Theoretical calculations rearrangement

Several ways to suppress the 2-oxonium-[3,3]-rearrangements might be envisioned. Apart from the introduction of a bulky substituent R at the aldehyde (Scheme 23) a similar steric repulsion between R and R might also be observed upon introduction of a bulky auxiliary at R. A proof-of-principle for this concept was observed upon by using of a trimethylsilyl group as substituent R in the alkyne moiety (Scheme 25, R = TMS). This improvement provided an efficient access to polysubstituted dihydropyrans via a silyl alkyne-Prins cyclization. Ab initio theoretical calculations support the proposed mechanism. Moreover, the use of enantiomerically enriched secondary homopropargylic alcohols yielded the corresponding oxa-cycles with similar enantiomeric purity [38]. 

Rearrangement of the cyclopropylcarbinyl chloride takes place over NaY zeolite, indicative of the formation of the bicyclobutonium cation. Theoretical calculations show that the bicyclobutonium is an intermediate on the zeolite surface and might be in equilibrium with the alkyl-aluminumsilyl oxonium ion. 

This chapter has attempted to demonstrate how secondary deuterium and tritium KIEs can be used to elucidate the mechanisms of reactions and determine the structure of their transition states. In particular, the advantages of using both theoretical calculations and experimental data to solve these problems has been emphasized. Unfortunately, several important topics where the combination of theoretical calculations and experimental work has been very useful in extending our understanding of KIEs could not be discussed. In particular, the extensive studies on the Diels-Alder and the Cope rearrangement by Houk and co-workers (Beno et al., 1996 Houk et al., 1992 Storer et al., 1994) are noteworthy. 

The initial adsorption of the oxime in zeolites was studied through a combination of solid-state NMR spectroscopy and theoretical calculations ". The calculated adsorption complexes formed over silanol groups and complexes over Brpnsted acid sites in zeolites are depicted. This study suggests that the A-protonated oxime is formed over Brpnsted acid centers, but not over weakly acidic silanol groups. It has been also suggested that weakly acidic or neutral silanol groups or silanol nests are active catalysts of the rearrangement reaction ... 

German chemists synthesised the tetraspiroketone 1 and carried out theoretical calculations in an attempt to decide whether acid-catalysed isomerisation would lead preferentially or exclusively to the bispropellanone 2 or to the pentacyclic ketone 3. Force field calculations failed to reveal any significant preference for rearrangement, and hence experimental clarification was sought. The ketone 1 was unstable to acid, and was quantitatively isomerised to 3 within 30 minutes at 80°C when treated with one equivalent of a 0.25 M solution of anhydrous p-toluenesulfonic acid in benzene. 

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