Eclipsed Isopropyl Conformations of Two Dimeric Hydrazines

Area of Science:

• Organic Chemistry
• Crystallography
• Computational Chemistry

Background:

• Investigating the conformational preferences of tetraazacyclotetradecane derivatives.
• Understanding the influence of alkyl substituents on molecular geometry.

Purpose of the Study:

• To determine the X-ray crystal structures of two specific tetraazacyclotetradecane isomers: s4iPr and a4iPr.
• To compare experimental conformational data with predictions from molecular mechanics (MM2) and semiempirical quantum mechanics (AM1) calculations.
• To elucidate the factors governing the conformational asymmetry observed in the a4iPr isomer.

Main Methods:

• X-ray crystallography to determine the solid-state structures of s4iPr and a4iPr.
• Molecular mechanics (MM2) and semiempirical quantum mechanics (AM1) calculations to predict low-energy conformations.
• (13)C-NMR spectroscopy to assess conformational stability in solution.

Main Results:

• The crystal structures of s4iPr and a4iPr reveal anti conformations for the N-alkyl groups.
• A key finding is the presence of an eclipsed conformation for one in-isopropyl group in both isomers.
• MM2 and AM1 calculations predict low energy for these eclipsed conformations.
• Solution NMR data indicate that the observed asymmetry in a4iPr (different isopropyl rotamers) is energetically favored, contradicting computational predictions.
• The bicyclooctane derivative (3) is identified as an inadequate model for 4iPr conformations.

Conclusions:

• The study highlights discrepancies between calculated and experimentally observed conformational preferences in solution for tetraazacyclotetradecane derivatives.
• Crystal packing forces do not explain the observed asymmetry in a4iPr; intrinsic molecular stability dictates the conformation in solution.
• Further refinement of computational methods may be needed to accurately predict the conformational behavior of such complex organic molecules.