Dienes linear dimerization

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. 

The linear dimerization of substituted conjugated dienes is difficult, but the Pd-catalyzed intramolecular dimerization reaction of the 1,3,9,11-tetraene 13 gives the 3-propenylidene-4-allylpiperidine derivative 14, which has the 1,3,7-octatriene system. The corresponding 1,3,8,10-tetraene also affords the 3-pro-penylindene-4-allylcyclopentane derivative[18]. 

There are two main types of reactions of conjugated dienes catalyzed by palladium complexes. The first type is the linear dimerization to form 1,3,7-octatriene (16) in the absence of a nucleophile ... 

Dimerization of conjugated dienes and trienes is generally accomplished at elevated temperatures or in the presence of metal catalysts. Linear dimerization of butadiene occurs readily at room temperature on nickel catalysts bearing aminophosphinite (AMP) ligands, and the reaction rate is reportedly twice that observed in other nickel systems employing either morpholine, ethanol or P-methyloxaphospholidines as modifiers62. 1,3-Pentadiene dimerizes in the presence of 1 mol% nickel catalyst to give a diastereomeric mixture of 4,5-dimethyl-l,3,6-octatriene as shown in equation 42. 

While linear dimerization of dienoic esters can also be accomplished with nickel-AMP systems, other functionalized dienes undergo little or no conversion. The reaction of methyl hexa-2,4-dienoate, 81, furnishes diastereomeric trienoic diesters (82) in high yields (equation 43). 

Under optimum reaction conditions (See Table IV.), selectivity to linear dimer is controlled by the choice of temperature, solvent and tertiary phosphine. Toluene and tetrahydrofuran are the best solvents. Temperatures between 25 to 60 C with a triphenyl or tributylphosphine/palladium acetate catalyst give linear dimer selectivities in the 80 s. At 25 C in toluene, a palladium acetate/tributylphosphine catalyst gave 98.7% conversion and 89.6% linear, 4.7% branched, 1.9% cyclic, and 3.8% heavies selectivity. The linear dimerization reaction was second order in diene with a 3.6 Kcal/mole activation energy. 

A related asymmetric dimerization is seen in the aminophosphinite-Ni(0) catalyzed dimerization of conjugated dienes (Scheme 61) 133). 1,3-Pentadiene forms head-to-head linked optically active 1,3,6-trienes that subsequently are isomerized to achiral 2,4,6-trienes. The linear dimerization is considered to proceed via a bis-ir-allylnickel intermediate, where the NH group in the ligand mediates proton transfer in the reaction. The reaction rate is one to two orders of magnitude higher than the reaction using morpholine, ethanol, or P-methyloxaphospholidines as modifiers. 

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. " ... 

Cyclo- and linear-dimerizations of 1,3-dienes were accomplished by means of a cationic ruthenium catalyst derived from [Cp RuCl(l,3-diene)] and AgOTf (Scheme 4.48) [96]. In THF, 1,3-butadiene was treated with the cationic ruthenium catalyst at 70 °C for 10 h to afford 1,5-cyclooctadiene in 89% yields. Similarly, isoprene underwent [4 -I- 4] cycloaddition in a head-to-tail fashion to yield quantitatively 2,6-dimeth-yl-l,3-cyclooctadiene and 3,7-dimethyl-l,5-cydooctadiene in a ratio of 21 79. On the O ther hand, a head-to-tail linear dimer was obtained in 95% yield from 1,3-pentadiene. 

Unlike nickel catalysts which form cyclic dimers and trimers (1,5-cyclooctadiene and 1,5,9-cyclododecatriene), palladium compounds catalyze linear dimerization of conjugated dienes. 1,3-Butadiene itself is converted to 1,3,7-octatriene. The reaction most characteristic of palladium is the formation of various telomers. 1,3-Buta-diene dimerizes with incorporation of various nucleophiles to form telomers of the following type ... 

Simple 1,3-dienes such as 1,3-butadiene, isoprene, and related compounds undergo efficient metal-catalyzed oligomerization. Under palladium catalysis, diene dimerization is the most common oligomerization reaction observed. Four modes of dimerization have been reported (Scheme 1) (i) [2 + 2] cycloaddition to afford 1,2-divinylcyclobutane (1) (ii) [4 -I- 2] cycloaddition to afford 4-vinylcyclohexene (2) (iii) [4 + 4] cycloaddition to afford 1,4-cyclooctadiene (3) and (iv) linear dimerization to afford 1,3,7-octatriene (4). 

Four related Pd-catalyzed linear dimerization modes have been observed for the intermole-cular reaction of 1,3-dienes (Scheme 2). While potentially of industrial importance for bulk... 

At first glance bisdiene substrates in which the two 1,3-diene subunits are substituted differently (e.g.. Scheme 6,17) appear to be improper candidates for Pd-catalyzed cycloisomerization, as they would probably lead to a mixture of isomers (e.g., 18). This is known for the linear dimerization of simple substituted 1,3-dienes (e.g., isoprene or piperylene). The attempted selective cross-coupling of different 1,3-dienes usually affords a complex mixture of isomeric products (vide infra). Nonetheless, the Pd-catalyzed cy-clization of bisdiene 17 does not form any of the isomeric 18 structures, but instead affords the single enediene 19 in near quantitative yield (95%). 

While the intermolecular reactions of butadiene and related methyl or simple alkyl-substituted dienes have been investigated extensively, relatively few examples of the Pd-catalyzed linear dimerization of higher dienes have been reported. Brun and co-work-ers in an isolated paper reported that, under palladium catalysis, reaction of methyl 2,4-pentadienoate (Scheme 9, 28) affords the linear dimer 29 in high yield (95%). Two aspects of this reaction are of particular interest, (i) The dimerization yields essentially only the tail-to-tail dimer 29, not the head-to-tail or head-to-head isomers (30 or 31, respectively). This is in contrast to the behavior of alkyl-substituted dienes under similar conditions vide infra), (ii) Although no details are given, the authors imply that 29 is... 

Metal-mediated cyclizations that rely on the initial complexation of an alkene or alkyne around a low oxidation state metal center are often sensitive to the presence of additional substituents (particularly electron-donating substituents), and relatively more stringent reaction conditions are often required for successful cychzation. This effect was noted in the Ni-catalyzed formal [4 -I- 4] cycloaddition reactions developed by Wender and Tebbe and is apparent when one compares the reported facility of Pd-catalyzed linear dimerization of 1,3-butadiene versus that of substituted 1,3-dienes. Similarly, the initial attempts at Pd-catalyzed cyclization of bisdiene 70a (Scheme 22) were rather disappointing. Using 0.05 equiv of [Pd(OAc)2/3 PhjP] (THF, 65 °C, 12 h), only a small... 

The linear dimerization of isoprene often affords mixtures of head/tail dimers. For example, [Pd(OAc)2, P(CH2CH2CN)3l catalyzes dimerization of isoprene with triethylammo-nium formate in dimethylacetamide (55 °C, 3 h), affording dimers in 91% yield as a 10 49 37 98/99/100 mixture of head/tail coupling regioisomers (Scheme 30). It is interesting to note that, as in the case of butadiene and formic acid in the presence of phosphine-modified Pd(OAc)2, only 1,7-dienes are formed in this reaction of isoprene with formic add. 

Disilane trapping reagents behave in a conceptually similar fashion. Tsuji and coworkers found that a variety of simple dienes (143, e.g., butadiene, isoprene, 2-phenyl-l,3-butadiene, 2-trimethylsilyloxy-l,3-butadiene) undergo efficient Pd(dba)2-catalyzed linear dimerization disilane trapping with a variety of simple disilanes 144 to afford bis(allylsi-lane) products 145 (41-92% yield) (Scheme 47). Cyclic disilanes afford macrocyclic bis(allylsilane) products. ... 

There are a number of remarkable features in this very facile linear dimerization reaction with trapping by disilanes, (i) In contrast to silanes with which substituted 1,3-dienes (e.g., isoprene, piperylene) give predominantly to exclusively simple hydrosilylation prodncts rather than linear dimerization products, disilanes afford only disilylated linear dimers—no disilylated monomer, (ii) Only the head-to-head dimerization products... 

The telomerization of dienes with alcohols as the nucleophile has now been conducted in a practical fashion. A palladium catalyst containing an N-heterocyclic carbene ligand has been shown to form linear dimeric ethers selectively from alcohols and butadiene with remarkably high turnover numbers (Equation 22.40). The activity of this catalyst is significantly higher than that of the more classical catalysts generated from Pd(OAc)j and PPhj. Under optimized conditions involving some added carbene precursor, presumably to... 

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