Trans-2-Methylcyclopropyl ketone

Highly enantioselective hydrogenation of simple dialkyl ketones is limited to a specific case. Cydopropyl methyl ketone or methyl 1-methylcyclopropyl ketone, for example, can be hydrogenated in 95-98% optical yield in the presence of trans-S)-xylbinap (S)-daipen ",4>RuCl2 (S)-xylbinap (S)-daipen /KOC(CH3)3 [67a, 81c[. 

The main products in the reaction mixtures of the 2,2-dimethylcyclopropyl ketone 39 and the cis-2-methylcyclopropyl ketone cis-40, respectively, result from C-l-C-2 bond breaking. In contrast, the frartj-2-methylcyclopropyl ketone trans-41 breaks the C-l-C-3 bond. This strongly suggests that steric factors control the direction of cleavage, presumably through asymmetric overlap of the carbonyl 7r-system with one of the cyclopropane bonds 43). In the absence of these steric effects, as in trans-41, the bond that cleaves is the one to give the more thermodynamically stable (= less substituted) carbanion intermediate. 

It has been found that cyclopropyl ketone exhibits a conformer mixture The preferred component has been identified as the bisected s-c/s conformer At the present time, however, it is not clear whether the second conformer (less stable) corresponds to the bisected s-trans form or anti-gauche form. On the other hand, on the basis of a theoretical analysis (on the semiempirical INDO-level) and a proton NMR investigation for the cis-trans isomeric 2-methylcyclopropyl-l (methyl) ketones 15 and 16 it was suggested that the trans compound 15 exists as an equilibrium of the bisected s-cis and s-trans conformers. The s-c/s and s-trans conformers are separated by a rotational barrier of 5.7 kcal mol ... 

In the case of 1-alkylated 1,2-cyclobutanediols, the tertiary hydroxyl is the preferred or exclusive leaving group. With 1,2-dialkylated diols, the trans isomers were normally found to rearrange somewhat more easily than the cis compounds. For instance, trans-1,2-dimethyl-l,2-cyclobutanediol (243) underwent immediate quantitative ring contraction to (1-methylcyclopropyl) methyl ketone (244) upon treatment with boron trifluoride etherate at room temperature while the ring contraction of the cis isomer (245) was observed only on heating at 70°C for 5 min in the presence of BF3 and Et20 (equation 166) . 

---

See also in sourсe #XX -- [ ]

---