Ring-opening reactions. 3. Mechanistic path vs. ring-strain control in elimination and substitution reactions of 1,1-dimethyl cyclic ammonium ions and their .alpha.,.alpha.'-dimethyl-substituted derivatives

Abstract

From the determination of product composition and overall second-order rate constants for the reactions of a number of cyclic quaternary ammonium ions with sodium methoxide in methanol and previous similar data, a complete set of partial rate coefficients for the ring-opening reactions (substitution and elimination) of 1,l-dimethyl cyclic ammonium ions and their a,&-dimethyl-substituted derivatives for ring sizes 4-6 was made available. Such reactions are sensitive probes for steric strain and reaction mechanism requirements in the @-elimination reaction and for the different stereochemical requirements of elimination vs. substitution.Ring opening of cyclic quaternary ammonium ions under the action of anionic reagents may occur by either substitution or elimination. In the preceding paper' we have reported that the ring-opening reactivity of such substrates appears to be affected by two major factors, i.e., the ring strain and the geometrical requirements of the reaction. The role of the latter factor was made apparent by the fact that the five-membered ring is more reactive in the ringopening substitution reaction and less reactive in the ring-opening elimination reaction than expected from ring strain effects. If methyl groups are present a t the a positions of the heterocyclic ring, a @-hydrogen is expected

Alkaline Stability of Quaternary Ammonium Cations for Alkaline Fuel Cell Membranes and Ionic Liquids

“…In DMPy, on the other hand, such a conformation is unfavorable due to ring strain. Freely rotating CH 3 groups with easily accessible b-protons then lead to rapid ring-opening elimination, [74] increasing the overall degradation rate by orders of magnitude because the molecular structure basically contains ethyl groups and the reaction can proceed as in an open-chain QA. The transition state for the elimination of a b-proton in DMP or ASU requires energetically expensive bond angles and lengths.…”

“…[45] Product analysis revealed disfavored elimination, with the rate of the ring-opening substitution related to ring strain. [45,73,74] The balance between ring strain and transitionstate energy determines which of the three possible degradation pathways ( Figure 10) predominates in an N,N-dimethylammonium heterocycle. For DMP this is substitution at the methyl groups whereas both ring-opening substitution and methyl substitution seem to occur in DMPy (see the Supporting Information); however, in alkaline methanolic solutions at T = 130 8C ring-opening substitution appears to dominate.…”

“…See also references by Lillocci et al[45,73,74] For DMP (a) is the dominating pathway, whereas both (a) and (c) occur in DMPy.…”

The synthesis of 16-dehydropregnenolone acetate (DPA) from potato glycoalkaloids

“…To avoid disturbing side reactions under the basic reaction conditions, the bromide Methylation with MeI and Na 2 CO 3 in water proceeded smoothly and gave the ammonium salts in high yields, which were subjected to treatment with base without further purification. This resulted in formation of the demethylated products 17 and 18 in 76% and 32% yield, 45,46 NaOH, 47,48 NaOMe, 49 or Et 3 N, 50 gave products which immediately decomposed during the isolation process. Only when 20 was treated with LDA the desired product 21 together with the N-monomethylated product 19 could be isolated in 32% and 26% yield, respectively (Scheme 5).…”

New self-crosslinkable copolymers based onN-methyl-N-vinylbenzylpyrrolidinium halide and methyl methacrylate

“…If the ring size increases to five or six, the CROP of cyclic amines is unfavourable due to the low ring‐strain. In the case of five‐membered pyrrolidine and its derivatives, the ring‐opening reaction for the pyrrolidinium ring requires the attack of relatively strong nucleophiles, such as RO − ,3–7 RS − 8 and RCOO − 9. Based on the nucleophilicity of the chosen anionic reagents, one can conduct the ring‐opening reactions under various conditions.…”