Method of Establishing the Lewis Acidity of a Metal Fragment Based on the Relative Binding Strengths of Ar-BIAN Ligands (Ar-BIAN = Bis(aryl)acenaphthenequinonediimine)

Abstract

The relative coordination strengths of a series of differently substituted Ar-BIAN ligands (Ar-BIAN ) bis(aryl)acenaphthenequinonediimine) to a series of palladium complexes in both the formal 0 and 2 oxidation states have been determined. In all cases a good to excellent linearity of log K eq with respect to the Hammett σ constants of the substituents on the aryl fragments of the ligands was observed. The resulting F constant is proposed to be a good indication of the Lewis acidity of the metal fragment, a physical quantity for which experimental parameters have been determined only for a limited class of compounds. The obtained parameters allow a comparison not only of different olefin complexes among themselves but also with respect to different metal fragments such as Pd(OAc) 2 , Pd(Me)Cl, and a π-allyl complex. The Lewis acidity of the olefin complexes is extremely variable and ranges from the less acidic (Pd(Ar-BIAN)(DMFU); DMFU ) dimethyl fumarate) to two of the most acidic (Pd(Ar-BIAN)(TCNE) and Pd(Ar-BIAN)(FN); TCNE ) tetracyanoethylene, FN ) fumarodinitrile) complexes among those examined. A cationic π-allyl complex has the highest Lewis acidity among the complexes examined. The importance of steric effects is examined in some cases.

Investigation of Lewis Acid versus Lewis Base Catalysis in Asymmetric Cyanohydrin Synthesis

“…In contrast, a reaction proceeding entirely by Lewis base catalysis would be expected to have a reaction constant of zero, since the rate-determining step of the mechanism involves activation of the trimethylsilyl cyanide and the aldehyde is only involved after the rate-determining step. Hammett plots have previously been used to determine the Lewis acidity of palladium complexes, [17] to study achiral Lewis acid- [18] or base-catalysed [19] reactions and to study asymmetric Lewis acid-catalysed, [20] Lewis base-catalysed [21] and carbene insertion reactions, [22] but do not appear to have previously been used to determine the relative importance of Lewis acid and Lewis base catalysis.…”

Catalyst activity or stability: the dilemma in Pd-catalyzed polyketone synthesis

“…In previous papers some of us have shown that the logarithm of the relative coordinating strength of substituted Ar-BIAN ligands shows a good correlation with the Hammett σ constants. 20,38 When two substituents were present, the correlation was maintained if the sum of the two individual σ was employed, whereas ligands having different substituents on the two aryl rings followed the same trend if the arithmetic average of the Hammett σ constants for the two rings was employed in the correlation. 16 In general, this corresponds to employing for both symmetrical and nonsymmetrical ligands the value of Σσ/2 (where the sum is extended to all substituents on the two rings),which is also shown in Table 2.…”

Iron‐Catalyzed Intramolecular Cyclotrimerization of Triynes to Annulated Benzenes.

“…[11] Other ligands 4a,b [11], 4c-e [12], 4f [13] and 6a-c [14] were prepared according to the reported procedure. The alkyne substrates were prepared by the conventional reactions.…”

“…A series of 1,2-diimines 4 could act as a good ligand in a nearly stoichiometric amount to FeCl 3 , where it was again observed that 4 with bulky N-substituents was a preferable ligand (entries [9][10][11][12][13][14]. 4,5,4',2']bioxazolyl (4f) was also a good ligand (entry 15).…”