2-Methylpropene dimerization

The two dimers of 2 methylpropene shown in the equation can be converted to 2 2 4 trimethylpentane (known by its common name isooctane) for use as a gasoline additive Can you suggest a method for this conversion ... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

FIGURE 6 16 Mechanism of acid catalyzed dimerization of 2 methylpropene... 

The first-formed cation (42) can add to a second molecule of 2-methylpropene (41) to yield the new (dimeric) cation (43) this in turn can lose a proton to yield the C8 alkene (44) or, alternatively, add to a third molecule of alkene to yield the (trimeric) cation (45), and so on. 

The dimerization of a,/3-unsaturated carbonyl compounds is a difficult reaction and where alternative processes are possible these will occur in preference. Thus, a mixture of propenal and 2-methylpropenal yields 2-formyl-5-methyl-3,4-dihydropyran no dimeric products are observed (equation 7). 

By tar the most widespread type of photocycioaddition in synthetic applications employs thealkene unit of an a, 3-unsaturated carbonyl compound os one of the addends. Simple examples of such reactions are the dimerization of cydopent-2-enone (2.71), and the photoaddition of 2-methylpropene to cyclohex-2-enone (2.72). The function of the carbonyl group is to bring the absorption of the alkene into... 

The mixed dimerization of perfluoro(2-methylpropene) (12) with fluorinated alkenes F2C = CFX (X = Cl, Br, CF3, Ph) in the presence of cesium fluoride leads to adducts 13 by nucleophilic addition of the (Cp3)3C anion and /1-elimination of fluoride anion. 

Perfluoroalkenes, such as hexafluoropropene (11) or perfluoro(2-methylpropene) (12), can be dimerized by heating with cesium fluoride.367-371... 

After three days on mixing, an organic phase was formed above the aqueous solution. The analysis of this new phase showed the presence of free butenes cis-2-butene (18%), trans-2-butene (41%) and 1-butene (41%). This aqueous catalytic dimerization of ethylene is not stereoselective (Scheme 1), all possible butenes, except for 2-methylpropene, being formed. No higher oligomers were detected, probably due to the low solubility of butenes in aqueous solutions and to the lack of their coordination to Ru(II). Besides [Ru(CH2=CH2)(H20)5]2+ and cis-[Ru(CH2=CH2)2(H20)4]2+, no more highly substituted Ru(II) complexes were isolated or detected. 

This aqueous catalytic dimerization reaction is not stereoselective, and all the possible butenes are formed except 2-methylpropene. These results show the utility of NMR studies under gas pressure for the study of catalytic reactions in aqueous solutions, which are increasingly attractive because of ecological considerations. 

Evidence for the intermediacy of a carbonyl oxide was found by Criegee in the ozonolysis of 2,3-dimethylbut-2-ene (5.100). In an inert solvent the cyclic peroxide and acetone were obtained. However, when formaldehyde was added to the reaction mixture, the known ozonide of 2-methylpropene was isolated. In the first case the intermediate carbonyl oxide dimerizes, but in the second it reacts preferentially with the reactive formaldehyde. 

Hydroboration of 1-Methylcyclopentene 248 6.10 Acid-Catalyzed Dimerization of 2-Methylpropene 262... 

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