Nonconjugated dienes, formation

Intramolecular [2 + 2] photocycloadditions of alkenes is an important method of formation of compounds containing four-membered rings.184 Direct irradiation of simple nonconjugated dienes leads to cyclobutanes.185 Strain makes the reaction unfavorable for 1,4-dienes but when the alkene units are separated by at least two carbon atoms cycloaddition becomes possible. 

Cyclopolymerization of Nonconjugated Dienes. Cyclopolymerization is an addition polymerization that leads to introduction of cyclic structures into the main chain of the polymer. Nonconjugated dienes are the most deeply studied monomers for cyclopolymerization and for cyclocopolymerizations with alkene monomers 66 In general, (substituted and unsubstituted) dienes with double bonds that are linked by less than two or more than four atoms cannot undergo efficient cyclization and result in crosslinked materials.12 In fact, efficient cyclopolymerization processes have been described, for instance, for a,oo-dienes like 1,5-hexadiene, 2-methyl-l,5-hexadiene, 1,6-heptadiene, and 1,7-octadiene,67 73 which lead to formation of homopolymers and copolymers containing methylene-1,3-cycloalkane units. 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

Dienes and polyenes undergo acyloxymercuration. Nonconjugated dienes afford either mono- or di-mercurated species, depending on the stoichiometry of the reaction (equation 288).511-513 Products involving carbon-carbon bond formation are occasionally observed during this process.512 Allenes afford vinylmercurials (equation 289).514,515... 

When nonconjugated dienes react with carbon monoxide and water in the presence of dicobalt octacarbonyl, saturated and unsaturated cyclic ketones are produced (55, 77). This appears to be due to the formation of unsaturated acylcobalt carbonyls followed by cyclization, as discussed in Section II, B,3. 

Abstraction of chloride from a vinylpalladium complex by silver acetate has been reported,45 as well as halide abstraction by silver triflate from arylpalladium halides.46 More interestingly in the context of creating C-C bonds, silver perchlorate was able to promote the reaction of (r -aryl)palladium bromide with dienes. Silver-promoted bromide abstraction led to the formation of (r 1-r 2-enyl)palladium complexes, which evolved further through the regioselective formation of a C-C bond between the aryl group and the former diene. Reactions with nonconjugated dienes suggested that the reaction proceeds via carbometallation of the less crowded double bond. Isomerization and (3 elimination led to a (ri3-allyl)palladium complex (Scheme 10.26).47... 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

In such olefin formation reactions, an electrodecarboxylation-deprotonation sequence is also involved. Electrochemical aromatization of the nonconjugated dienes (LXIV and LXVI) can be successfully performed by this method as in Eqs. (34) and (35) [154]. Decarboxylation of the y-lactone a- and )6-carboxylic acids (LXVIII and LXX) in a Py-H20-Et3N-(Pt or C) proceeds smoothly, yielding the corresponding bute-... 

If monoolefin-Fe(CO)4 complexes are involved in oleate hydrogenation, similar complexes may be invoked as intermediates in the formation of diene-Fe(CO).3 complexes from linoleate (Scheme III). Iron tetracarbonyl complexes were also suggested by Pettit and Emerson (19) in the formation of conjugated diene-Fe(CO)a from nonconjugated dienes. 

Aryl halides, nonconjugated dienes, and stabilized enolates can be coupled in high yields. Apparently, this process consists of an addition of an arylpalladium halide intermediate to the less hindered C,C double bond, palladium migration by a sequence of /3-hydride elimination, and readdition steps with eventual formation of a Tr-allylpalladium complex and its coupling with a nucleophile (Scheme... 

Cycloaddition of carbon-carbon double bonds can also occur intramolecularly. Direct irradiation of simple dienes leads to cyclobutanes. This is a singlet-state process and is concerted. The stereochemistry of the cyclobutane can be predicted on the basis of orbital-symmetry rules (Part A, Section 10.1). Nonconjugated dienes can also undergo photochemical cyclization employing mercury or carbonyl compounds as sensitizers. Cyclobutane formation is usually unfavorable with 1,4-dienes because it would result in a very strained ring system. When the alkene units are separated by at least two carbon atoms, cyclization becomes more favorable sterically ... 

The initial stage involves the formation of a highly reactive anion Fe(CO)4 , which interacts with isolated double bonds, followed by double bond isomerization and the formation of n-complexes with the iron tricarbonyl residues. The final product is composed of the ri" -(butadienyl)irontricarbonyl units with both trans-trans- and cis-trans-tetramethylene groups. Since the iron tricarbonyl complexes with two noncon-jugated double bonds are unstable, no complexes between two polymer chains are formed. The reaction of iron carbonyls with low-molecular-weight nonconjugated dienes is accompanied by the double bond migration. 

As a rule, MCMs based on polyvalent metals are bifimctional or trifimctional monomers with noncqjugated double bonds. The mechanism of formation of cychc units is close to that of the polymerization of nonconjugated diene monomers (e.g., 1,5- or 1,6-dienes) and acryhc or methacrylic anhydride. The cychzation is based on the alternation of intermolecular and intramolecular chain growth events (scheme 36). 

Analogous intermediates could possibly be involved in the formation of conjugated diene complexes from nonconjugated dienes. Thus the formation of fra r-l,3-pentadiene-iron tricarbonyl from 1,4-pentadiene could proceed by the route shown in Eq. (3). 

Heck reactions on nonconjugated dienes create an electrophilic Jt-aUyl complex that is susceptible to nucleophilic attack. When the compound contains a conjugated diene, the regioselectivity becomes a very important issue, which must be considered. Cascade sequence involving a Heck reaction and a Jt-allyl reaction could be found in the formal total synthesis of Morphine by Overman [15]. Treatment of dienes with PdCOCOCFjj CPPhj) and NEtj in toluene led to formation of the backbone of Morphine in 56% yield (Scheme 7.6). 

The double bonds in a conjugated diene are hydroborated separately, that is, there is no 1,4 addition. However, it is not easy to hydroborate just one of a conjugated system, since conjugated double bonds are less reactive than isolated ones. Thexylborane °(48) is particularly useful for achieving the cyclic hydroboration of dienes, conjugated or nonconjugated, as in the formation of 53." ... 

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