Dionis E. Sunko

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.

1H NMR study of hydrogen bonding of fluorinated alcohols with ethers

Abstract The hydrogen bonding of TFE and HFIP with t-BuOCH 3 (1) and PhOCH 3 (2) was studied using 1 H nmr. The Δ δ values of carbon-bound hydrogens in 1 and 2 show good linear correlation with the alcohol acidity.

Confirmation of the H-bridged structure of the 2-butyl cation by comparison of experimental and ab initio IR frequencies

The IR spectrum of the 2-butyl cation, measured at –125 °C in an SbF5 matrix and calculated ab initio(MP2(FULL)/6-31G*), is characterized by a C(H [graphic omitted] ) C peak at 2175 cm–1 assigned to the nonclassical H-bridged (C2-symmetry) structure 1.

Oxygen Bonding in Haemoglobin

The binding of oxygen molecules to the iron atom In haemoglobin molecules is not fully understood. Nuclear magnetic resonance of bovine oxyhaemoglobin incorporating oxygen-17 can throw some light on the problem.

Neighboring sulfur participation in the solvolysis of 2-(ω-alkylthioalkyl)-3-methyl-2-cyclohexenyl p-nitrobenzoates

Abstract As shown by kinetic and product analysis, the solvolysis of 7 and 8 in 97% TFE includes several competitive reactions, one of them being neighboring sulfur participation and formation of intermediate cyclic sulfonium cation.

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.

Characteristic frequencies of hydrogen-bridged carbocations. IR spectra and ab initio calculations of the nonclassical structures of the cyclooctyl cation

The symmetrically H-bridged structure of the cyclooctyl cation is confirmed definitively by the characteristic IR vibration at 1845 cm–1 and by the agreement of the IGLO chemical shifts calculated with MP2/6-31G* geometry of the chair–boat (CS) conformation with the experimental NMR values.

Rearrangement of bicyclo[3.2.0]heptyl precursors to the 7-norbornyl cation in the cryogenic SbF5 matrix

The first direct observation by IR spectroscopy in the cryogenic SbF5 matrix was made of the rearrangement of 2- and 3-chlorobicyclo[3.2.0]heptanes (3) and (4), respectively, to the 7-norbornyl cation 1 and its subsequent transformation into the 2-norbornyl cation 5.