Rearrangement reactions, solvent effects

The list of publications in the obituary of the organic chemist Otto Dimroth (1872-1940)139 has eleven papers that are classified as physikalisch-organische Chemie. They are mainly about tautomerism, intramolecular rearrangements, and solvent effects on tautomeric equilibria. One paper,140 published in 1933, deals with relationships between reaction velocities and oxidation-reduction potentials for quinone systems and it is evident that Dimroth was a pioneer in developing linear free-energy relationships. He was Professor in Wurzburg from 1918 to 1940. 

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

The solvent effects observed in the photochemical rearrangements of cross-conjugated dienones suggest that the availability of a proton to an excited state of the ketone may be an important controlling factor. It is interesting to consider the possibility that protonation of an excited state may lie behind the apparent polar reactions. Triplet states are likely to be more polarizable and to be stronger bases than the corresponding... 

Base-catalysed ring fission of 3,4-diphenylcyclobut-3-ene-l,2-diones (103) in 50% (v/v) aqueous DMSO proceeds by rapid reversible addition of hydroxide ion followed by rate-determining benzilic acid-type rearrangement to form an intermediate 1-hydroxycyclopropane-1-carboxylic acid which ring opens to the corresponding (Z)-2-oxo-3,4-diphenylbut-3-enoic acid (Scheme 8).173 This is supported by the value of Hammett p = 1.3 (for variation of substituents on one or both rings), the kinetic solvent effects, and the three-oxygen enrichment of (107) from reaction of (103) in 50% H2 180-DMSO. 

Thus, ketenes (2) can react as dienophiles with (E)-l,3-diazabuta-l,3-dienes (E)-(25) to yield either [4 + 2] cycloaducts (26) or (27) depending on the participation of the C = C or C = O moieties of the ketenes (Scheme 7). Claisen rearrangement of 3,6-dihydro-2-methylene-2//-l,3,5-oxadiazines (27) yields the p-lactams (28). Alternatively, reaction between ketenes (2) and (Z)-l,3-diaza-buta-1,3-dienes (Z)-(25) leads to the usual zwitterionic intermediates (29), whose conrotatory electrocyclation leads to p-lactams (28). No computational data including solvent effects have been reported for these reactions. 

Detailed mechanisms of intramolecular rearrangement reactions have been difficult to determine. Classical rate measurements seldom lead to unambiguous mechanistic predictions. Generally only after extensive examination of concentration, solvent, and substituent effects on the reaction rate can a general mechanistic class be proposed for example, intra vs intermolecular or bond rupture of a bidentate chelate vs non-bond rupture twist pathways. Indeed, only two examples of slow complexes are known where detailed rate comparisons for geometrical and optical iso-merizations were made and used to eliminate several mechanisms however, a single most probable pathway was not demonstrated in either case.12,13 Only with DNMR can detailed environmental site interchanges be directly observed and with this in-... 

Alkenyl(phenyl)iodine(III) compounds can also serve as starting materials in rearrangements. Allenyl(aryl)iodine(III) compounds of type 86 can be synthesized from (diacetoxyiodo) derivatives 85 and propargylsilanes [145]. It depends on the leaving group ability of the aromatic substituent on iodine in 86 as to whether the reaction proceeds via nucleophilic substitution to compounds of type 87 or by an iodonio-Claisen rearrangement to compounds 88, Scheme 37 [146,147]. The easy access to propynyl compounds 87 has been shown [148] and solvent effects in these reactions have been investigated as well [149,150]. 

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