Vögtle and Goldschmitt [1] were the first to use hydrogen bonds to drive diaza-Cope rearrangement reactions to completion over thirty years ago. Although many interesting publications on the topic have since appeared, [2] no other weak forces have been found to drive the rearrangement reaction to completion. The diaza-Cope rearrangement is useful for synthesizing a wide variety of chiral vicinal diamines (Scheme 1) [3] that may be valuable for the development of catalysts [4] and drugs. [2,5] [3,3] Sigmatropic reactions, including the Cope, Claisen, oxy-Cope, and aza-Cope reactions, have received much interest on both a theoretical [6] and a practical [7] level. The diaza-Cope rearrangement provides an ideal platform for studying the effects of weak forces in [3,3] sigmatropic reactions, as the systematic variation in the structure of the starting materials that is needed for these investigations can be achieved readily through simple synthesis. It is well known that strained molecules show remarkable reactivity. [8] Herein, we report the effect of steric strain on the rate and equilibrium constants of the diaza-Cope rearrangement reaction. Although ring strain has been used to drive [3,3] sigmatropic rearrangements, [1a, 9] this reaction is the first example of a [3,3] sigmatropic rearrangement driven by steric strain.We prepared (S,S)-1,2-bis(2,4,6-trimethylphenyl)-1,2-diaminoethane (TPEN) [10] through the diaza-Cope rearrangement of the diimine formed from (R,R)-1,2-bis(2-hydroxyphenyl)-1,2-diaminoethane (HPEN) and mesitaldehyde. [3] The addition of benzaldehydes (2.5 equiv) with electrondonating or electron-withdrawing substituents to (S,S)-1,2bis(2,4,6-trimethylphenyl)-1,2-diaminoethane in ethanol and subsequent stirring of the reaction mixture overnight at ambient temperature gave the corresponding rearranged diimines 1 b-3 b in good yields (80-85 %). The identity of products 1 b-3 b of the rearrangement reaction was confirmed by comparing their 1 H NMR spectra with those of the diimines prepared from the corresponding diamines and mesitaldehyde. Although the initial diimines 1 a-3 a were not isolated, their clean formation (d H = 5.6 ppm for 3 a, Figure 1) and conversion into the product diimines (d H = 4.8 ppm for 3 b) could be monitored readily by 1 H NMR spectroscopy. The equilibrium constants for the rearrangement reactions in Scheme 1 must be greater than 10 2 , as we did not observe any of the initial diimines by 1 H NMR spectroscopy after equilibration. We were pleasantly surprised that the steric effect was so dramatic and complete for the rearrangement reactions (Figure 1). Figure 2 a shows the crystal structure of 1 b formed through the rearrangement of 1 a. [11] This structure is "preorganized" [3c] for the reversible rearrangement reaction and resembles the computed transition-state (TS) structure for the rearrangement of 3 a (Figure 2 b). [12] We investigated the effect of steric strain on the rate and equilibrium constants for the rearrangement of 3 a to 3 b by DFT computation (...