Isoprene dimerization

In the absence of air or peroxides, only cycHc dimers are formed in the thermal dimerization of isoprene (33). Six cycHc dimers are formed in good yields four substituted cyclohexenes (3—6) and two dimethylcyclooctadienes (7—8). The latter two are, of course, not Diels-Alder dimers. There is some evidence that the isoprene dimerization mechanism differs from the usual Diels-Alder route. 

Mixtures of benzene and isoprene yield products (61) and (62) plus isoprene dimers 0,71 ... 

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, ... 

In addition to (3-carotene there are a variety of other C j pro-vitamin A carotenes that differ from (3-c.arotene in the nature of the terminal cyclic moieties. Thus, representing the right cyclic moiety as X, we can represent (3-carotene as X—(IP)2—(PI)2—X that yields two molecules of vitamin A or X—(IP)2—OH. Carotenes can have different cyclic moieties X (where X X) or no cyclic isoprene dimer moieties. Other C () pro-vitamin A carotenes that yield only one vitamin A molecule on oxidation include a-carotene, (3-cryptoxanthin, (3-c.arotene epoxide, echinenone and mutachrome (generalized structure X—(IP)2—(PI)2—X ) and y-carotene and torulene (X-(IP)2-(PI)4). 

Two years later, the transformation of the isoprene dimer (91) to (94) (mixture of stereoisomers) was observed and interpreted as a conjugate addition of Bu"Li to diene (91) followed by a (surprisingly smooth) intramolecular lithium-ene addition (92) (93) (Scheme 21). ... 

This type of reaction explains (in part) the formation of the isoprene dimer during rubber pyrolysis. The same compound may also be formed from other reactions such as isoprene dimerization. 

A Py-GC study [7] with pyrolysis done at 500° C showed numerous peaks corresponding to the isoprene dimers, trimers. .. up to hexamers eluting in clusters of peaks. The separation was done on a methyl silicone 5% phenyl silicone type column with FID detection. The results from a Py-GC/MS study [8] where natural rubber was pyrolysed at 580° C in a Curie point Py-GC/MS on-line system showed similar results. The TIC trace of the pyrolysate with the separation done on a 60 m Carbowax column, 0.32 mm i.d., 0.25 pm film thickness, with the temperature gradient of the GC oven between 40°... 

Commercial isoprene, obtained from Phillips Petroleum Company, should be freshly distilled before use, in order to eliminate isoprene dimers and polymers which are likely to accumulate in storage. If peroxides are present, they should be destroyed before distillation of the isoprene by washing with 10% acidified ferrous ammonium sulfate. As an added precaution, the distillation flask should not be permitted to go dry. 

In liquid ethanol, the yield of quenchable triplets is <0.03 (benzophenone 1.00) for HB, 4-methoxy HB and 2,2 dlhydroxybenzo-phenone (8). The yield of Isoprene dimers relative to benzophenone (50% isoprene) is found to be 0.03 in cyclohexane and 0.15 in ethanol if HB is used as sensitizer. The lack of plperylene sensitization even in ethanol Indicates a very short lifetime of T of HB even in its broken H-bond form (8). 

As an example, myrcene, an isoprene dimer, is found in many natural oils. Most of the polymerizations of myrcene were affected by a metal catalyst in organic solvents [36-39], We tried to synthesize copolymers containing myrcene, styrene and diethyl fumarate (DEF) by using Me-p-CD in water (Fig. 8) [40],... 

With the modem thermostated densimeters it is much easier to determine the temperature dependence of partial molar volumes and, hence, of reaction volumes. From these data El yanov extrapolated a generally applicable equation shown in Table 2.3 (footnote a) to describe the temperature dependence of activation and reaction volumes. The temperature dependence obtained experimentally for isoprene dimerization is in accord with the El yanov equation. 

The telomerization of a mixture of isoprene and prenyl acetate in ethyl acetate, catalysed by boron trifluoride, gives the usual complex mixture of acetates this includes geranyl and a-terpinyl acetates. Isoprene dimerizes in the presence of certain titanium or zirconium catalysts, e.g. ZrBusCl -f AlClEt2 + Ph3P, giving 70% of the hydrocarbon (8), 8% of (9), and 22% of trimers. Heggie and... 

Isopentenylhydroquinone, 397, 398 Isopentyl nitrite, 48 Isophorone, 287 oxide, 282, 283, 394 Isoprene dimers, 4-5 Isoprene epoxide, 79 isoprenoid quinones, 250 Isoprenylation, 397-398 N-Isopropylideneanilines, 3 Isopropylidenecarbene. 358 Isup ropy I idenecy do propanes, 358... 

Since isoprene was the principal desired product from methyl pentene pyrolysis it was essential to determine its stability under reaction conditions. Isoprene degradation proceeds via second-order rate processes. Isoprene dimer and related compounds formed during the pyrolysis are dependent on the isoprene partial pressure in the reaction system. 

These features can only be explained in terms of a cyclization process. Using the isoprene dimer, dihydromyrcene, as a model system Bloomfield (1943) obtained a ring structure by the following reaction ... 

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)... 

Further interesting possibilities to functionalize isoprene dimers are mentioned by Morikawa and Kitazume [120]. They started with 2,6-dime-... 

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