H. Steinberg

The chemistry of small ring compounds—XIV : Acidity of cyclopropanes with electron-withdrawing substituents

Abstract The hydrogen isotope exchange rates of α-deuterio- and α-tritiocyclopropanes with electronwithdrawing substituents and the corresponding open 2-substituted propanes have been measured in solutions of sodium methoxide in methanol. Substituents fall in two classes: CF3, CN and ∅SO2 make the α-hydrogen in cyclopropanes more acidic than in the open chain analogues. On the other hand acyl groups (t-C4H9CO, iso-C3H7CO, C6H5CO) make the cyclopropanes less acidic than the corresponding 2-substituted propanes. An explanation based on inductive and mesomeric stabilisation of carbanions is given. A large isotope effect has been found for bulky isopropyl t-butyl ketone, suggesting rate limiting hydrogen abstraction by base. In most other cases (Table 2) isotope effects are small, presumably because anions remain strongly associated with methanol by hydrogen bonding (e.g. R− … DOCH3) and consequently solvent exchange (with MeOH) becomes rate limiting for D/H (T/H) exchange.

The chemistry of small ring compounds—XX : Cidnp effects in the oxidation of 2,2-dimethylcyclo- propanone methyl hemiacetal

Abstract Mild oxidation of 2,2-dimethylcyclopropanone methyl hemiacetal ( 1 ) with oxygen, t-butyl- hydroperoxide or di t-butylperoxalate leads to hydrogen abstraction from the OH group of 1 . The cyclopropyl ring probably breaks simultaneously to give ring-opened tertiary radical 3a and primary radical 3b . In CDCl 3 , CIDNP effects are observed in the saturated disproportionation product of 3a and also in products resulting from disproportionation and combination of 3a with solvent derived CCl3 3 and CDCl 2 radicals. The large difference in g -values between individual radical components in each of these latter pairs explains the observed emissions. In benzene, solvent does not interfere and CIDNP effects mainly arise from radical pairs, consisting of identical components, e.g. two radicals 3a or at least of components with practically equal g -values. Thus disproportionation leads to absorption-emission (multiplets effects) for the unsaturated esters 5 and 6 and the isovalerate 8 .

Base catalysed hydrogen exchange in cyclopropylcyanide

Abstract The exchange of the α-hydrogen atom in cyclopropylcyanide has been studied in methanol and sodium methoxide as a base. The reaction is both first order in base and in cyclopropylcyanide. Using base concentrations from 0·01 M up to 3·0M the logarithm of the exchange rate constant depends linearly on the H − function. For the H/D exchange, the following activation parameters have been calculated: ΔH * = 27·2 ± 0·2 kcal/mol, ΔS * = 15·7 ± 0·8 e.u. (326°K). The kinetic isotope effect is found to be k D / k T = 1·33 and k H / k D = 1·9. This suggests that the slowest reaction step is not proton abstraction, but the exchange between labelled and unlabelled solvent molecules in a relatively rapidly formed solvated carbanion.

Reaction of alkylidenemalononitriles with diazomethane

At −15° C isopropylidenemalononitrile (5) reacts with anequimolar amount of diazomethane to yield the pyrazoline8, which loses nitrogen upon heating, to give 2,2-dimethyl-1,1-cyclopropanedicarbonitrile (9), and 2-butylidenemalononitrile (6). At room temperature with anexcess of diazomethane5 gives a number of homologous 2-methyl-2-n-alkyl-1,1-cyclopropane-dicarbonitriles9–13 via the corresponding homologous alkylidenemalononitriles, as supported by the similarity of products obtained from the reaction of 2-pentylidenemalononitrile (7) and diazomethane.

REDUCTIVE REMOVAL OF HALOGEN BY THIOLATES FROM 1-HALOCYCLO-PROPYL SULFIDES

Abstract Nucleophilic substitution of cyclopropyl halides or tosylates is often accompanied by ring rupture, but this can be prevented by the presence of an electron releasing substituent in α-position (NR2, OR, SR). Alkylthio and arylthio groups are particularly effective in stabilizing intermediate cyclopropyl cations. These give with most nucleophiles e.g. alkoxide, azide, fluoride, in relatively rapid reactions good yields of substitution products.