The Hammond-Leffler Postulate was formulated by G. S. Hammond (then at the California Institute of Technology) and J. E. Leffler (Florida State University) and states that the structure of a transition state resembles that of the species nearest to it in free energy. That is to say that the transition state of an endergonic reaction resembles the products, while that of an exergonic reaction resembles the reactants.
References
Solomons, T.W. Graham & Fryhle, Craig B. (2004). Organic Chemistry (8th ed.). John Wiley & Sons, Inc. ISBN 0-471-41799-8.