Isoprene, cyclodimerizations

Coupling of Cl and Cg in 61 provides a likely path to VCH, the major Cg product with this ligand. Structure 62, the analog of 61 produced by isoprene cyclodimerization, has been established by X-ray diffraction. 

The cyclodimerization of 1,3-butadiene and isoprene by zinc reduction of an iron nitrosyl complex dispersed in [BMIM]BF4 (and alternatively [BMIMJPFg) showed... 

Nickel(0)-catalyzed dimerization of isoprene (3 2-methylbuta-l.3-diene) has been used as the key step in the synthesis of (+)-grandisol (8), a male boll weevil pheromone. It is interesting to note that the cyclobutane 4 was formed by unsymmetrical and head-to-head cyclodimerization and that cycloadduets 5-7 were also formed in this transformation.9... 

Cyclodimerization of isoprene to 1,5-dimethylcycloocta-l,5-diene and disproportion with a rhenium oxide catalyst and isobutene produce 2,6-dimethylhepta-l,5-diene. The diene is hydroformylated to citronellal, which after hydrogenation produces citronellol (137). 

Catalytic cyclodimerization of dienes can also be performed selectively. 1,5-Cyclooctadiene, dimethylcydooctadienes, and 6-methyl-2,4,7-nonatriene can be obtained from butadiene, isoprene, and 1,3-pentadiene, respectively, upon treatment with a catalytic amount of (C5Me5)RuCl(diene) and AgOTf... 

The cyclooligomerization reaction is not confined to BD as the monomer. Activated or monosubstituted 1,3-dienes also react, but reaction rates are usually slow, and selectivity and turnover numbers (TONs) are low. Cyclotrimerization and cyclodimerization of substituted 1,3-dienes - either alone or in admixture with BD - give numerous isomers of substituted CDT, COD, VCH and divinyl-cyclobutane (DVCB). For example, isoprene [34], 1,3-pentadiene [35], 2,3-dimethylbutadiene [36], 1,3-hexadiene [37], and even 1-vinyl-1-cyclopentene [38] do react (eqs. (2)-(6)). 2,4-Hexadiene is inert. 

Fe compounds have received much less significant attention than Ni or Co compounds as the diene polymerization catalyst. FeEt2(bpy)2 catalyzes cyclodimerization of 1,3-butadiene [79] and polymerization of vinyl monomers such as acyclic ester [80]. Recently, FeEt2(bpy)2/MAO was found to show high catalytic activity toward 1,2-polymerization of 1,3-butadiene and 3,4-polymerization of isoprene at -40 to +25 °C (Eq. 14) [81]. The crystalline polybutadiene prepared below 0 °C is composed of... 

Isoprene is cyclodimerized by Ni(0)-L catalysts to a mixture of methylsubstituted cod, vch, and dvcb derivatives, the distribution of which depends on the ligand used and... 

Tab. 10.3. Cyclodimerization of isoprene. Comparison between external and internal pressure.

During a joint venture with Naarden International, Shell International developed a new route to citronellol, but this was never commercialized. The route starts with cyclodimerization of isoprene (1) to give 1,5-dimethylcycloocta-1,5-diene (137). Olefin metathesis with isobutene in the presence of a rhenium oxide catalyst produces 2,6-dimethylhepta-1,5-diene (138). The diene is hydroformylated to citronel-lal (139), which after hydrogenation produces citronellol (85) (169). Alternatively, the 1,5-dimethylcycloocta-1,5-diene can be partially hydrogenated and the monoene (140) then pyrolyzed to 2,6-dimethylocta-l,7-diene (141), which can be converted subsequently to citronellol in several steps (170). These processes are shown in Fig. 8.28. 

A catalyst for the selective cyclodimerization of butadiene to 4-vinylcyclo-hexene can be generated by the reduction of [Fe(NO)2Cl], with metal carbonyls or electrochemically. Addition of cyclic amines to the isoprene dimerization catalyst [NiCl2(PPhs)2]-NaBH4-H20 enhances the formation of cyclic dimers the product distribution has been correlated with an electronic and structural properties of the amine. [Ni P(n-OC6H4R)3 J (R = Ph or SiMes) catalyses the selective (95 %) dimerization of butadirae to cyclo-octa-1,5-diene the catalysts appear to operate via two active centres, probably the Ni and P atoms. When solutions of (44) are allowed to stand and then treated with HCl or Brj signficant amounts of [1,1,3,3- H4]- and [2,2,3,3- H4]-butane are produced in addition to the anticipated [1,1,4,4- H4]-isomer. This would seem to indicate that there is an equilibrium (14) and in keeping with this (44)... 

Cyclodimerization and Cycloaddition. Tris(l,l,l,3,3,3-hexafluoro-2-propyl)phosphite serves as a ligand in Ni-catalyzed cyclodimerization reactions as well as Rh- or Ni-catalyzed [4-f2] cycloaddition processes. The first example is Ni-catalyzed dimerization of isoprene with tris(l,l,l,3,3,3-hexafluoro-2-propyl) phosphite ligand. The main product is l,4-dimethyl-4-vinyl-cyclohexane (eq 4). 

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