2-Methylpropene preparation

Problem 31.6 Draw the structure of an alternating segment of butyl rubber, a copolymer of iso-prene (2-methyl-],3-butadiene) and isobutylene (2-methylpropene) prepared using a cationic initiator. 

Tertiary butyl alcohol, trimethyl carbinol, tertiary butanol. 2-methyl-2-propanol, Me3COH. Colourless prisms, m.p. 25°C, b.p. 83°C. Prepared by absorbing isobutene (2-methylpropene) in sulphuric acid, neutralizing and steam distilling the liquor. Converted to isobutene by heating with oxalic acid. Potassium-/-buloxide is a very strong base. 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

The [l,3]oxazino[3,4- ]azepinone 42, isolated as a byproduct (8%) during the preparation of 2//-azepine from the 1-carboxylate 40, is believed to arise by a [2 + 4] cycloaddition of 2-methylpropene (formed in situ) with the l-acyl-2/7-azepinium ion 40 via intermediate 41.290... 

Hodgson, P. K. G. et al., J. Chem. Soc., Perkin Trans. 2, 1978, 854 Preparation of /er/-butyl esters from the acid and 2-methylpropene under pressure in glass vessels led to explosions. A safe procedure in a steel cylinder is described. 

Exercise 15-11 A suitable method of preparing ferf-butyl esters is to add the carboxylic acid to 2-methylpropene. Good yields can be obtained if a strong acid catalyst is used, if water is excluded, and if the temperature is kept low ... 

The Sn1-E1 behavior of tertiary alcohols in strong acids can be used to advantage in the preparation of tert-butyl ethers. If, for example, a mixture of tert-butyl alcohol and methanol is heated in the presence of sulfuric acid, the tertiary alcohol reacts rapidly but reversibly to produce 2-methylpropene by... 

Methyl ferf-butyl ether (Problem 8.71) is prepared by reaction of methanol (AH°f = —238.7kj/mol) with 2-methylpropene, according to the equation... 

Murray and Benavali59 have recently reported the preparation of 1-benzoylphenanthrene 9,10-oxide (125) by photolysis in the presence of oxygen of 1-phenanthrylphenyldiazomethane (126) and also-by ozonolysis of 1-(1-phenanthryl)-l-phenyl-2-methylpropene (127) in 7 and 4% yield, respectively. The carbonyl oxide 128, formed in both cases, reacts intramolecularly to give 125. 

Preparation of ferf-butyl esters from the acid and 2-methylpropene under pressure in... 

The desired alkyl bromide may be prepared by free-radical addition of hydrogen bromide to 2-methylpropene. 

Another good method for the preparation of this ether is the acid-catalyzed addition of ethanol to 2-methylpropene ... 

Butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) are antioxidants that are added to foods and many other organic materials to inhibit decomposition caused by reactions with oxygen. Perhaps you have seen these compounds listed among the ingredients on your cereal box at breakfast. (The mechanism of operation for these antioxidants is described in Section 21.8.) Both of these compounds are prepared by Friedel-Crafts alkylation reactions. BHT is synthesized by the reaction of p-methylphenol with 2-methylpropene in the presence of an acid catalyst. 

Addition of a proton to 2-methylpropene produces the /-butyl carbocation, which then alkylates the ring. Conditions are adjusted so that two /-butyl groups are added. BHA is prepared in a similar manner by the reaction of />-methoxyphenol with 2-methylpropene and an acid catalyst. In this case conditions are adjusted so that only one /-butyl group is added. Because the hydroxy group and the methoxy group are both activating groups, a mixture of products is formed in this case. 

More recently a large variety of different carbonium ions have been prepared in various highly acidic media. For example t-butanol is completely converted to the trimethylcarbonium ion in HS08F— SbF5—S02 at — 60° (Olah et al., 1967). Stable solutions of the same carbonium ion can be obtained by protonation of 2-methylpropene in HF—SbF8 at low temperature (Brouwer et al., 1968). 

BHT is prepared by Friedel-Crafts alkylation of p-cresol with 2-methylpropene. 

With 2-methylpropene as M, both linear and star macromers have been prepared [92-94]. Many kinds of inifers may, of course, be used. For example Kress and Heitz prepared macromers from poly(oxytetramethylene) chains with acrylate or methacrylate end groups, by THF polymerization initiated by superacids with anhydrides as co-initiators - transfer agents [95]. 

Methylcyclopropene has also been prepared by reacting 3-chloro- or bromo-2-methylpropene with sodium amide or lithium di-isopropylamide in mineral oil and the product trapped in an a-dextrin matrix (3). In addition, it has been prepared in mineral oil using catalytic amounts of hexamethyldisilazane (4) as described below ... 

A sample of hexafluoropropene (19 g, 0.13 mol) was introduced with stirring and cooling to O C to a mixture of CsF (1 g, 6.5 mmol), abs diglyme (30 mL), and perfluoro(2-methylpropene) (32 g, 0.16 mol). After 2 h, the reaction mass was poured into a solution prepared from one part coned HCl and five parts HjO. The organic layer was distilled over coned H2SO4 in vacuo. Fractional distillation gave the title compound yield 37 g (84% relative to hexafluoropropene) bp 72 C. 

Cyclopropanation. The cationic Cu(I) complex, which is readily prepared from (5, 5)-r-Bu-box and CuOTf, is the most efficient catalyst available today for the cyclopropanation of mono-and 1,1-disubstituted olefins with diazoacetates. For example, in the reaction of ethyl diazoacetate with 2-methylpropene, >99% ee and high yields can be obtained with this catalyst using substrate to catalyst ratios as high as 1000 1. 

The food preservative BHT is prepared by Priedel-Crafts alkylation < p-methy) phenol (p-cresol) with 2-methylpropene in the presence of acid BHA is prepared similarly by alkylation of p-methoxyphenol. 

Attempts to prepare tetrazolo(l,5-a)pyrimidines in the same manner, from 5-aminotetrazole 329), failed. As a weak acid 329) forms a scarcely soluble sodium salt which in addition shows only poor nucleophilic properties. A conversion could be achieved with 3,3-diethoxy-l-dimethylamino-2-methylpropene 35 b) and 329) (whereas the unsubstituted 35 a) does not give reproducible results) leading to colorless needles of sharp melting point, mp. 116° (82%) which shows by H NMR examination to be in deuteriochloroform an equilibrium mixture of 6-methyltetra-... 

The cleavage of silane into alkylfluorocarbene requires temperatures of at least 140°C. A competitive reaction is the rearrangement of carbene to fluoroalkene. 1-Fluoro-l-fluoromethyl-2,2-dimethylcyclopropane was prepared from trifluoro(l,l,2-trifluoroethyl)silane and 2-methyl-propene see Houben-Weyl, Vol. E19b, pp 1466-1467. Similarly, fluoro(methyl)carbene was generated and underwent addition to 2-methylpropene to give 2. ... 

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