The Isoinversion Principle—a General Model of Chemical Selectivity

Abstract

Dedicated to Vladimir Prelog and to the memory of Horst PracejusA large number of successful methods for chirality transfer, using either stoichiometric or catalytic chiral auxiliaries, are in use today. However, there is a lack of practical and dynamic selectivity models, i.e. models which take into account the entire reaction sequence, and which allow simple and reliable assessment, optimization and prediction of selectivity in asymmetric syntheses. The models that are available are either too strongly biased to the steric requirement of the particular molecules reacting, but do not go beyond classical considerations of static facial differentiation, or they take a demanding, theoretical approach, which because of its inherent limitations and the great theoretical effort required has not yet found its way into the practical world of the synthetic chemist. The "Isoinversion Principle", developed on the basis of Eyring's theory, closes this gap. With its aid, the synthetic chemist can determine the characteristic isoinversion temperature for the reaction type of interest from a few temperature-dependent measurements of selectivity parameters. then affords information on interesting questions such as optimization etc. The advantage of this method is that it is useful not only for stereoselectivity, but for any kind of process where selectivity in general (regio-, chemo-, etc) is generated at two or more stages of a reaction sequence, regardless of whether these reactions involve the ground state or a diabatic photoprocess. The present review will demonstrate that this generation of selectivity at two or more stages of a reaction sequence occurs more commonly than is generally thought.

Coordination Chemistry and Asymmetric Catalysis with a Chiral Diphosphonite.

“…Furthermore, the enantioselectivity of the (R)-enantiomer increased with reaction temperature, contradictory to the ''normal' behavior seen with CHIRAPHOS. This phenomenon, already discussed by Scharf in a review on dramatic temperature effects on selectivities in catalysis, [35] was explained by the existence of competing pathways of diastereomeric intermediates of a single chelate conformation of the catalyst system, hence indicating kinetic control.…”

Dual stereocontrolled alkylation of aldehydes with polystyrene-supported nickel complexes derived from α-amino amides

“…It must be noted that the addition of dialkylzinc to aldehydes usually reaches a maximum ee value at a certain temperature, which is called the isoinversion temperature. 25 As described above, initial experiments were carried out at 0 C. A further decrease in the reaction temperature to À25 C did not provide a signicant variation of the enantioselectivity, but, on the contrary much lower reaction rates were observed (entry 6, Table 1). On the other hand, when the reaction was carried out at room temperature, the enantioselectivity decreased to 59% in the case of 3b.…”

(R,R)-N-phenyl-3,4-bis(diphenylphosphino)pyrrolidine: an N-aryl pyrrolidine ligand for enantioselective transfer hydrogenation

“…The formation of a five-membered chelate between the ligand and the metal centre, the existence of a rigid cyclic backbone structure and also chiral centres close to the co-ordination sites [6,7] correspond to general characteristics of these chirality inducing ligands which proved to be highly favourable in this particular case.…”

“…The preferential formation of one of the two possible enantiomers of a reaction product strongly depends on the characteristics of the catalyst namely, the tridimensional structure of the chiral ligand and its electronic properties. High efficiency has been observed with diphosphine ligands which simultaneously have rigid backbone structures and form five-membered chelate rings with the metal centre [6,7].…”