Orbital symmetry theory fails to account for the stereochemical characteristics of thermal [1,3] carbon shifts. John Baldwin at Syracuse University, US, and Phyllis Leber at Franklin & Marshall College, US, examine the body of experimental evidence surrounding the reaction mechanism of this interesting molecular rearrangement reaction. Baldwin discusses his work in the interview below. 

1. Could you explain the significance of your article to the non-specialist?

Experimental evidence and theory-based studies agree: thermal isomerizations of simple hydrocarbons through [1,3] carbon shifts occur by way of conformationally flexible diradical intermediates. This mechanistic insight refutes the long-standing consensus judgment that such isomerizations are controlled by orbital symmetry factors. Application of the conservation of orbital theory paradigm to [1,3] sigmatropic carbon shifts fails to account for the observed stereochemical and kinetic characteristics of real examples of these reactions. These findings prompt a renewed consciousness of the pitfalls that may attend applications of any theory taken to be a conceptual absolute. 

2. What motivated you to undertake this work? 

Three primary motivations prompted our studies: curiosity about the mechanisms of simple thermal reactions, ones simple enough for modern computational methods to engage with fair prospects of making fine distinctions reliably; curiosity about issues at the interface of applications of orbital symmetry theory and experimental evidence; and the inherent attraction of experimental studies presenting serious design and technical challenges. 

3. Where do you see this work developing in the future?

What challenges might there be? There remain numerous fundamental mechanistic questions posed by organic chemical reactions that have not been seriously addressed by experiments or by computational efforts. The experimental methods suitable for a given unsolved problem are typically not at all obvious. Meeting the challenges often requires new strategies and new combinations of technical resources. Ideally, such work will enhance the repertoire of physical organic chemical tactics that can be invested in careful studies of chemical transformations. Thus we aspire to gain well-grounded understandings of fundamental chemical reactions and augment the armamentarium available for such work. 

4. How long have you been involved in this area of research? 

For a long time. My first publication related to a simple thermal reaction of a deuterium-labeled hydrocarbon, cyclopentene, and the relevant insights of orbital symmetry theory applied to its conversion to hydrogen and cyclopentadiene, appeared in 1966. Over time, more and more difficult mechanistically fascinating thermal reactions of relatively small hydrocarbons have been investigated.