Isomerization 2,3-dimethyl-2-butene

The Arrhenius parameters for the gas-phase unimolecular structural isomerizations of 1,1,2-trimethylcyclopropane28 to three isomeric methylpentenes and two dimethyl-butenes, and of 1,1,2,2-tetramethylcyclopropane29 to 2,4-dimethylpent-2-ene have been determined over a wide range of temperatures. Despite previous reports on substantial decreases in activation energies for structural isomerizations of methyl-substituted cyclopropanes, this study has revealed that the trend does not continue beyond dimethylcyclopropane isomerization. 

Typic2il examples of acid-catalysis of heteropoly compounds are as follows Dehydration of methanol, - > ethanol, - - propanol - - - -"- "- > and butanol, conversion of metanol or dimethyl ether to hydrocarbons, etheration to form methyl /-butyl ether, esterifications of acetic acid by ethanol and pentanol, decomposition of carboxylic acid and formic acid, alkylation of benzene by ethylene and isomerization of butene, o-xylene and hexane. ... 

The following points may be noted. First, in 7.5 and 7.7 other /3-carbons are also available and eliminations from those will produce isomers of alkenes, which are not shown. Second, 7.1 can catalyze the isomerization of the products into the other isomers of -hexenes, methyl pentenes, and dimethyl butenes by the chain walk type of mechanism discussed earlier. 

Reaction of 2,3-dimethyl- 1-butene with HBr leads to an alky) bromide, CfcH Br. On treatment of this alkyl bromide with KOH in methanol, elimination of HBr to give an alkene occurs and a hydrocarbon that is isomeric with the starting alkene is formed. What is the structure of this hydrocarbon, and how do you think it is formed from the alkyl bromide ... 

Branching at an olefinic carbon atom inhibited the reaction markedly, the most dramatic case being that of 2,3-dimethyl-2-butene. It should be noted that the product in this case is nearly exclusively 3,4-dimethylpen-taldehyde for either cobalt or rhodium catalysis (7). Thus, a general rule that products containing a formyl group attached to a quaternary carbon atom are not formed (49) remains valid. Hydroformylation proceeds only after isomerization has occurred. 

In contrast to the examples of selectivity control discussed in the previous sections, the problem here is control of the regioselectivity of the individual reaction steps. This is evident from the Scheme 5. In the first reaction step the nickel-hydride species adds to propene forming a propyl- or isopropyl-nickel intermediate this step is reversible, and the ratio of the two species can be controlled both thermodynamically and kinetically. In the second step, a second molecule of propene reacts to give four alkylnickel intermediates from which, after j8-H elimination, 8 primary products are produced (Scheme 5). 2-Hexene and 4-methyl-2-pentene could be the products of either isomerization or the primary reaction. Isomerization leads to 3-hexene, 2-methyl-2-pentene (the common isomerization product of 2-methyl-1-pentene and 4-methyl-2-pen-tene), and 2.3-dimethyl-2-butene. It can be seen from the Scheme 5 that, if the isomerization to 2-methyl-2-pentene can be neglected, the distribution of the products enables an estimate to be made of the direction of... 

Ceric ammonium nitrate promoted oxidative addition of silyl enol ethers to 1,3-butadiene affords 1 1 mixtures of 4-(/J-oxoalkyl)-substituted 3-nitroxy-l-butene and l-nitroxy-2-butene27. Palladium(0)-catalyzed alkylation of the nitroxy isomeric mixture takes place through a common ij3 palladium complex which undergoes nucleophilic attack almost exclusively at the less substituted allylic carbon. Thus, oxidative addition of the silyl enol ether of 1-indanone to 1,3-butadiene followed by palladium-catalyzed substitution with sodium dimethyl malonate afforded 42% of a 19 1 mixture of methyl ( )-2-(methoxycarbonyl)-6-(l-oxo-2-indanyl)-4-hexenoate (5) and methyl 2-(methoxycarbonyl)-4-(l-oxo-2-indanyl)-3-vinylbutanoate (6), respectively (equation 12). 

Ruthenium complexes B also undergo fast reaction with terminal alkenes, but only slow or no reaction with internal alkenes. Sterically demanding olefins such as, e.g., 3,3-dimethyl-l-butene, or conjugated or cumulated dienes cannot be metathesized with complexes B. These catalysts generally have a higher tendency to form cyclic oligomers from dienes than do molybdenum-based catalysts. With enol ethers and enamines irreversible formation of catalytically inactive complexes occurs [582] (see Section 2.1.9). Isomerization of allyl ethers to enol ethers has been observed with complexes B [582]. 

In 1962, Kennedy reported the first isomerization polymerization using 3-methyl-1-butene to give a 1,1-dimethyl PP as below ... 

Clinoptilolite Isomerization of n-butene, the dehydration of methanol to dimethyl ether, and the hydration of acetylene to acetaldehyde... 

Section I1,D,1 The kinetics have been studied for competitive formation of 3//-pyrazoles and cyclopropenes thermally from the isomeric vinyldiazo compounds 2,3-dimethyl-l-phenyl-l-diazo-2-butene and 4-methyI-3-phenyl-2-diazo-3-pentene. The higher (12 kJ/mol) ground state energy of the latter accounts almost entirely for its larger (x68) rate of cyclization to a 3H-pyrazole, relative to its isomer.172... 

For example, the ene reaction proceeds stereoselectively in a suprafacial manner specifically, oxygen attack and hydrogen removal take place on the same side of the double bond. As a result, neither racemization of optically active com-pounds nor cis-trans isomerization occurs. Diradical or dipolar intermediates are not consistent with these observations. The lack of a Markovnikov-type directing effect also rules out dipolar intermediates.360 399 Isotope effects observed in the reaction of different deuterium-labeled 2,3-dimethyl-2-butenes indicated that... 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of ds-2,3-dimethyl thiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

The olefin 3,3-dimethyl-l-butene (III) was studied since it gave almost exclusively the expected product 3,3-dimethyl-l-buten-2-yl acetate (VII) with only traces of ketone or 3,3-dimethyl-l-buten-l-yl acetate. With a single product the test of material balance was simplified since it was possible to run a blank on the wet separation and then check the yield of VII by isotope dilution using (CH3)3CC(OCOCD3)=CH2. Experiments with III are not complicated by isomerization or allylic palladium complex formation and are unlikely to involve free radical reactions. 

Fluoride ion produced from the nucleophilic addition-elimination reactions of fluoroolefms can catalyze isomerizations and rearrangements The reaction of per fluoro-3-methyl-l-butene with dimethylamine gives as products -N,N dimeth-y lamino-1,1,2,2,4,4,4-heptafluoro-3-trifluoromethylbutane, N, JV-dimethyl-2,2,4,4,4-pentafluoro 3 trifluoromethylbutyramide, and approximately 3% of an unidentified olefin [JO] The butylamide results from hydrolysis of the observed tertiary amine, and thus they share a common intermediate, l-JV,AJ-dimethylamino-l,l 24 44-hexafluoro-3-trifluoromethyI-2-butene, the product from the initial addition-elimination reaction (equation 4) The expected product from simple addition was not found... 

When either an alcohol or an amine function is present in the alkene, the possibility for lactone or lactam formation exists. Cobalt or rhodium catalysts convert 2,2-dimethyl-3-buten-l-ol to 2,3,3-trimethyl- y-butyrolactone, with minor amounts of the 8-lactone being formed (equation 51).2 In this case, isomerization of the double bond is not possible. The reaction of allyl alcohols catalyzed by cobalt or rhodium is carried out under reaction conditions that are severe, so isomerization to propanal occurs rapidly. Running the reaction in acetonitrile provides a 60% yield of lactone, while a rhodium carbonyl catalyst in the presence of an amine gives butane-1,4-diol in 60-70% (equation 52).8 A mild method of converting allyl and homoallyl alcohols to lactones utilizes the palladium chloride/copper chloride catalyst system (Table 6).79,82 83... 

Scheme 9.17 Product distributions from the reaction of isomeric d6-2,3-dimethyl-2-butenes with N-methyltriazolidinedione. Products a, b, c and d arise form H or D transfer from sites a, b, c and d in the isomeric butenes.

For example, molecular mechanics calculations (MM2) showed that the methyl group geminal to the neopentyl group in 2,3,5,5-tetramethyl-2-hexene (51, Scheme 18) has the lowest rotational barrier and is the most reactive [79], Furthermore, the ethyl group in 2,3-dimethyl-2-pentene (52) has a much higher rotational barrier (5.76 kcal/mol) than the methyl groups and is totally inactive to 102. Similar trends hold with 2-methyl-2-butene (4). However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of 102 with alkenes. As it was shown later [62], it is the nonbonded interactions in the isomeric... 

It is, however, also possible to find elementary reactions with very similar rates in gas and solution phases. Thus, the thermally-activated unimolecular isomerization of some substituted cyclo-butenes in a solution (of dimethyl phtalate at temperatures around 275°C) has been found to proceed at, essentially, the same rate as in the gas phase. 

The isomerized structure dominates even at - 100° C, but accounts for only 70% of the repeat units at - 130° C. Similar but more complicated structures are formed in 4-methyl-1-butene polymerizations by competing hydride and methide shifts [298]. Other monomers whose propagating carbenium ions isomerize include 5-methyl-l-hexene, 4,4-dimethyl-1-pen-tene and some terpenes [299]. 

Dimethyl hexanes are believed to result mainly from codimerization of butene-l and isobutene, followed by abstraction of a hydride ion from isobutane. This mechanism differs significantly from previously published theory. Thus, high initial concentrations of butene-l favor dimethylhex-ane formation. Some isomerization of dimethylhexyl carbonium ions occurs. 

Cyclohexene exists only as internal cw-olefin and is moderately reactive. In contrast, -hexene, regardless of whether charged as 1- or internal olefin or a mixture of these, is quickly isomerized to a near-equilibrium mixture containing some 5 to 10% of the isomer with terminal double bond, whose reactivity is about two orders of magnitude higher than those with internal ones. Accordingly, -hexene is more reactive than cyclohexene with only internal double-bond positions. Lastly, neohexene (3,3 -dimethyl-l-butene) has its double bond locked in the terminal position —no double bond can exist adjacent to a quaternary carbon atom—and so should have the highest reactivity if not sterically hindered. (This is an unsubstantiated prediction, as the hydroformylation reactivity of that olefin seems not to have been studied to date.)... 

Less stable alkenes can be isomerized to more stable alkenes by treatment with strong acid. For example, 2,3-dimethyl-1-butene is converted to 2,3-dimethyl-2-butene when treated with H2SO4. Draw a stepwise mechanism for this isomerization process. 

Both T-substituted 1-pentene and 3-methyl-1-butene were observed in yields comparable to the other T-pentene isomers. This observation clearly indicates that the primary source of T-pentene in these systems is through the loss of H atoms from the excited radicals formed by hot tritium addition with opening of the cyclopropane ring. The almost lack of isomerization of the T-substituted parent molecule set an upper limit for the energy left in the T-substituted parent molecule due to the reaction with energetic tritium the upper limit is equal to the activation energy for isomerization (65 kcal mol for cyclopropane and methylcyclopropane and about 62 kcal mol for ethylcyclopropane and dimethyl-cyclopropane. ... 

As catalytic tests four reactions, isomerization of 1-butene and methyloxirane, dehydration of 2-propanol and the pinacol rearrangement of 2,3-dimethyl-2,3-butanediol were used. 

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