Isoprene hydrochlorination

Synthesis of P-Methylheptenone from Petrochemical Sources. p-MethyUieptenone (1) is an important intermediate in the total synthesis of terpenes. Continuous hydrochlorination of isoprene [78-79-5] produces prenyl chloride [505-60-6] which then reacts with acetone with a quaternary ammonium catalyst and sodium hydroxide to give P-methyUieptenone (6-methyIhept-5-en-2-one [110-93-0]) (eq. 1) (16—19). 

Synthetic polyisoprene rubbers were found by Harries during 1910 [19], to be successfully hydrochlorinated. These synthetic polyisoprenes resemble natural rubber, which is also a polymer of isoprene which occurs naturally and therefore hydrogen chloride is rapidly added when the rubber is exposed to it thus forming a protective film of chlorinated rubber or hydrogen chloride. Polybutadiene, and butadiene acrylonitrile rubbers do not add hydrogen chloride from hydrochloric acid. 

The addition of hydrogen chloride to unsaturated elastomers has also received considerable attention. Extensive work has been done on the hydrochlorination ofHevea [poly(ds-l,4-l,4-isoprene)] andBalata [poly(rraMs-l,4-isoprene)] mbbers since 1940 (Staudinger, 1944 Gordon and Tyler, 1953). Both cis-1,4 and trans c/s-l,4-polyisoprenes readily add hydrogen chloride following Markovnikov s rules with only a small amount of cyclization. 

Characteristic examples of industrial fast chemical reactions are the electrophilic polymerisation of isobutylene [7], its copolymerisation with isoprene [10], chlorination of olefins [17] and butyl rubber [18], ethylene hydrochlorination [17], sulfation of olefins [19], neutralisation of acidic and basic media [20], isobutene alkylation (production of benzines) [21-23], and so on. These examples of fast liquid-phase reactions and a variety of such processes assume a formal approach for their calculation and modelling, based on material and heat balance in the industrial implementation of respective products. It is a priori acknowledged that is not difficult to achieve an isothermic mode for fast chemical exothermic processes if you are aware of the process behaviour and can control it. 

Hydrochlorination of isoprene [78-79-5] (1) produces prenyl chloride (56), together with some of the isomeric 3-chloro-3-methylbut-l-ene (57), the ratio between the two depending on reaction conditions. The former undergoes Sn2 reactions while the latter prefers Sn2, hence both alkylate preferentially at the primary carbon atom. Therefore, treatment of the chlorides with acetone in the presence of base, gives methylheptenone [110-93-0] (19), as shown in Fig. 8.13 (62). This is the basis of a process developed by Rhone-Poulenc in which a phase-transfer catalyst is used to assist in the alkylation of acetone [63-65]. A similar process is operated by Kuraray (66). Linalool produced in this way can be isomerized to geraniol using an orthovanadate catalyst (67). 

In addition to the methods described above, prenol (51) can be prepared from methyl-butynol (43) by rearrangement to prenal (52) using a titanium alkoxide/copper chloride catalyst [69, 70] followed by selective hydrogenation using a ruthenium rhodium tris( 7-sulfonatoyl)phosphine trisodium salt (TPPTS) catalyst [71, 72]. However, it is more usual to prepare the prenyl esters by nucleophilic substitution of a carboxylate anion on prenyl chloride [503-60-6] (56) which, in turn, is available through hydrochlorination of isoprene [78-79-5] (1). This hydrochlorination often employs copper ions as catalysts. These processes are shown in Fig. 8.14. 

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