Cyclopropanes, alkyl-substituted, hydrogenation

Relatively few studies of the hydrogenation of substituted cyclopropanes are extant. From the studies which have been reported [26,248, 258], it would appear that, with alkyl-substituted cyclopropanes, ring cleavage occurs by rupture of the bond opposite the carbon atom carrying the greatest number of substituent groups. Thus in the platinum on... 

Catalytic hydrogenation of alkyl-substituted cyclopropanes usually breaks the least hindered bond . This constitutes a nice method for converting ketones into geminal dimethyl compounds (equation 13) , a principle realized for the syntheses of natural and unnaturaP products. 

When 7,7-dichlorobicyclo[4.1.0]heptane was reacted with potassium tert-butoxide in dimethyl sulfoxide in the presence of a C-H acid, elimination of hydrogen chloride, giving 7-chloro-bicyclo[4.1.0]hept-l(7)-ene, was followed by addition to the cyclopropene of the carbanion obtained from the C-H acid. The course of reaction was sensitive to several factors, among which the C-H acid employed was of significant importance. -Thus, when diethyl mal-onate was used several benzene derivatives were obtained and alkyl-substituted cyclopropanes were afforded, but in low yields. However, dehydrochlorination in the presence of 2-phenyl-propanonitrile gave 2-(bicyclo[4.1.0]hept-5-en-l-yl)-2-phenylpropanonitrile (1) and minor amounts of 2-ethyltoluene. ... 

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. ... 

Figure II.6. Hydrogenation of alkyl-substituted cyclopropanes dependence of rate of the preferred splitting route on hydrogen pressure for silica-supported metals (curves for fresh and run-in catalysts are usually similar).

Lactobacillic acid, Ci9H3g02, isolated by Hofmann et al. (1952) is a saturated acid with two less hydrogen atoms than a normal C19 saturated acid. These facts suggest a cyclic structure. A band found at 1020 cm is characteristic of alkyl-substituted cyclopropanes. The structure of lactobacillic acid (XXVIII) is, therefore... 

Usually, the formation of a new chiral centre involves the conversion of a prochiral sp carbon atom into one with sp hybridisation, the methods most generally used being the aldol and related condensations, pericyclic reactions (especially the Diels-Alder reaction), epoxidation, cyclopropanation and additions to double bonds (hydrogenation and hydroboration). Another possibility is the conversion of a prochiral sp carbon atom into a chiral centre, as for instance in the a-substitution (alkylation, halogenation, etc.) of a ketone. 

The reaction of acceptor-substituted carbene complexes with alcohols to yield ethers is a valuable alternative to other etherification reactions [1152,1209-1211], This reaction generally proceeds faster than cyclopropanation [1176], As in other transformations with electrophilic carbene complexes, the reaction conditions are mild and well-suited to base- or acid-sensitive substrates [1212], As an illustrative example, Experimental Procedure 4.2.4 describes the carbene-mediated etherification of a serine derivative. This type of substrate is very difficult to etherify under basic conditions (e.g. NaH, alkyl halide [1213]), because of an intramolecular hydrogen-bond between the nitrogen-bound hydrogen and the hydroxy group. Further, upon treatment with bases serine ethers readily eliminate alkoxide to give acrylates. With the aid of electrophilic carbene complexes, however, acceptable yields of 0-alkylated serine derivatives can be obtained. 

A series of substituted 2,2,4,4-tetramethylcyclobutanones in which the substituent is alkyl, aryl, hydroxy, amino, or imino yield on photolysis in methanol or ethanol the corresponding alkoxytetrahydrofuran [Eq. (112)] in addition to a cyclopropane derivative formed by decarbonylation.429 Earlier examples of this process occurring in 7,7-dimethyl[3.2.0]bicyclohept-2-en-6-one430 and in (+)-cyclocam-phanone431 have been reported. In all cases, intramolecular hydrogen abstraction is unfavorable, and a carbene is postulated as the intermediate species. 

In 2004, Vignola and List [111] demonstrated the ability of proline-derived catalysts to overcome drawbacks associated with the stoichiometric alkylation of preformed aldehyde enolates when they described an elegant amino acid catalyzed intramolecular a-alkylation reaction of haloaldehydes. The reaction furnished substituted cyclopentanes, cyclopropanes, and pyrrolidines in good yields and good enantio-selectivities (Scheme 8.23), when commercially available (5)-a-methyl proline (LV) as catalyst was used. The presence of a stoichiometric amount of additional base (tertiary amine) was required, not only to trap the hydrogen halide produced in the reaction but also because it has also significant effect on the stereoselectivity of the C—C bond-formation process by stabilizing the ant/ -TS of the /ra 5-enamine intermediate. Nevertheless, an intermolecular version of the reaction remains still elusive, mainly because of the deactivation of the amine catalyst by A -alkylation with the alkyl halide [112]. 

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