Stanko Borcic

Secondary hydrogen isotope effects—IX: Solvolysis rates of methyl and methyl-d3 substituted cyclopropylcarbinyl and cyclobutyl derivatives☆

Abstract Methyl and methyl-d3 substituted cyclopropylcarbinyl and cyclobutyl derivatives (I–IV) were prepared and their solvolysis rates measured. It was found that methyl substitution on the ring in cyclopropylcarbinyl derivatives (I, III) produces small but significantly larger rate enhancements than an analogous phenyl substitution. Methyl deuteration has no significant effect on the solvolysis rates. Methyl substitution at the carbinyl position (IV) accelerates the solvolysis rate by a factor of 2. 103 to 4. 103 which is less than usually observed (104–106). 1-Methyl-d3-cyclobutyl methanesulfonate (IIb-D) and 1-cyclopropylethyl-2-d3 chloride (IVc-D) display upon solvolysis in 96% ethanol only one fourth (kH/kD = 1·09) resp. one half (kH/kD = 1·19) of the kinetic secondary β-deuterium isotope effect usually observed for a CD3-group (1·30–1·40). The results are discussed in terms of transition states and carbonium ion intermediates related to bicylobutonium ions (V). The reduced magnitude of the secondary deuterium isotope effects is correlated with small methyl- and phenyl-substitution rate effects. The hypothesis is advanced that secondary β-deuterium isotope effects might be a criterion for charge delocalization such as occurs in the formation of non-classical carbonium ions.

Competitive reactions of nucleophiles : Solvolyses of cyclopropylcarbinyl and cyclobutyl methanesulfonates in the presence of NaBH4

Abstract Cyclopropylcarbinyl (I) and cyclobutyl (II) mesylates were solvolysed in aqueous diglyme with or without added NaBH 4 under a variety of conditions. The reaction products were isolated and analyzed by VPC. A mixture containing cyclopropylcarbinyl, cyclobutyl and minor quantities of homoallyl products, was obtained in all cases. The proportion of the unsaturated compound was greater in hydrocarbon than in carbinol products, the composition of the mixture depending upon reaction conditions and the starting mesylate. Thus, solvolysis of both I and II yielded cyclopropylcarbinol and cyclobutanol in a ratio of nearly 1:1, while in the hydrocarbon products methylcyclopropane predominated over cyclobutane by a factor of 3 in the reactions of I and by a factor of 2 in the reactions of II. It was shown that a direct displacement reaction of the BH 4 ion on the primary mesylate I is not competitive with the solvolysis and cannot therefore be responsible for the above results. It was found that a better yield in hydrocarbons was obtained with I and II than with (1-methylcyclopropyl)carbinyl (III) and 1-methylcyclobutyl (IV) mesylates. The unexpected fact that the BH 4 − ion competes more efficiently with water for intermediates formed in solvolyses of I and II than for those of III and IV was confirmed by determination of competition factors of N 3 − ion vs water. In all cases, intermediates formed from I and II showed more discrimination in their reactions with reagents differing in nucleophylicity than did those formed from III and IV. The results indicate that the attack of the nucleophile occurs mostly on the ion pair formed in the rate determining step. The latter reaction is apparently subject to reactivity relationships typical for a direct displacement reaction and not characteristic for free carbonium ions. These facts should be borne in mind when conclusions concerning the structure of carbonium ions are drawn from the composition and structure of solvolysis products. It is concluded that the ion pairs trapped by NaBH 4 have different structures, depending upon the starting isomeric mesylate. It is shown that the formation of an equilibrating mixture of classical ions or ion pairs as intermediates in these reactions cannot account for experimental results.

The “bicyclobutonium ion”: Reaction of (1-methylcyclopropyl)carbinyl and 1-methyl cyclobutyl methanesulfonates with sodium borohydride under solvolytic conditions

Abstract The reaction of both, (1-methylcyclopropyl)carbinyl mesylate and 1-methylcyclobutyl mesylate, with NaBH 4 in aqueous diglyme affords a mixture of 1,1-dimethylcyclopropane and methylcyclobutane in addition to 1-methylcyclobutanol. The hydrolysis of both mesylates resulted in the formation of only one product, i.e. 1-methylcyclobutanol. The results obtained when the amounts of BH 4 - and water were varied indicate that the reaction of the primary mesulate with NaBH 4 proceeds by two competitive mechanisms (S N 1 and S N 2). The results are rationalized in terms of nonclassical ions and the inadequacy of the equilibrating classical ion pair hypothesis is discussed. The suggestion is made that the described method might be of use in some cases for distinguishing between classical and nonclassical carbonium ions.

Competitive reactions of nucleophiles—III : The azide probe

Abstract The azide competition factors (kN3,−/kSOH) of cyclopentyl mesylate (1), 1-methylcyclopentyl chloride (2), cyclohexyl brosylate (3), benzhydryl chloride (4), benzhydryl bromide (5), 3β-cholestanyl brosylate (6), 3α-cholestanyl brosylate (7), 2-methyl-2-adamantyl chloride (8), 1-adamantyl bromide (9) and 2-adamantyl tosylate (10) were determined. Tertiary substrates (2,8,9) invariably gave lower kN3−/kSOH values than secondary ones (1, 3, 4, 5, 7); opposite from what is expected on the basis of the stabilities of the respective free carbanium ions. This was explained by the attack of the nucleophile on ion pair(s) rather than on free carbonium ions. The magnitude of the competition factor seems to yield useful mechanistic information only in two extreme cases, i.e. direct displacement (SN2) and free stable carbonium ions (tertiary aromatic substrates-dissociative SN1 mechanism). In all other cases, especially with secondary substrates, the mechanistic evaluation of the kN3−/kSOH values is ambiguous and in our opinion of doubtful value.

A case of extended π-participation. Solvolysis rate of 1-phenyl-5,9-dimethylundeca-5,9-dienyl chloride

The title compound (2) solvolyses nine times faster than 1-phenyl-5-enyl chloride (1U) owing to participation of both aliphatic double bonds.

Competitive reactions of nucleophiles—II: Solvolysis of α,α,- and γ,γ-dimethylallyl chlorides☆

Abstract α,α,-Dimethylallyl (IT) and γ,γ-dimethylallyl (IP) chlorides were solvolyzed in water, absolute and aqueous ethanol in the presence of an excess of base (CaCO3, NaOEt or NaBH4). The reaction products were isolated and analyzed by glc. Both primary (P) and tertiary (T) products were obtained in all cases. The P/T product ratio from both chlorides increases with increase of nucleophilicity of attacking reagent, but this increase is considerably faster with the primary chloride (IP) indicating an SN2 reaction of IP with stronger nucleophiles. Under the same conditions the Sn2′ reaction seems to be unimportant. The results indicate that stronger nucleophiles attack intimate ion pairs and the undissociated primary substrate rather than dissociated solvated cations. The composition of the solvolysis product mixture depends, therefore, upon both the charge distribution in the cationic part of the ion pair (favoring tertiary products) and on reactivity relationships characteristic for direct displacement reactions (favoring primary products). t-Butanol and borohydride yield almost exclusively primary products under solvolytic condition indicating the importance of steric factors. The difference in behaviour between the solvated cation and an ion pair should be considered whenever experimental results are interpreted in terms of structure of solvolysis intermediates.

Lack of secondary β-deuterium kinetic isotope effect in the solvolysis of 2-chloro-2,6,10-trimethyldodeca-6,10-diene. Indication of extended Π-participation

The title compound (3) with two deuteriated methyl groups attached to the reaction centre undergoes solvolysis without a kinetic isotope effect indicating participation of both double bonds.

Rate enhancements and secondary β-deuterium kinetic isotope effects as criteria of neighbouring group participation. Solvolysis of some tertiary alk-5-enyl chlorides

Rate enhancements in solvolysis of several of the title compounds are small or zero but significantly reduced secondary β-deuterium kinetic isotope effects indicate participation of the double bond.