyenes are a consequence of the nature of radical processes in general. Most synthetically useful radical reactions are chain reactions. In its most simple form, the radical chain consists of only two chain-carrying steps as shown in Scheme 1 for the addition of reagent R—X to a substituted polyene. In the first of these steps, addition of radical R. (1) to the polyene results in the formation of adduct polyenyl radical 2, in which the unpaired spin density is delocalized over several centers. In the second step, reaction of 2 with reagent R—X leads to the regeneration of radical 1 and the formation of addition products 3a and 3b. Radical 2 can also react with a second molecule of diene which leads to the formation of polyene telomers. 

Based on the data collected in this section, one must conclude that the addition of radicals to dienes is certainly rapid enough to compete against the typical chain-breaking processes and that especially the addition of electrophilic radicals to polyenes appears to bear significant potential. Terminally substituted polyenes are likely to be unsuitable for radical addition reactions due to their lower addition rates and to undesirable side reactions. 

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

In this chapter, nucleophilic l,n-additions (n = 4, 6, 8,. ..) to acceptor-substituted dienes, enynes and polyenes are presented2. Addition reactions which obviously proceed via non-nucleophilic pathways (e.g. catalytic reductions, electrophilic or radical additions3), as well as 1,2-additions to the acceptor group, are not covered. 

The reactions of dienes and other polyenes can be broadly classified as either addition reactions, coupling (or substitution reactions) or rearrangements (including metathesis reactions). This chapter will present recent examples from the literature of synthetic transformations involving polyenes. Cycloaddition and ring closing metathesis reactions appeared in volume one of this series and therefore will not be covered in this chapter. Citations for more detailed descriptions of the individual reactions discussed in this chapter and for more comprehensive reviews appear in the text. 

Like CgQ most of the characterized higher fullerenes can be considered as electron-deficient polyenes. They exhibit similarities in their chemical behavior [1-3]. Preferred primary addition reactions are, for example, addition of nucleophiles, as well as cycloadditions at the bonds adjacent to two six-membered rings ([6,6]-bonds). The most important difference from the chemistry of Cgg is due to the less symmetrical cages of the higher fullerenes, which exhibit a broad variety of bond environments with unequal reactivity towards addition reactions. Consequently... 

Behavior of Carbethoxynitrene toward Polyisoprene Structures. After studying the reaction of a number of divalent carbon derivatives with polyenes (17, 18), attention was directed to monovalent nitrogen derivatives. Of these, carbethoxynitrene, N—C02Et, gives addition reactions with olefins (12, 13). It can be produced either by photolysis of ethyl azidoformate (12), or by a-elimination (13) (Figure 5). 

Because they are not as strongly stabilized as benzene, anthracene and phenanthrene can undergo addition reactions that are more characteristic of their nonaromatic polyene relatives. Anthracene undergoes 1,4-addition at the 9- and 10-positions to give... 

This is cyclooctatetraene. However, Willstatter was distrusted because it was considered, in accordance with the conceptions of his time, that a cyclic polyene must exhibit the benzene-like properties, that is, readily enter into substitution reactions and very reluctantly into addition reactions. As to the compound obtained, it behaved quite differently, not entering into substitution reactions and easily adding bromine. 

The pattern of addition in polyenes in which the double bonds are neither contiguous nor conjugated does not require special comment. Compounds with conjugated double bonds are somewhat more susceptible to electrophilic addition than simple alkenes but nevertheless there appear to be no literature reports of such compounds undergoing addition reactions with hydrazoic acid in the absence of a catalyst. 

In one of the earliest investigations of regioselectivity in radical addition reactions to polyenes, the addition of hydrogen bromide to 1,3-butadiene was observed to yield mainly the 1,4-addition product in the presence of peroxides. The preference for attack at the Cl position of 1,3-butadiene has subsequently been observed for a large number of radicalsOnly for the addition of the methyl radical has the ratio of addition to the Cl vs C2 actually been measured. A value of Cl C2 = 1.0 0.01 has been found . For all other cases, products arising from attack at C2 have not been reported. This is also true for radical addition to 2,3-dimethyl-l,3-butadiene . Additions to 1,3-pentadiene occur predominantly at the Cl position due to the steric effect exerted by the terminal methyl This is a reflection of the reduced... 

This base is generally superior to pyridine for dehydrohalogenation. Addition of A% of 3,5-lutidine is advantageous (synthesis of carotinoid polyenes by reaction at 140 under nitrogen). ... 

Electrophilic additions to dienes and polyenes latter reaction (equation 104). 

Only few examples have been reported so far on nucleophilic addition reactions to acceptor-substituted polyenes 23,i24,i86-i88 1933, Farmer and Martin examined... 

Only few examples have been reported so far on nucleophilic addition reactions to acceptor-substituted polyenes " . In 1933, Farmer and Martin examined the reaction of methyl 2,4,6-octatrienoate with sodium dimethyl malonate and isolated the 1,4-adduct as major product (equation 81). In contrast to this, 3,5,7-nonatrien-2-one and ethyl 2,4,6-octatrienoate react with organocuprates under 1,8-addition to provide the 4,6-dien-2-ones and 3,5-dienoates, respectively (equation 82). ... 

Recent additions to the family of alkene complexes are fullerene derivatives such as Rh(CO)(q -Cgo)(H)(PPh3)2 Pd(q -Cgo)(PPh3)2 (Figure 23.17b) and (q -Cp)2Ti(q2-Cgo). The Cgo cage (see Section 13.4) functions as a polyene with localized C=C bonds, and in Cgo Pt(PEt3)2 6, six C=C bonds (remote from one another) in the Cgo cage have undergone addition. Reaction 23.73 illustrates CgQ-for-ethene substitution (the 16-electron centre is retained), and reaction 23.74 shows addition to Vaska s compound (a 16- to 18-electron conversion). Equation 23.75 shows the formation of the first fullerene complex of titanium, by fullerene displacement of a coordinated alkyne. 

Three major reaction sequences or strategies dominate steroid syntheses (1) Diels-Alder reactions, (2) all kinds of aldol and Michael addition reactions, and (3) biomimetic syntheses corresponding to polyene cyclizations. We shall give a few examples of each of these approaches, starting with the polyene cyclizations (Akhrem and Titov, 1970 Anand et al., 1970 Blickenstaff et al., 1974). 

There are many possible schemes for addition reactions of diene monomers from electronical and steric viewpoints. Because the monomer molecules arrange along the direction of the channels, a,co-addition may selectively take place in one-dimensional inclusion polymerization. Therefore, conjugated polyenes, such as dienes and trienes, may selectively polymerize by 1,4- and 1,6-addi-tion, respectively. 1,3-Butadiene polymerized via 1,4-addition exclusively in the chaimels of urea and perhydrotriphenylene. while the same monomer polymerized via both 1,2- and 1,4-additions in the channels of deoxycholic acid and apocholic acid. Moreover, we have to evaluate head-to-tail or head-to-head (tail-to-tail) additions in the case of dissymmetric conjugated diene monomers such as isoprene and 1.3-pentadiene. 

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