Issue in Honor of Prof. B. S. Thyagarajan ARKIVOC 2001 (xi) 1-11
influenced by proton donor acidity and steric demand. The team advanced the hypothesis that the conjugate base of the sulfone was more strongly influenced by sp3 hybridized a-carbon resonance form than the nitro or benzoyl analogs. The hybridization difference results in an axial proton-accepting sp3 orbital leading to a trans product. Conversely, the nitro and benzoyl analogs have increased proportion of sp2 carbanion hybridization leading to a pi accepting orbital. Differences in the steric effects of the proton donors were well accommodated by this model, while differences in the electronic properties of the proton donor were ascribed to a later transition state.
In a supporting study, Professor Thyagarajan and Zimmerman compared the acidities of isopropyl- and cyclopropylphenylsulphone to examine resonance contributions to the stability of the conjugate bases of these species (scheme 6).15 The previously described study had suggested a limited resonance contribution from the carbanion into the sulfone moiety. Thus, the cyclopropyl analog was not expected to significantly stabilize the a carbon of the cyclopropyl species and the cyclopropyl analog was expected to be more acidic due to increased s-character of cyclopropyl C-H bonds. However, the acidities of the two species were very similar, suggesting that the resonance stabilization of the propyl analog approximately equaled the stability associated with the cyclopropyl anion. The two studies with Zimmerman required challenging syntheses, in addition to the development and careful execution of difficult monitoring protocols. These studies exemplified the inventive experimental design and superb laboratory skills that characterized the collaboration of Professor Thyagarajan and Zimmerman.16
Pericyclic reactions. Professor Thyagarajan examined the intricacies of Claisen rearrangements of 2-butyn- and buten-1,4-diyl bisaryl ethers as an independent investigator. For the former, it first appeared that a rearrangement was taking place that involved a double-Claisen mechanism17 (scheme 7, route a). However, some expert detective work revealed the true character of this reaction.18 The first intermediate of route b was synthesized and easily converted to product, while the second intermediate of route a resists any reaction when submitted to the normal conditions. Therefore, route b prevails, involving a single Claisen rearrangement and two addition steps. Considerable synthetic and analytical skills were employed to unravel this mystery.
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