Conjugated diene complexes reduction

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Non-conjugated dienes isomerize during complexation to afford tricarbonyliron-coordinated conjugated dienes. This isomerization has been applied to a wide range of substituted cyclohexa-1,4-dienes available by Birch reduction from aromatic... 

Diene iron tricarbonyl complexes are prepared by thermal or photochemical reaction of conjugated dienes with iron pen-tacarbonyl in the presence of TMANO, triiron dodecacarbonyl, ()]" -benzylidenacetone)iron tricarbonyl, diiron nonacarbonyl, or diiron nonacarbonyl absorbed on silica gel in the absence of solvent. The latter method is particnlarly usefiil for the preparation of complexes from polar electron-rich dienes and heterodienes. A reductive complexation of cycloheptatrienes using iron tricarbonyl and sodium borohydride to give cyclo-heptadiene iron tricarbonyl has been developed (Scheme 126). 

Mixed cyclopentadienyl-diene titanium complexes, Cp TiX(diene)(X = Cl, Br, I), have been prepared in 30-60% yield by the stoichiometric reaction of CpTiXs with (2-butene-l,4-diyl)magnesium derivatives or by the reduction of CpTiXs with RMgX (R = i-Pr, f-Bu, Et X = Cl, Br, I) in the presence of conjugated dienes, as shown in Scheme 4. The butadiene, 1,3-pentadiene, and 1,4-diphenylbutadiene complexes of Cp TiX exhibit a unique prone (endo) conformation (13), while the isoprene, 2,3-dimethylbutadiene, and 2,3-diphenylbutadiene complexes prefer the supine (exo) conformation (14). Reduction of Cp TiX(diene) with RMgX or Mg gives a low-valent species, which catalyzes a highly selective (>99%) tail-to-head linear dimerization of isoprene and 2,3-dunethylbutadiene. " ... 

Photochemical generation of the bent metallocene (39) has turned out to be very useful for the preparation of 3. Thus, (s-tra j-i7 -butadiene)ZrCp2 (3a) is obtained in good yield from the photochemically induced reductive elimination of biphenyl from diphenylzirconocene (10) at low temperatures in the presence of 1,3-butadiene. This is probably the most versatile and most widely applicable method to prepare (s-fr nj-i7 -diene)zirconocenes. A wide variety of examples of this class of complex [and of (s-cis-diene)zirconocenes 5, as well] has been prepared by this route using various substituted conjugated dienes (22, 23) (Scheme 2). 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

In the polymerization of ethylene by (Tr-CjHsljTiClj/AlMejCl [111] and of butadiene by Co(acac)3/AlEt2Cl/H2 0 [87] there is evidence for bimolecular termination. The conclusions on ethylene polymerization have been questioned, however, and it has been proposed that intramolecular decomposition of the catalyst complex occurs via ionic intermediates [91], Smith and Zelmer [275] have examined several catalyst systems for ethylene polymerization and with the assumption that the rate at any time is proportional to the active site concentration ([C ]), second order catalyst decay was deduced, since 1 — [Cf] /[Cf] was linear with time. This evidence, of course, does not distinguish between chemical deactivation and physical occlusion of sites. In conjugated diene polymerization by Group VIII metal catalysts -the unsaturated polymer chain stabilizes the active centre and the copolymerization of a monoolefin which converts the growing chain from a tt to a a bonded structure is followed by a catalyst decomposition, with a reduction in rate and polymer molecular weight [88]. 

This complex reached a maximum of about 10% in the early stages of reduction and then decreased. Initially the relative specific radioactivity was 1.93 for methyl linoleate and zero for conjugated diene. Relative specific radioactivity in the products at 50% reduction was about equal in the monoene (1.0), stearate (0.96), and diene-Fe(CO)3 (1.0), but only minor in the free conjugated diene (0.14). Although the diene-Fe(CO)3 complex is a significant intermediate in the reduction of linoleate, the free conjugated diene is not. 

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