With isoprene

Hydroboration of conjugated dienes proceeds without a catalyst to give 1,2-adducts. However, the less reactive catecholborane reacts with isoprene with catalysis by Pd(PhiP)4, yielding the 1,4-adduct 73[66]. 

The reaction proceeds readily, depending on the nature of the dienophile, and normally no catalyst or inhibitor is effective. The substituents, A and B, in the adduct retain their configuration relative to the double bond originally present in the dienophile (22). Many materials having the general stmcture CH2=CH—X react with isoprene to give mixtures of two isomers (1) and (2). 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

The rates of these two reactions have been studied for the attack of trifluoromethyl (51) and methyl radicals (52) in isoprene that has been dissolved in 2,3-dimethylbutane and isooctane, respectively. The rate constants for the reactions with isoprene are much greater than those for the attack on the solvent. The ratio between the two rates for the attack of trifluoromethyl radicals varies from 1090 at 65°C to 233 at 180°C. For the corresponding reaction involving methyl radicals, the ratio is 2090 at 65°C. 

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

Alkylation of cyclohexane with isoprene can be carried out with alkyl radicals formed at 450°C and 20.3 MPa (200 atm) (73). 40% Pentenylcyclohexanes, 20% dipentenes (ie, substances having the general formula C qH ), and 40% higher boiling compounds are obtained using a 6.8 molar ratio of cyclohexane to isoprene and a space velocity of 2.5. Of the pentenylcyclohexanes, the head and tail products are in equal amounts. Even... 

Other Compounds. Primary and secondary amines add 1,4- to isoprene (75). For example, dimetbylamine in ben2ene reacts with isoprene in the presence of sodium or potassium to form dimetby1(3-metby1-2-buteny1)amine. Similar results are obtained with diethylamine, pyrroHdine, and piperidine. Under the same conditions, aniline and /V-metbylaniline do not react. Isoprene reacts with phenol in the presence of aluminum phenoxide (76) or concentrated phosphoric acid (77) to give complex products. 

Trimetbylsilane does not react under these conditions. However, under similar conditions, heptamethylcyclotetrasiloxane reacts with isoprene by... 

The principal route for production of isoprene monomer outside of the CIS is recovery from ethylene by-product C streams. This route is most viable where ethylene is produced from naphtha or gas oil and where several ethylene plants are located in relatively close proximity to the isoprene plant. Although the yield of isoprene per mass of ethylene is quite low, there is enough ethylene produced to provide a large portion of demand. Because of the presence of / -pentane in these streams which a2eotropes with isoprene, extractive distillation must be used to recover pure isoprene. Acetonitrile is the most common solvent, but dimethylformamide is also used commercially. 

Polymers account for about 3—4% of the total butylene consumption and about 30% of nonfuels use. Homopolymerization of butylene isomers is relatively unimportant commercially. Only stereoregular poly(l-butene) [9003-29-6] and a small volume of polyisobutylene [25038-49-7] are produced in this manner. High molecular weight polyisobutylenes have found limited use because they cannot be vulcanized. To overcome this deficiency a butyl mbber copolymer of isobutylene with isoprene has been developed. Low molecular weight viscous Hquid polymers of isobutylene are not manufactured because of the high price of purified isobutylene. Copolymerization from relatively inexpensive refinery butane—butylene fractions containing all the butylene isomers yields a range of viscous polymers that satisfy most commercial needs (see Olefin polymers Elastomers, synthetic-butylrubber). 

Butyl rubber and other isobutylene polymers of technological importance iaclude various homopolymers and isobutylene copolymers containing unsaturation achieved by copolymerization with isoprene. Bromination or chlorination of the unsaturated site is practiced commercially, and other modifications are beiag iavestigated. 

Mikami et al. have reported that the chiral titanium reagent 12 derived from bi-naphthol and TiCl2(0-i-Pr)2 catalyzes the Diels-Adder reaction of a-bromoacrolein or methacrolein with isoprene or 1-methoxy-l,3-butadiene to afford the cycloadducts with high enantioselectivity [18] (Scheme 1.25). 

Another issue important to the success of this chiral titanium reagent 31 was the discovery of a marked solvent effect. When the fumaric acid derivative is reacted with isoprene in the presence of 10 mol% of the titanium reagent 31 in toluene, poor optical purity results (36-68% ee). Interestingly the optical purity of the adduct greatly increased in the order benzene, toluene, xylenes, and mesitylene, with 92% ee obtained in the last. Mesitylene is difficult to remove, because of its high boiling point, and other solvents were screened in detail. As a result, the mixed solvent system toluene petroleum ether (1 1) was discovered to be very effective. 

The Diels-Alder reaction catalyzed by this chiral titanium catalyst 31 has wide generality (Scheme 1.53, 1.54, Table 1.22, 1.23). Acryloyl- and fumaroyl-oxazolidinones react with isoprene giving cycloadducts in high optical purity. 2-Ethylthio-l,3-buta-diene can also be successfully employed as the diene [42]. 

The interest in chiral titanium(IV) complexes as catalysts for reactions of carbonyl compounds has, e.g., been the application of BINOL-titanium(IV) complexes for ene reactions [8, 19]. In the field of catalytic enantioselective cycloaddition reactions, methyl glyoxylate 4b reacts with isoprene 5b catalyzed by BINOL-TiX2 20 to give the cycloaddition product 6c and the ene product 7b in 1 4 ratio enantio-selectivity is excellent - 97% ee for the cycloaddition product (Scheme 4.19) [28]. 

Ionol is a commercial antioxidant, 2,6-di-/cr/-butyl- -cresol, manufactured by Shell Chemical Corp. Inhibitors appear to minimize formation of polymeric side products, although with isoprene the effect is often small. 

It has been established that alkoxy alkenylcarbene complexes participate as dienophiles in Diels-Alder reactions not only with higher rates but also with better regio- and stereoselectivities than the corresponding esters [95]. This is clearly illustrated in Scheme 51 for the reactions of an unsubstituted vinyl complex with isoprene. This complex reacts to completion at 25 °C in 3 h whereas the cycloaddition reaction of methyl acrylate with isoprene requires 7 months at the same temperature. The rate enhancement observed for this complex is comparable to that for the corresponding aluminium chloride-catalysed reactions of methyl acrylate and isoprene (Scheme 51). 

The presence of two substituents at C-4 also strongly influences the regios-electivity as shown in the cycloaddition of dienone 13 with isoprene (2) (Equation 3.1). In violation of the para-rule for Diels-Alder reaction, only metfl-adduct was obtained [19,20]. 

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... 

Similarly a marked increase of regioselectivity has been shown in the catalyzed Diels-Alder reactions of the chiral bicyclic lactame 24 (Scheme 3.9) with a variety of dienes [27] (isoprene, mircene, (E,E)-L4-dimethylbutadiene, 2,3-di-methylbutadiene, 2-siloxybutadiene). The catalyzed reactions were more regio-selective and totally enJo-antz-diastereoselective anti with respect to the bridgehead methyl group). The results of the cycloadditions with isoprene and mircene are reported in Scheme 3.9. The cycloadducts have then been used to provide interesting fused carbocycles [28] with high enantiomeric purity as shown in Scheme 3.10. 

Supported Lewis acids are an interesting class of catalysts because of their operational simplicity, filterability and reusability. The polymer-bound iron Lewis-acid 53 (Figure 3.8) has been found [52] to be active in the cycloadditions of a, S-unsaturated aldehydes with several dienes. It has been prepared from (ri -vinylcyclopentadienyl)dicarbonylmethyliron which was copolymerized with divinylbenzene and then treated with trimethylsilyltriflate followed by THF. Some results of the Diels-Alder reactions of acrolein and crotonaldehyde with isoprene (2) and 2,3-dimethylbutadiene (4) are summarized in Equation 3.13. 

Fluoboric acid is also an efficacious promoter of cyclic oxo-carbenium ions (Scheme 4.24) bearing an activated double bond which, in the presence of open-chain and cyclic dienes, rapidly undergo a Diels-Alder reaction [91]. Chiral a, -unsaturated ketones bearing a -hydroxy substituents, protected as acetals, react with various dienes in the presence of HBF4, affording Diels-Alder adducts that were isolated as alcohols by hydrolysis of the acetal group by TsOH. Some examples of reactions with isoprene are reported in Table 4.23. The enantios-electivity of the reaction is dependent on the size of the substituent R on the of-carbon high levels of asymmetric induction were observed with R = z-Pr (90 1) and R = t-Bu (150 1) and low levels with R = Me (2.7 1) and R = Ph (3.0 1). Scheme 4.24 shows the postulated reaction mechanism. 

Recently Nafion-H was successfully used in the Diels-Alder reaction of olefin acetals with isoprene and cyclopentadiene (Scheme 4.27). The reactions work well in DCM at room temperature and Nafion-H did not cleave the acetal group [96]. The recovered Nafion-H was used four or five times without affecting the yield of the cycloadducts. 

A convenient alternative to LP-DE is lithium trifluoromethanesulfonimide (LiNTf2) in acetone or diethyl ether (LT-AC, LT-DE). Representative examples are the Diels-Alder reactions of citraconic anhydride with cyclopen-tadiene and of dimethyl acetylenedicarboxylate with isoprene [47] (Scheme 6.26). 

The use of Lewis acids (ZnU, BF3 Et20) in ionic liquids, tested in the cycloaddition of but-3-en-2-one with isoprene, increases both the rate and selectivity of the reaction. The ionic liquid remains catalytically active after the work-up and can be reused. 

The Diels Alder reactions of maleic anhydride with 1,3-cyclohexadiene, as well the parallel reaction network in which maleic anhydride competes to react simultaneously with isoprene and 1,3-cyclohexadiene [84], were also investigated in subcritical propane under the above reaction conditions (80 °C and 90-152 bar). The reaction selectivities of the parallel Diels-Alder reaction network diverged from those of the independent reactions as the reaction pressure decreased. In contrast, the same selectivities were obtained in both parallel and independent reactions carried out in conventional solvents (hexane, ethyl acetate, chloroform) [84]. 

O3 + terpene products Rate =. [03] [terpene] We expect the reaction rate to depend on two concentrations rather than one, but we can isolate one concentration variable by making the initial concentration of one reactant much smaller than the initial concentration of the other. Data collected under these conditions can then be analyzed using Equations and, which relate concentration to time. For example, an experiment could be performed on the reaction of ozone with isoprene with the following initial concentrations ... 

With isoprene, 2,3-dimethylbuta-1,3-diene and cyclopentadiene, if the ozone concentration in the ozone/oxygen mixture exceeds a certain limit (not stated), the medium immediately combusts when incorporating this mixture at -78°C. 

The present homoallylation with isoprene under Ni-Et3B catalysis shows marginal success for the reaction with aliphatic aldehydes. Results are summarized in Table 6. Primary alkyl aldehydes (bearing no a-substituents) and sterically less-hindered secondary alkyl aldehydes undergo the homoallylation successfully to provide the expected products in good yields with excellent stereoselectivity (runs 1-5). The results in runs 3-5 indicate that the present reaction shows almost no diastereofacial selectivity with respect to the a-stereo centers of secondary alkyl aldehydes. Sterically demanding aldehydes, such as cyclohexanecarbaldehye and pivalaldehyde, provide the... 

Table 5 Ni-catalyzed homoallylation of aromatic and unsaturated aldehydes with isoprene promoted by Et3B ...

Et2Zn also participates in the reductive coupling as a formal hydride source. Results for the Ni-catalyzed, Et2Zn-promoted homoallylation of carbonyl compounds with isoprene are summarized in Table 7 [30]. Et2Zn is so reactive that for the reaction with reactive aromatic aldehydes it causes direct ethylation of aldehydes, and the yields of homoallylation are diminished (runs 1 and 2). Unsaturated aldehydes seem to be subject to the Michael addition of Et2Zn. Accordingly, for the reaction with cinnamaldehyde, none of the expected homoallylation product is produced instead, the 1,4-addition product of Et2Zn, 3-phenylpentanal is produced exclusively (run 3). 

The erosion in regioselectivity observed for ketones may be due to 1,3-diaxial repulsion in an intermediate 45 (indicated by shading in Scheme 12), that is inevitable when isoprene reacts with ketones at Cl position. Accordingly, the reaction may optionally proceed through an intermediate 45", which is formed by the reaction of a ketone with isoprene at the C4 position and may be sterically less congested than 45. ... 

Table 8 Ni-catalyzed homoallylation of anisidine-imine with isoprene promoted by Et2Zn...

Likewise, thiazolol3,2-<7 1,4,2 diazaphospholes and their 5,6-dihydro and benzo derivatives (87) reacted with 2,3-dimethylbutadiene and with isoprene with or without sulfur/selenium to give [2+4] cycloadducts 88 and 88 diastereo- and regioselectively (Scheme 27) [100],... 

Table 7 Polar cycloadditions of 1 -benzothiopyrylium salts 43 with isoprene 41 b (Equation 13)...

---