The concept of hard and soft acids and bases (HSAB) proved to be useful for rationalizing stability constants of metal complexes. Its application to organic reactions, particularly ambident reactivity, has led to exotic blossoms. By attempting to rationalize all the observed regioselectivities by favorable soft-soft and hard-hard as well as unfavorable hard-soft interactions, older treatments of ambident reactivity, which correctly differentiated between thermodynamic and kinetic control as well as between different coordination states of ionic substrates, have been replaced. By ignoring conflicting experimental results and even referring to untraceable experimental data, the HSAB treatment of ambident reactivity has gained undeserved popularity. In this Review we demonstrate that the HSAB as well as the related Klopman-Salem model do not even correctly predict the behavior of the prototypes of ambident nucleophiles and, therefore, are rather misleading instead of useful guides. An alternative treatment of ambident reactivity based on Marcus theory will be presented.

Farewell to the HSAB Treatment of Ambident Reactivity

Theoretische Uberlegungen und neue praparative Ergebnisse haben in den letzten Jahren die Kenntnisse von der Struktur und Reaktivitat ambifunktioneller nucleophiler Verbindungen vertieft und erweitert. Es wird versucht, einen Uberblick uber die Theorien zu geben, die zur Deutung des Verhaltens dieser Verbindungen gegenuber elektrophilen Agentien entwickelt worden sind, und Regeln aufzustellen, die als Leitfaden fur die Durchfuhrung neuer Reaktionen dienen konnen.

Beziehungen zwischen Struktur und Reaktivität ambifunktioneller nucleophiler Verbindungen

Chemoselectivity of organometallic reactions : A hsab appraisal

A series of 2-allyl-phenols and 2-but-3′-enyl-phenol were photochemically rearranged to the corresponding cyclic ethers. The isomer distribution was shown to be similar to that observed for the acid catalysed cyclisation.

Über die photochemische Cyclisierung von 2-Allylphenolen

A general mechanism to ra- tionalize Ru IV -catalyzed isomerization of the C=C bond in O-allylic substrates is proposed. Calculations supporting the proposed mechanism were per- formed at the MPWB1K/6-311 + G- A + SDD level of theory. All experi- mental observations in different sol- vents (water and THF) and under dif- ferent pH conditions (neutral and basic) can be interpreted in terms of the new mechanism. Theoretical analy- sis of the transformation from precata- lyst to catalyst led to structural identifi- cation of the active species in different media. The experimentally observed induction period is related to the mag- nitudes of the energy barriers comput- ed for that process. The theoretical energy profile for the catalytic cycle re- quires application of relatively high temperatures, as is experimentally ob- served. Participation of a water mole- cule in the reaction coordinate is mech- anistically essential when the reaction is carried out in aqueous medium. The new mechanistic proposal helped to de- velop a new experimental procedure for isomerization of allyl ethers to 1- propenyl ethers under neutral aqueous conditions. This process is an unique example of efficient and selective cata- lytic isomerization of allyl ethers in aqueous medium.

E. Tietze

Papers

Über C‐Alkylierung (Kernalkylierung) von Phenolen

Über anormale Umsetzugen bei Derivaten des Isoprens und des β,γ‐Dimethylbutadiens