Isoprene selective reduction

Many of these cobalt complexes will catalyze the reduction of organic compounds by borohydride, hydrazine, thiols, etc. Cobalt cyanide complexes will catalyze the reduction of a,j8-unsaturated acids by borohydride (105) DMG complexes the reduction of butadiene and isoprene by borohydride, but not by H2 (124) Co(II) salen, the reduction of CHCI3 and CH3CCI3 to the dichloro compounds by borohydride (116) and cyanocobalamin, the selective reduction of -CCI2- by borohydride to -CHCl- in compounds such as aldrin, isodrin, dieldrin, and endrin without... 

Heck tried the reductive dimerization of isoprene in formic acid in the presence of triethylamine at room temperature using 1% palladium phosphine catalysts to give dimers in up to 79% yield (95). Better selectivity to the head-to-tail dimer was obtained by using Pd(OAc)2 with 1 1 ratio of arylphosphines. THF as solvent showed a favorable effect. In a scaled-up reaction with 0.5 mole of isoprene using 7r-allylpalladium acetate and o-tolyphosphine, the isolated yield of the dimers was 87%. The dimers contained 71% of the head-to-tail isomers. The mixture was converted into easily separable products by treatment with concentrated hydro-... 

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. " ... 

Analogous additions of isoprene or 2-[(trimethylsilyl)methyl]-1,3-butadiene (182 M = SiMea) to dienophile (198) resulted in 75 25 or 95 5 mixtures of para and meta adducts, respectively. Although obtained with good selectivity, the major silyl-substituted adduct furnished, on prot esilylation, an inseparable mixture of endo- and exo-cyclic alkenes. In favorable contrast, the exclusively formed adduct (199) allowed a quantitative reductive removal of the sulfur auxiliary without migration of the alkenic bond. 

C = C double bonds in SCCO2 [47]. Isoprene was selectively reduced to a mixture of iso-pentenes with very little over-reduction to iso-pentane. 

The reaction of [Bu3Sb] in heptane solution with a reduced Pd/Al203 catalyst yields a supported alloy. " When these final solids are reduced at 573 or 773 K, the second metal locates preferentially at the outer layer of the bimetallic aggregates. After reduction at 773 K, large metallic aggregates are obtained (particle size around 15 nm). The specific activity of the Pd surface atoms for isoprene hydrogenation is then lowered and the selectivity increased. 

Reactions performed with methyl vinyl ketone and metacrolein as ethylenic dienophiles also revealed the clear advantage of SMWI conditions over conventional heating (Table 9.2, entries 4-6) [31]. In the reaction of isoprene with methyl vinyl ketone (Table 9.2, entry 6), selectivity for the para adduct (54%) was much better than when conventional heating was used (26%), probably owing to the reduction of the reaction time. 

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)... 

Diene Dimerization. The reductive dimerization of isoprene to its head-to-tail dimer has been achieved in high yield using di-/x-acetatobis(allyl)dipalladium (4) as the catalyst in combination with phosphine ligands. To achieve optimal selectivity for the desired dimers, a large number of phosphines have been examined in this reaction. Use of the complex (4) with the phosphine allows rapid access to each of the desired catalysts, as well as maximum flexibility with respect to Pd/L ratio (eq 21). ... 

---