Isoprene, telomerization

In contrast to butadiene, isoprene is an vmsymmetrical molecule. The connection of two isoprene molecules can therefore occur on the head h or on the tail t of the molecule. Consequently, in the telomerization of isoprene the tail-to-tail tt)-, tail-to-head th), head-to-tail ht)- and head-to-head (hh)-products are possible. Because the nucleophile HY can attack the chain at positions 1 and 3, eight telomers occur. The fact that the primary telomers possess an iimer-standing double bond means that these molecules exist as cis- and trans-isomers and that, on the whole, twelve different molecules can be formed in isoprene telomerization. 

Leca F, R6au R (2006) 2-Pyridyl-2-phospholenes new PJM ligands for the palladium-catalyzed isoprene telomerization. J Catal 238 425... 

The dimerization of isoprene is possible, but the reaction of isoprene is slower than that of butadiene. Dimerization or telomerization of isoprene, if carried out regioselectively to give a tail-to-liead dimer 18 or a head-to-tail... 

A telomerization reaction of isoprene can be carried out by treatment with 2-chloro-3-pentene, prepared by the addition of dry HCl to 1,3-pentadiene (67). An equimolar amount of isoprene in dichi oromethane reacts with the 2-chloro-3-pentene at 10°C with stannic chloride as catalyst. l-Chloro-3,5-dimethyl-2,6-octadiene is obtained in 80% yield by 1,4-addition. 

Isoprene (2-methyl-1,3-butadiene) can be telomerized in diethylamine with / -butyUithium as the catalyst to a mixture of A/,N-diethylneryl- and geranylamines. Oxidation of the amines with hydrogen peroxide gives the amine oxides, which, by the Meisenheimer rearrangement and subsequent pyrolysis, produce linalool in an overall yield of about 70% (127—129). 

Synthetic methods for the production of citroneUal iaclude the catalytic dehydrogenation of citroneUol (110), the telomerization of isoprene (151), and the Utbium-catalyzed reaction of myrcene with secondary alkylamines (128). 

The isomeric N,N-diethylnerylamine could be obtained in 99% isomeric purity (77-86% yield) by telomerization of isoprene with diethylamine in the presence of n-BuLi [215]. 

A successful selective head-to-tail telomerization to give 1-methoxy-2,6-dimethyl-2,7-octadiene (89) by the reaction of isoprene with methanol at room temperature was reported by Yamazaki (59). Although the reaction is the selective head-to-tail addition, unfortunately the methoxy group was introduced at the position opposite to oxygen function in natural products ... 

Asymmetric telomerization of isoprene and methanol by using chiral phosphines, such as menthyldiphenylphosphine, gave an optical yield of 17.6%. The telomerization of methanol and isoprene using w-allylpalla-dium chloride and PBu3 in the presence of sodium methoxide in a mixed solvent of methanol and isopropyl alcohol at room temperature for 2 days produced l-methoxy-2,6-dimethyl-2,7-octadiene (89) (80%) and 1-meth-oxy-2,7-dimethyl-2,7-octadiene (91) (15%) (91). After 2 days, the reaction mixture was heated at 80°C for 8 hours, and 2,6-dimethyl-l,3,7-octatriene (88) (75%) and 2,7-dimethyl-1,3,7-octatriene (85) (14%) were obtained. Also, NiCl2(Bu3P)2 was used as a cocatalyst for the formation of 88. 

The results in Table VI were obtained by the reactions at 80°C with Pd(acac)2 using different ligands in a mixture of methanol and isopropyl alcohol. From these results, it seems likely that reaction temperature has large influence on the regiospecificity of the telomerization. As another example, a mixture of isomeric telomers was obtained by the reaction of methanol and isoprene at 100° (95). 

The telomerization of the lower reactive isoprene with glycerol was achieved in the presence of palladium-carbene complex but in a dioxane/PEG solvent [15]. Under such conditions, both glycerol and PEG are converted. After 24 h, in the presence of 0.06% [Pd(acac)2/IMes.Cl] (1/1.5) at 90°C, the telomerization of isoprene with glycerol (glycerol/PEG/dioxane/isoprene = 1/2.5/2.5/5) yields 70% of the linear monoether glycerol together with 29% of PEG telomer. 

The telomerization of butadiene by means of water in ILs was described by Dullius et Rottger et al. report a process for the telomerization of acyclic olefins having at least two conjugated double bonds, or their mixtures, using a palladium-carbene complex as catalyst in an IL solvent. The nucleophiles included water, alcohols, phenols, polyols, carboxylic acids, ammonia and primary and secondary amines. The acycylic olefins could be either 1,3-butadiene or isoprene. 

Much of the telomerization work this year is either closely related or similar to work reported previously. This includes tail-to-tail isoprene dimerization [Pd(PEt)j-C02] to (37) which is extensively isomerized on prolonged reaction, ... 

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. 

Fig. 1 Possible isomers that can be obtained in the telomerization of isoprene with NuH...

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. 

Jackstell R, Grotevendt A, Michalik D, El Firdoussi L, Beller M (2007) Telomerization and dimerization of isoprene by in situ generated palladium-carbene catalysts. J Organomet Chem... 

Keim W, Kraus A, Huthmacher K, Hahn R (1999) Method and catalysts for the preparation of 6,10- and 6,9-dimethyl-5,10-undecadienyl-2-ones by the telomerization of isoprene with alkyl acetylacetonates. DE 19730546... 

Maddock SM, Finn MG (2000) Palladium-catalyzed head-to-head telomerization of isoprene with amines. Organometallics 19 2684—2689... 

Telomerization of Isoprene with Dialkylamine- N,N-Diethylnerylamine K. Takabe, T. Yamada, T. Katagiri, and J. Tanaka, Department of Synthetic Chemistry, Faculty of Engineering, Shizuoka University, Johoku, Hamamatsu 432, Japan... 

Telomerization of Isoprene.—Reviews have appeared on isoprene and chloro-prene, and on the complex reactions of isoprene to form terpenoids (in Japanese). Isoprene reacts with magnesium, especially in the presence of Lewis acids, and the resulting complex gives adducts with aldehydes. As usual in this type of reaction, a very complex mixture is obtained. The palladium-chloride-catalysed reaction of isoprene with acetic acid gives different products in different solvents. Monomers predominate in benzene [2-methylbut-2-enyl acetate (5) and 3-methylbut-2-enyl acetate (6)] while dimers [(7), (8), neryl (9), and geranyl (10) acetates] tend to be formed in tetrahydrofuran. Further details of the synthesis of Cio alcohols from isoprene and naphthyl-lithium are available, as well as of the in situ oxidation,but there is little of novelty (see Vol. 1, p. 17). 

In a related study, Trost and Zhi [24] showed that the use of dppp(l,3-(diphenylphosphino)propane) as a ligand on palladium also led to a high selectivity for 1,4-addition of active methylene compounds to 1,3-dienes. For example, 2,3-dimethylbuta-1,3-diene gave an excellent yield with a number of active methylene compounds [Eq.(9)]. Interestingly, the reaction temperature is of importance for the 1,4-selectivity. Thus, in the reaction of (PhSOjICHa with isoprene employing the Pd(0)-dppp system, the ratio between the desired 1,4-addition product and the telomerization product was 73 27 at 70 °C but increased to 95 5 at 100 °C. Also, cyclic dienes gave an excellent yield of the 1,4-addition products [Eq.(lO)]. 

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