Piperylene regioselectivity

In the polymerisation of piperylene regioselectivity, addition of 1,4-units according to head-to-tail type, is caused by the fact that only the double bond of diene bearing no methyl substituent can react with the metal-carbon o-bond of the active centre. Involvement of the double bond of piperylene bearing the methyl substituent in the reaction of insertion is considerably hampered because of the steric repulsion between the methyl groups of the terminal unit of the growing chain and the diene. The smaller difference in energy between the two possible o-forms of the terminal unit in the polymerisation of piperylene, as compared to butadiene, is responsible for the increased lifetime of the active centre in the o-form, in which the Nd atom is linked to the atom. This results in an increased content of trans-1,2- and trans-1,4 units in polypiperylene. 

Strong effects of the catalyst on the regioselectivity have been observed in the cycloadditions of a variety of heterocyclic dienophiles. Some results of the BF3-catalyzed reactions of quinoline-5,8-dione (21) and isoquinoline-5,8-dione (22) with isoprene (2) and (E)-piperylene (3) [25], and of the cycloadditions of 4-quinolones (23a, 23b) as well as 4-benzothiopyranone (23c) with 2-piperidino-butadienes, are reported [26] in Scheme 3.8 and Equation 3.2. The most marked... 

Copper-catalyzed monoaddition of hydrogen cyanide to conjugated alkenes proceeded very conveniently with 1,3-butadiene, but not with its methyl-substituted derivatives. The most efficient catalytic system consisted of cupric bromide associated to trichloroacetic acid, in acetonitrile at 79 °C. Under these conditions, 1,3-butadiene was converted mainly to (Z )-l-cyano-2-butene, in 68% yield. A few percents of (Z)-l-cyano-2-butene and 3-cyano-1-butene (3% and 4%, respectively) were also observed. Polymerization of the olefinic products was almost absent. The very high regioselectivity in favor of 1,4-addition of hydrogen cyanide contrasted markedly with the very low regioselectivity of acetic acid addition (vide supra). Methyl substituents on 1,3-butadiene decreased significantly the efficiency of the reaction. With isoprene and piperylene, the mononitrile yields were reduced... 

The effects of Lewis acids on regioselectivity are shown by the reaction of piperylene with methyl acrylate giving mainly the ortho adducts 6.203, as usual, but this preference is increased with Lewis acid catalysis. Similarly, as an... 

Diels-Alder reactions (6, 65-66). 2-Methoxy-5-methylbenzoquinone shows no regioselectivity in reactions with piperylene or isoprene. These cycloadditions are catalyzed by both BF3 and SnCl4, but they favor different adducts in each case. The difference is believed to arise from different types of complexes with the quinone.1... 

Davies and coworkers , for example, used iV-enoyl derivatives of a cw-l-aminoindan-2-ol based L3-oxazolidin-2-one (222) as chiral dienophiles in the Diels-Alder reactions with isoprene (91a) and piperylene (91b) which give 223 (equation 62). Their results have been summarized in Table 6. The reactions proceeded with high endo/exo and regioselectivities. Bidentate co-ordination of the catalyst to both carbonyl groups kept the dienophile in a rigid conformation, which gave rise to the high de values observed. 

Phosphonodithioic anhydrides react regioselectively with unsymmetri-cal dienes to give Diels-Alder adducts.24 For example, isoprene and frans-piperylene react to form adducts shown in Scheme 6-1V. Yields... 

As for the regioselectivity, C2 electron-donating substituents strongly direct dienes to react with benzaldehyde at the Cl position (runs 2, 3, and 5). Cl substituents are not so influential as the C2 substituents and tend to promote the reaction at the distal C-4 diene terminal. For example, piperylene forms a mixture of the C4 and Cl adducts in ca. 2 1 ratio (run 4). The relative directing abiUty of the Cl and C2 substituents may be demonstrated by the reaction of l,2-dimethyl-l,3-butadiene, where both substituents work oppositely, the former directing the reaction at Cl, while the latter at C4 (run 6). The exclusive formation of the Cl adduct clearly indicates that C2-Me overrides Cl-Me in controlling the regioselectivity. 

As for the regioselectivity, it is important that in the presence of lanthanide catalysts the dienes can enter insertion, upon isomerisation from the transoid to cisoid conformation, only via the double bond containing no methyl group. The reactions for this phenomenon are different for piperylene and isoprene. 

Naturally, these changes are extended in time and need considerable expenditure of energy. In our opinion, it is this factor that is responsible for the participation of only the unsubstituted double bond of piperylene in the insertion and ensures the regioselectivity, that is, the addition of 1,4-units only according to the head-to-tail type. 

Diels-Alder Reactions. Bp3-OEt2 is used to catalyze and reverse the regiospecificity of some Diels-Alder reactions, e.g. with pen-hydroxylated naphthoquinones, sulfur-containing conpounds, the reaction of 1-substituted trans-1,3-dienes with 2,6-dimethylbenzoquinones, and the reaction of 6-inethoxy-l-vinyl-3,4-dihydronaphthalene with p-quinones. BFs-OEta has a drastic effect on the regioselectivity of the Diels-Alder reaction of quinoline- and isoquinoline-5,8-dione with piperylene, which produces substituted azaanthraquinones. This Lewis acid is the most effective catalyst for the Diels-Alder reaction of furan with methyl acrylate, giving high endo selectivity in the 7-oxabicyclo[2.2.1]heptene product (eq 35). ... 

It reacts with traws-piperylene with high regioselectivity and endo stereoselectivity no exo product is observed (Table 30.6) [23]. However, the major isomer is reversed from that observed with vinyl-9-BBN, which has afforded only meta substituted products [5]. 

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