Methylcyclopropane, hydrogenation

There is one more surprise to come. On two single-crystal faces of iridium, n-butane was the major product of methylcyclopropane hydrogenation at about 400 K, its selectivity being approximately 99%. It is difficult to be precise because results are only shown graphically, and n-butenes were also formed at higher temperatures. Such a complete reversal of normal behaviour is quite astonishing. 

The radical is generated by photolytic decomposition of di-/-butyl peroxide in methylcy-clopropane, a process that leads to selective abstraction of a methyl hydrogen from methylcyclopropane ... 

Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene rearrangement, there is no general method for the rearrangement of alkynyl-cyclopropane to cyclopentene derivatives. One specific example is the pyrolysis of l-ethynyl-2-methylcyclopropane to methylenecyclopentene and other compounds [5]. At 530°C, l-ethynyl-2-methylcyclopropane (1) undergoes a [1,5]-hydrogen shift to give hexa-l,2,5-triene (2), which further isomerizes to methy-lenecyclopentenes 3 and 4 in 38 and 29% yield, respectively (Scheme 1). 

The recent study by Lias and Ausloos at 1470 and 1236 A has confirmed many of the results of other workers. The molecular elimination of methane by reaction (5) and the elimination of molecular hydrogen by reaction (3) were confirmed. Methylcyclopropane and butene-2 were noted as products. An important finding of this study was evidence for the elimination of methylene from isobutane, viz. 

The fact that the decomposition of ionized methylpropene (106) at longer lifetimes is preceded not only by hydrogen randomization but also by participation of the C(2) carbon atom (indicated with an asterisk in Scheme 14) in the expulsion of ethylene has been explained by invoking the intermediate existence of methylcyclopropane (107) and 2-butene (108) (Scheme 14). This is in line with the finding that direct ionization of neutral methylcyclopropane gives 107 which is known to undergo ethylene loss. The isomerization sequence described in Scheme 14 can be viewed as a further example for a mass spectrometric dyotropic rearrangement ". ... 

Other findings which show the difficulty in forming the cyclopropyl radical by some radical molecule reactions are the failure of chlorine atoms to abstract the tertiary ring hydrogen from methylcyclopropane and the failure of r-butoxy radicals to abstract the... 

The effect of spillover onto the aluminas included unusual selectivities in the isomerization of methylcyclopropane and in the hydrogenation of butenes and benzene. 

Many cyclopropyl chlorides and bromides have been converted to alkoxycyclopropanes by treatment with a strong base, in most cases potassium rerf-butoxide, in an appropriate organic solvent (Table 13). Under such conditions, hydrogen halide elimination takes place, yielding strained cyclopropene intermediates, which are trapped by nucleophilic attack of the alkoxide. Overall, a simple substitution occurs when a bond is formed between the alkoxide group and the carbon atom to which the halide was attached. This is the case when l-chloro-5-methyl-exo-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (1) was reacted with potassium /ert-butoxide l-/er/-butoxy-5-methyl-ent/o-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one (2) was isolated in 94% yield.If a C-O bond is established at the other olefinic carbon atom, a C H bond is concomitantly formed at the carbon atom, to which the halide was attached. The result is a double substitution which is discussed elsewhere (see Section 5.2.1.3 ). When the substrate contains more than one halogen atom, several elimination reactions usually take place. Thus, treatment of 1 -bromo-2-chloro-2-methylcyclopropane (3) with an excess of potassium /er/-butoxide gave l-ter/-butoxy-2-methylenecyclopropane (4) in 30% yield. 

In contrast to the methylene hydrogens, ring hydrogens can be quite labile when substituted by deuterium atoms. Thus, in the case of ethyl (2-bromo-2-methylcyclopropane-l-(/)carboxylate (20), reaction with potassium hydride in dimethyl sulfoxide led to substantial loss of the deuterium label. However, use of deuterated dimethyl sulfoxide as the solvent led to 99% deuterium retention in the product, ethyl (2-methylenecyclopropane-l-[Pg.1441]

If other reactive moieties are present a cyclopropanecarbonyl compound can be converted to the corresponding cyclopropylalkane using various conditions. Thus, 3,4-benzotricy-clo[4.3.1.0 ]dec-3-en-2-one yielded 3,4-benzotricyclo[4.3.1.0 ]dec-3-ene in excellent yield on treatment with sodium in liquid ammonia. di-l-Methylcyclopropane-l,2-dicarboxylic anhydride underwent a similar reaction to afford isomeric lactones on treatment with lithium aluminum hydride or sodium borohydride in tetrahydrofuran. On the other hand, ionic hydrogenation (triethylsilane in trifluoroacetic acid and water) of cyclopropyl phenyl ketone gave a complex reaction mixture and very little benzylcyclopropane. ... 

Hydrogen bromide underwent rapid addition to methylcyclopropane in the presence of bromine to give predominantly 2-bromobutane (la, 50% yield). A small amount (10%) of disubstituted butanes 3a and 4a was also isolated. A 100% excess of bromine over the cyclopropane was added in order to convert alkene products into dibromides. The reaction was carried out in the dark to avoid radical reactions of bromine. A similar result was obtained with ethylcy-clopropane. cis- and /ran5-l,2-Dimethylcyclopropane gave a mixture of four isomeric bro-mopentanes in 76% and 89% yield, respectively, on treatment with hydrogen bromide/bromine at -78°C. 3... 

The reactions of alkylcyclopropanes with bromine and with hydrogen bromide have been investigated in detail. Methylcyclopropane (4, R = Me) scarcely underwent addition of bromine in the cold when light and catalysts were excluded. Similar observations were made with other alkyl-substituted cyclopropanes 4 such as ethylcyclopropane, butylcyclopropane, cis-and trans-1,2-dimethylcyclopropane. ... 

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