The Retro–Diels–Alder Reaction Part I. C ? C Dienophiles

The Diels–Alder (“DA”) reaction is so familiar to organic chemists that the retro-Diels–Alder or retro-diene reaction (hereafter “rDA”) requires no conceptual introduction. However, a working definition is needed to indicate the coverage of this review. The intent is to include all reported examples of the general skeletal process excluded in this chapter. Any of the atoms in the starting material (cycloadduct) may be carbon or heteroatom, and substituents on all positions are allowed. In addition, any bond order available to the element in any oxidation state is included, as well as bonding to non-nearest neighbor atoms (bicyclics, etc.). The volume of literature required separation of this review into two parts. Part I covers all expelled C–C dienophiles, i.e. those reactions that generate a new carbon-carbon double or triple bond in the dienophile that is formed. Part II, to appear later, covers those expelled dienophiles in which one or both atoms is a heteroatom. Certain specific topics are intentionally omitted. These are: rDA reactions invoked under mass spectral conditions; rDA reactions of polymeric substrates; Electrocyclic processes, some of which are arguably very similar to rDA reactions (these are not addressed unless needed for clarification of an rDA sequence); and a thorough discussion of homo-rDA reactions. CAS-Online searches resulted in ca. 1,300 references, of which ca. 900 proved pertinent to the rDA topic as defined and delimited above. Active literature searching was halted in April 1995, but occasional more recent articles are included. Over 2,500 pertinent references were eventually found. Most were obtained by perusal of primary literature (articles) and secondary sources (books and reviews). Computer based search success was limited mainly by the failure of authors or abstractors to key-word this topic, an especially common occurrence when the expelled dienophile was a simple substance such as N2 or CO2. Overall, approximately 3,500 books, chapters, reviews, articles, and abstracts were consulted in an effort to make this review as comprehensive as possible. This was done with the certainty that some pertinent literature would be missed, tempered by the view that these omissions are probably also lost to any future rational search method. Many secondary sources proved valuable not only for in-depth discussion of certain rDA reaction types, but also for providing comparisons with related topics. To assist the reader, these secondary sources are referenced (alphabetically by first author within broad subject areas) with a title or brief note on the topic(s) addressed. The first group lists earlier reviews with primary focus on rDA reactions, followed by items of Historical/General Interest, reviews that deal with Experimental Methods, DA reactions (general and specific), reviews of Related Topics (these impinge in more or less significant ways on rDA reactions), and general treatments of Theory and Mechanism. Keywords: retro Diels-Alder reactions; dienophiles; mechanism; scope; limitations; acid catalysis; base induced reactions; scavengers; reactivity comparisons; acetylenic dienophiles; alkyl ethylenes; alkyl alkenes; aryl substituted alkenes; vinyl substituted alkenes; heteroatom substituted alkenes; ketenes; experimental conditions; tabular survey