Abstract
AbstractAn intramolecular base‐catalyzed nitroaldol reaction of three newly prepared 2‐(2‐nitroalkyl)benzaldehydes (1a‐c) giving corresponding 2‐nitroindan‐1‐ols (2a‐c) was studied in water and five non‐aqueous solvents. Whilst for the parent 2‐(2‐nitroethyl)benzaldehyde (1a) the reaction takes place in two kinetically discernible steps – deprotonation and cyclization, for 2‐(2‐nitropropyl)‐ and 2‐(2‐nitro‐2‐phenylethyl)benzaldehydes 1b and 1c general base‐catalyzed formation of a reactive carbanion is rate‐limitting. An unusually high Brønsted coefficient (βB=1.12 ± 0.03) was found for 1b indicating smaller imbalance of the transition state whose structure therefore resembles a nitronate. Cyclization products 2a‐c prefer relative trans‐configuration in ratios from 55:45 to 83:17 depending on the solvent. Mutual interconversion of the cis‐/trans‐diastereoisomers occurs either via nitronate species 2a– or via a reverse ring opening/ring closing pathway through the nitronate species 1b– and 1c–. The mechanism was also supported by quantum calculations.
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