Dyotropic Rearrangements, a New Class of Orbital-Symmetry Controlled Reactions. Type I

In 1972, M. T. Reetz defined dyotropic (from the greek dyo, meaning two) rearrangements as a new class of pericyclic valence isomerizations in which two σ-bonds simultaneously migrate intramolecularly.1,2 Reactions in which the two migrating groups interchange their relative positions were designated as type I (Scheme 1A) whereas those of type II involve migration to new bonding sites without positional interchange (Scheme 1B,C). The uncatalyzed and concerted nature of these processes was also proposed.3,4 Nowadays, dyotropic transformations are standard tools for the construction of organic and organometallic molecules. Frequently, this unique rearrangement is the single entry to an efficient preparation of target molecules. Moreover, the discovery of new dyotropic reactions involving transition metals and excited states had widened the mechanistic scope of these processes to new reaction pathways well away from the original definition of uncatalyzed and concerted processes. This review comprehensively reports the dyotropic reactions studied since the seventies, accounts for the different reaction mechanisms for these rearrangements, and discusses their synthetic applications in organic and organometallic chemistry.

Dyotropic Reactions: Mechanisms and Synthetic Applications†

Although recent years have witnessed significant advances in the development of catalytic, enantioselective halofunctionalizations of alkenes, the related dihalogenation of olefins to afford enantioenriched vicinal dihalide products remains comparatively underdeveloped. However, the growing number of complex natural products bearing halogen atoms at stereogenic centers has underscored this critical gap in the synthetic chemist's arsenal. This Review highlights the selectivity challenges inherent in the design of enantioselective dihalogenation processes, and formulates a mechanism-based classification of alkene dihalogenations, including those that may circumvent the "classical" haliranium (or alkene-dihalogen π-complex) intermediates. A variety of metal and main group halide reagents that have been used for the dichlorination or dibromination of alkenes are discussed, and the proposed mechanisms of these transformations are critically evaluated.

Catalytic, Stereoselective Dihalogenation of Alkenes: Challenges and Opportunities.

Dyotropic processes involving unprecedented 1,2-acyl migrations provide access to novel spirocyclic, bridged keto-γ-lactones from a series of fused, tricyclic-β-lactones, available via biscyclization of ketoacids including a new asymmetric variant. In addition, a spirocyclic bis-γ-lactone was generated via a dyotropic process involving a 1,2-β-lactone/δ-lactone interchange. Overall, this sequence provides a simple, two-step process for conversion of diketoacids to complex spiro[5.n]alkanes bearing a contiguous tertiary carbon center, a quaternary carbon center, and a tertiary alcohol in the form of a bridging γ-lactone.

Concise Synthesis of Spirocyclic, Bridged γ-Butyrolactones via Stereospecific, Dyotropic Rearrangements of β-Lactones Involving 1,2-Acyl and δ-Lactone Migrations

Neighboring methoxyl participation in solvolytic nucleophilic substitution

The mutarotation of 5 α, 6 β-dibromocholestane in a series of solvents and that of cholesterol dibromide in chloroform has been studied polarimetrically. The rate of mutarotation is quite insensitive to the addition of hydrogen bromide, benzoyl peroxide or catechol, the kineties being well formulated by reversible first order reactions.

Mechanismus der Mutarotation von 5,6-Dibromcholestan

The solvolysis of 14C-labelled 2-p-anisyl-1-ethyl p-toluenesulfonate p-CH3O–C6H4–CβH2–*CαH2–OTs (IA) leads to product in which 14C-scrambling between Cα and Cβ is complete when the solvent is formic acid or acetic acid, but only partial when the solvent is ethanol. These results confirm previous conclusions, based on kinetic evidence, that solvolysis of 2-p-anisyl-1-ethyl toluenesulfonate is almost exclusively anchimerically assisted in formic and acetic acid solvents, but not in the more nucleophilic solvent, ethanol.

S. Winstein

Papers

Mechanismus der Mutarotation von 5,6-Dibromcholestan

14C‐Umlagerung, Salzeffekte und Ionenpaar‐Rückkehr in der Solvolyse von [2‐(p‐Anisyl)‐äthyl]‐p‐toluolsulfonat