1.3- Dienes 2 molecules

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Figure 17.2 Maleimide groups provide good dienophiles for a Diels-Alder reaction. Biotin-PEG2-maleimide can react with an oligo-diene molecule to form a covalent cycloaddition product, which adds the biotin tag to the oligo.

In a molecule of this type, trans conformational preference is not only due to effects inherent in any diene molecule but also to an attractive sigma nonbonded interaction between the Xpx lone pair and the Is AO of the hydrogen attached on C3. Consequent-... 

These results have been interpreted in terms of trans addition of mercuric ion and nucleophile where the attack of the mercuric ion takes place from the more hindered side of the diene molecule. A transition state 197, involving an endo attack of mercuric ion with some stabilization by coordination to the 8,9-ethylenic bond to the mercury atom, has been proposed to support the suggested mechanism. Analogously, and in sharp contrast to the results obtained167 in the mercuration of norbomadiene which reacts with mercury salts via the usual scheme of exo-syn addition, the principal pathway in the mercuration of bicyclo[2.2.2]octa-2,5-diene is the formation of endo-syn products (equation 165). 

Early work on the mechanisms of alkene cleavage by RuO has been briefly reviewed [50]. In the oxidation of 1,5-dienes to cA-tetrahydrofurandiols by RuO / aq. Na(10 )/EtOAc-acetone it is likely that there is cyclo-addition of RuO to one double bond of two 1,5-diene molecules to give a Ru(lV) diester this is oxidised by Na(lO ) to a Ru(Vl) diester, which is then hydrolysed to the organic product (Fig. 3.12) [345], and indeed Ru(Vl) diesters RuOlO R) have been isolated (Fig. 1.31) [323, 346]. ... 

A related phenomenon has been observed in the benzophenone sensitized isomerization of c/y-piperylene.150 The measured quantum yield of cis to trans isomerization increased from 0.55 to 0.90 as the concentration of piperylene increased from 0.08 to lOAf. This observation can be rationalized as arising from addition of the piperylene triplet to a ground state diene molecule to give a biradical intermediate which can either cyclize to the dimer151 or dissociate to give two molecules of the more thermodynamically stable trans-isomer. This mechanism predicts that the quantum yield for the isomerization of /runs-piperylene to cw-piperylene should decrease with increasing diene concentration, an experiment that has not yet been reported. 

Figure 8.14 Packing of unsymmetrically substituted diene molecules in a crystal, favouring photopolymerization leading to asymmetric synthesis. (After Addadi et al, 1980.)...

The mechanism for the polymerization of dienes to the ds 1,4 structure is also parallel. Sterns and Foreman (112) presented a cyclic 6-membered transition state to produce the cis structure. Orr (113) concluded that the configuration of the monomer itself has no role to play in determining the amount of cis or trans isomer produced from isoprene and butadiene. There is no requirement for the prealignment of the diene monomer by coordination between a diene molecule and the gegen ion. The diene must only assume the ds transition state during reaction. 

The titanium trichloride-diethylaluminum chloride catalyst converted butadiene to the cis-, trans,-trans-cyclododecatriene. Professor Wilke and co-workers found that the particular structure is influenced by coordination during cyclization between the transition metal and the growing diene molecules. Analysis of the influence of the ionicity of the catalyst shows effects on the oxidation and reduction of the alkyls and on the steric control in the polymerization. The lower valence of titanium is oxidized by one butadiene molecule to produce only a cis-butadienyl-titanium. Then the cationic chain propagation adds two trans-butadienyl units until the stereochemistry of the cis, trans, trans structure facilitates coupling on the dialkyl of the titanium and regeneration of the reduced state of titanium (Equation 14). 

This reaction elucidates the mechanism of the photoreaction of 73 with dienes. In the first step, 73 loses one carbonyl ligand with formation of the reactive 16-electron species [0/5-C5Hj)Mo(CO)2CH3] (87) (109-113), which adds a diene molecule. jj2-Diene complexes [( 5-C5H5)Mo(CO)2CH3-( 72-diene)] (88) are quite likely as intermediates. Coordination of the free C=C double bond of the r 2-diene ligand causes insertion of CO into the Mo—C [Pg.338]

The insertion of the coordinating diene monomer proceeds via an t]3-allylic species. The metal atom with the attached copolymer chain terminated by such a species gives its two coordination sites to be occupied by the -allylic group. This allows the next coordination of only one additional a-olefm. After insertion of the coordinating 7-olefin, two coordination sites at the metal atom are again available, and they are preferably complexed by another conjugated diene molecule. 

It should be noted that when an R group, e.g. a methyl group, is present at the internal unsaturated carbon atom in a terminal diene molecule, as in the CH2=C(Me)—(CH2)x-C(Me)=CH2 monomer, the cyclic diene metathesis ceases [7,8]. The steric effect exerted by the R substituent can even be important at the a position to the double bond in the monomer. Sterically encumbering this position hinders polymer formation [9]. 

Secondary unimolecular reactions in these systems usually result either from production of hot energized species by chemical reaction or from conventional thermal activation. In a few systems, residual excitation from the original photochemical process may be of importance. An interesting and potentially valuable example, due to Srinivasan, is the production of highly vibrationally excited, ground electronic state diene molecules by internal conversion which follows photoexcitation. 

In addition to the complexes listed in Table VII, we have used 13C NMR to determine the structure of the product of the reaction of (CDT)Ni with 2,3-dimethylbutadiene (65). The spectrum (-40°C) consists of six signals—two unprotonated carbon atoms at 102.3 and 101.7 ppm, two methylene carbon atoms at 61.1 and 48.4 ppm, and two methyl carbon atoms at 21.8 and 19.4 ppm. A determination of J(CH) for the methylene C atoms gave values of 156 and 157 Hz (both 3 Hz), clearly indicating that they are both sp2-hybridized and thereby eliminating structures such as 20b. The spectrum is consistent with a pseudotetrahedral arrangement of the two diene molecules about the nickel atom (20a). 

Three-component additions, comprising two conjugated diene molecules and a nucleophile, can be catalyzed by palladium salts, such as palladium acetate. The major products are 1-substituted 2,7-octadienes, along with minor amounts of 3-substituted 1,7-octadienes (equation 65). Water, alcohols, phenols, carboxylic acids, and amines are some of the nucleophiles that have been used in this reaction. 

At a temperature below —2()°C, a butadiene complex [(C4Ha)PdCl2]2, in which only one of the double bonds of each diene molecule is complexed can be obtained by ligand exchange with the 1-pentene complex 180). 

An X-ray analysis (26,32) of the yellow, monomeric (norbornadiene)-dichloropalladium(II) (1, 6) has shown the diene molecule chelated to palladium with both double bonds perpendicular to the coordination plane. The distance from the Pd atom to the trigonal carbons is about 2.16 A and the angle subtended at Pd(II) by the coordinate bonds is 71.8°, The bond lengths within the norbornadiene molecule appear to be changed very little by coordination. 

With 1,5-hexadiene, the compound K2[(CaHio)Pt2Cle] with a bridging diene molecule can be obtained (283) this diene will form the complex (CeHio)PtCl2 as well in which it is chelated, probably with a gauche configuration (283) and which is less stable than the analogous complex with chelated 1,5-cyclooctadiene (284). 

With AgC104, 1,5-hexadiene, 1,7-octadiene, and 1,9-decadiene form 3 2, 1 1, and 1 1 complexes, respectively (85a). The 1 1 complexes are considered (85a) to be coordination polymers while the 3 2 complex of 1,5-hexadiene has been shown (45a) to have chelated diene-silver ion entities connected through a bridging diene molecule to produce a distorted trigonal coordination about the metal. 

It is also possible that the presence of an active catalytic (de)hydrogenation function results in a higher concentration of diene molecules, which are potential coke precursors when interacting with acid sites. 

The efficiency in the transfer of energy from the triplet states of various compounds to ground singlet states of isoprene, 1,3-pentadiene, and 2,3-dimethylbutadiene have been studied. The diene molecules become excited to their lowest triplet states and undergo chemical reactions which may be dimerization or stereoisomerization. As in the case of butadiene, the energy level of the sensitizer in its lowest triplet state determines the magnitude of the energy that is transferred to the diene and the course of its subsequent reactions. Typically, in the case of... 

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