Nonconjugated diene complexes

Very few i 4-diene halocarbonyl complexes of molybdenum(II) and tung-sten(ll) had been reported up to mid-1995. This section describes conjugated diene complexes, in Section VI,A, and nonconjugated diene complexes, in Section VI,B. 

As mentioned above nonconjugated dienes give stable complexes where the two double bonds can form a chelate complex. A common pathway in palladium-catalyzed oxidation of nonconjugated dienes is that, after a first nucleophilic addition to one of the double bonds, the second double bond inserts into the palladium-carbon bond. The new (cr-alkyl)palladium complex produced can then undergo a /(-elimination or an oxidative cleavage reaction (Scheme 2). An early example of this type of reaction, although not catalytic, was reported by Tsuji and Takahashi (equation 2)12. 

Nonconjugated dienes are rearranged to iron carbonyl complexes of conjugated dienes when treated with FefCOjs or Fe3(CO)i2. Thus 1,4-pentadiene gives tmw -l,3-pentadieneiron tricarbonyl (34) possibly by the route ... 

NaOH-HiOz) or to carboxylic acids (with w-chloroperbenzoic acid).-190 Double bonds can be hydroborated in the presence of triple bonds if the reagent is 9-BBN.191 On the other hand, dimesitylborane selectively hydroborates triple bonds in the presence of double bonds.192 Furthermore, it is often possible to hydroborate selectively one particular double bond of a nonconjugated diene.191 When the reagent is catecholborane, hydroboration is catalyzed by rhodium complexes, such as Wilkinson s catalyst.194 Enantioselective hydro-boration-oxidation has been achieved by the use of optically active rhodium complexes.195... 

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. 

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... 

B. Nonconjugated Diene Halocarbonyl Complexes of Molybdenum(ll) and Tungsten(ll)... 

In the epoxidation of olefins not bearing a complexing group, there is not a great difference in stereoselectivity in comparison with the peracids. The c/s-oxirane is formed from cis-butene-2 and the trans-oxirane from trans-butene-2. Due to steric hindrance, regioselectivity may be observed for nonconjugated dienes. °° ° In terpenes, the attack takes place from the less hindered side. Double bonds in bridged cyclic systems have an effect on the reaction rate. ... 

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. 

Regio- and stereoselectivity are important considerations in the epoxidation of more complex olefin substrates. In this context it is important to distinguish between two types of effect those which are attributable to electronic and steric factors inherent in the substrate and those which are derived from a particular property of the reagent. An example of the former is the difference in reactivity of double bonds in nonconjugated dienes due to increasing alkyl substitution, as illustrated with limonene (eq. (13)) [31]. 

Copolymerization of alkenes with nonconjugated dienes is conducted by using zirconocene, CGC catalyst, and the catalyst composed of iron di-iminopyridine complexes (Eq. 25). The former two catalysts give the copolymer,... 

Copolymerization of Nonconjugated Dienes with CO Catalyzed by Pd Complexes... 

Coordination polymerization of dienes has progressed significantly within the last decade. Selective polymerization of 1,3-dienes is reinforced by conventional transition metal catalysts and by new organolanthanide catalysts. Nonconjugated dienes also polymerize selectively to produce polymers with cyclic units or vinyl pendant groups. Living polymerization of dienes has become common, which enabled preparation of block copolymers of dienes with alkenes and other monomers. Another new topic in this field is the polymerization of allenes and methylenecycloalkanes catalyzed by late transition metal complexes. These reactive dienes and derivatives provide polymers with novel structure as well as functionalized polymers. The precision polymerization of 1,2-, 1,3-, and l,n-dienes, achieved in recent years, will be developed to construct new polymer materials with olefin functionality. 

Similar stereoselectivities arc observed with nonconjugaled dienes which give bicyclic cyclobu-tanes with trans fusion of the annulated ring. Thus, 1,7-octadiene, treated with the same photochemically activated nickel complexes, gives trans-bicyclo[4.2.0]octane (6) in 21 % yield, after the resulting metallacycle is exposed to air24. 

Nonconjugated dienes bound to electrophilic transition-metal centers are activated toward nucleophilic attack analogous to olefin activation by transition metals discussed in 5.8.2.3.4. Although secondary reactions or rearrangements can occur, the pattern of the nucleophilic addition step (Nuc = nucleophile) leading to a, n complexes is shown... 

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