Isoprene Substrates

Narasaka et al.16 reported that 53 catalyzes Diels-Alder reactions of 54-type substrates with diene in the presence of 4 A molecular sieves (Scheme 5-18). A remarkable solvent effect on the enantioselectivity is observed. High enantio-selectivity is attained using mesitylene as the solvent. As shown in Scheme 5-18, the reaction of 54a with isoprene proceeds smoothly in this solvent, affording product 55a with 92% ee. Other 3-(3-substituted acryloyl)-l,3-oxazolidin-2-ones 54b-d also give good results (75-91% ee) when reacted with cyclopentadiene. 

Diels—Alder reactivity was also reported for a cationic zirconocene alkoxide (A Scheme 8.46) at a 10 mol% level for the substrate combination methyl acrylate/isoprene [84]. Whereas the regioselectivity (para/meta = 96.2 3.8) in this process compared favorably to that with traditional Lewis acids (A1C13 in C6H6 regioselectivity = 95 5), the activity was quite low. The substrates methyl acrylate and cydopentadiene (Scheme 8.46 R =... 

Table 9.4 Characteristics of arborescent polystyrene substrates used in the preparation of isoprene graft copolymers (adapted from ref. 16)...

The synthetic importance of non-nucleophilic strong bases such as lithium diisopro-pylamide (LDA) is well known but its synthesis involves the use of a transient butyl lithium species. In order to shorten the preparation and make it economically valuable for larger scale experiments an alternate method of synthesis has been developed which also involves a reaction cascade (Scheme 3.14) [92]. The direct reaction of lithium with diisopropylamine does not occur, even with sonication. An electron transfer agent is necessary, and one of the best in this case is isoprene. Styrene is used in the commercial preparation of LDA, but it is inconvenient in that it is transformed to ethylbenzene which is not easily removed. It can also lead to undesired reactions in the presence of some substrates. The advantages of isoprene are essentially that it is a lighter compound (R.M.M. = 68 instead of 104 for styrene) and it is transformed to the less reactive 2-methylbutene, an easily eliminated volatile compound. In the absence of ultrasound, attempts to use this electron carrier proved to be unsatisfactory. In this preparation lithium containing 2 % sodium is necessary, as pure lithium reacts much more slowly. 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

The Pt(CH2 = CH2)(PPh3)2-catalyzed dehydrogenative double silylation of olefins and dienes with o-bis(dimethylsilyl)benzene was also examined by Tanaka and co-workers.61 The major product of the reaction with dienes, such as isoprene and penta-1,2-diene, is a result of 1,2-addition to the less substituted double bond. The reaction pathway for simple alkenes, shown in Eq. (19), appears to be dependent on the alkene substrate and, in some cases, on reaction temperature. Products resulting from 1,2-addition, 1, and 1,1-addition, 2, are detected for various substrates. In addition, hydrosilylation may occur to give the simple hydrosilylated product, 3, or a by-product, 4, derived from 1,4-migration of a methyl group in 3. 

Grange and Adair73 isolated 2,3-dehydrodolichyl phosphate from in vitro studies on hen oviduct. It may be derived from 2,3-dehydrodolichyl diphosphate, a likely intermediate in the biosynthesis of dolichyl phosphate. Many questions still remain unanswered, such as is the substrate for the a-isoprene reductase, the diphosphate or the monophosphate of 2,3-dehy-drodolichol Are the unsaturated polyprenols a reserve pool for the synthesis of dolichol in plants What is the function of unsaturated, plant polyprenols How is the synthesis of saturated and unsaturated polyprenols regulated ... 

Although various transition-metal complexes have reportedly been active catalysts for the migration of inner double bonds to terminal ones in functionalized allylic systems (Eq. 3.2) [5], prochiral allylic compounds with a multisubstituted olefin (Rl, R2 H in eq 2) are not always susceptible to catalysis or they show only a low reactivity [Id]. Choosing allylamines 1 and 2 as the substrates for enantioselective isomerization has its merits (1) optically pure citronellal, which is an important starting material for optically active terpenoids such as (-)-menthol, cannot be obtained directly from natural sources [6], and (2) both ( )-allylamine 1 and (Z)-allylamine 2 can be prepared in reasonable yields from myrcene or isoprene, respectively, The ( )-allylamine 1 is obtained from the reaction of myrcene and diethylamine in the presence of lithium diethylamide under Ar in an almost quantitative yield (Eq. 3.3) [7], The (Z)-allylamine 2 can also be prepared with high selectivity (-90%) by Li-catalyzed telomerization of isoprene using diethylamine as a telomer (Eq. 3.4) [8], Thus, natural or petroleum resources can be selected. 

In addition to the industrial developments listed above, much academic efforts have also been devoted over the years to (1) the expansion of the scope of the telomerization reaction, (2) the elucidation of the details of the reaction mechanism and (3) process modifications that allow more efficient production and separation of the desired products. The scope of substrates that can be used in this reaction has indeed been shown to be very broad. 1,3-Butadiene is most often used as the conjugated diene, since it is cheap, readily available and provides a linear octadienyl chain. The use of other dienes, such as isoprene [13-16], piperylene [17] and myrcene [18,19], has also been described, but they have been far less commonly studied. Such substituted telogens come with an additional selectivity challenge as many more isomers can potentially be obtained, which is illustrated for isoprene in Fig. 1. 

In all examples of the palladium-catalyzed telomerization discussed up till now, the nucleophile (telogen) can be considered renewable. The taxogens used (butadiene, isoprene), however, are still obtained from petrochemical resources, although butadiene could, in principle, also be obtained from renewable resources via the Lebedev process that converts (bio)-ethanol into 1,3-butadiene. Limited attention has been given in this respect to the great family of terpenes, as they provide direct access to renewable dienes for telomerization. In particular, those terpenes industrially available, which are derived mostly from turpentine, form an attractive group of substrates. Behr et al. recently used the renewable 1,3-diene myrcene in the telomerization with diethylamine, for instance [18]. The monoterpene myrcene is easily obtained from (3-pinene, sourced from the crude resin of pines, by pyrolysis, and is currently already used in many different applications. 

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