Woodward‐Hoffmann or Hoffmann‐Woodward? Cycloadditions and the Transformation of Roald Hoffmann from a “Calculator” to an “Explainer”

• Published in: Chemical record Vol. 24; no. 8; pp. e202300181 - n/a
• Main Author: Seeman, Jeffrey I.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.08.2024
• Subjects: Calculators; Careers; Chemical reactions; Cycloaddition; cycloadditions; History of chemistry; Hoffmann, Roald; pericyclic reactions; Woodward-Hoffmann rules; pericyclic reactions; Woodward-Hoffmann rules; cycloadditions; History of chemistry
• ISSN: 1527-8999; 1528-0691; 1528-0691
• DOI: 10.1002/tcr.202300181

On May 1, 1965, Roald Hoffmann and R. B. Woodward published their second joint communication, Selection Rules for Concerted Cycloaddition Reactions, in the Journal of the American Chemical Society. Herein is presented a historical analysis of Woodward and Hoffmann's determination of the mechanism of cycloadditions. This analysis is based on thorough analyses with Roald Hoffmann of his 1964 and 1965 laboratory notebooks and his archived documents and on numerous in‐person, video, and email interviews. This historical research pinpoints several seminal moments in chemistry and in the professional career of Hoffmann. For example, now documented is the fact that Woodward and Hoffmann had no anticipation that their collaboration would continue after the publication of their first 1965 communication on electrocyclizations. Also pinpointed is the moment in Hoffmann's professional and intellectual trajectories that he became a full‐fledged, equal collaborator with Woodward and Hoffmann's transition from a “calculator” to an “explainer.” It was early February 1965. The Woodward‐Hoffmann collaboration had ended with the publication of their January 1965 JACS communication. Hoffmann was applying extended Hückel calculations to many topics in organic chemistry. But the moment he saw the molecular orbital energy level crossing, of MOs from bonding to antibonding MOs in the thermal [2+2] cycloaddition, a Eureka moment arrived. He knew this was the reason those reactions were thermally forbidden and photochemically allowed.