Allylation of aromatic aldehydes 1a-m with allyl- and crotyl-trichlorosilanes 2- 4, catalyzed by the chiral N-oxide QUINOX (9), has been found to exhibit a significant dependence on the electronics of the aldehyde, with p-(trifluoromethyl)benzaldehyde 1g and its p-methoxy counterpart 1h affording the corresponding homoallylic alcohols 6g, h in 96 and 16% ee, respectively, at -40 degrees C. The kinetic and computational data indicate that the reaction is likely to proceed via an associative pathway involving neutral, octahedral silicon complex 22 with only one molecule of the catalyst involved in the rate- and selectivity-determining step. The crotylation with (E) and (Z)-crotyltrichlorosilanes 3 and 4 is highly diastereoselective, suggesting the chairlike transition state 5, which is supported by computational data. High-level quantum chemical calculations further suggest that attractive aromatic interactions between the catalyst 9 and the aldehyde 1 contribute to the enantiodifferentiation and that the dramatic drop in enantioselectivity, observed with the electron-rich aldehyde 1h, originates from narrowing the energy gap between the (R)- and (S)-reaction channels in the associative mechanism (22). Overall, a good agreement between the theoretically predicted enantioselectivities for 1a and 1h and the experimental data allowed to understand the specific aspects of the reaction mechanism.

A dynamic kinetic asymmetric transformation (DYKAT) technique has been designed for the synthesis of 2'-substituted 2-aryl pyridines/isoquinolines and related heterobiaryls. In this way, the Pd(0)-catalyzed coupling of racemic 2-triflates with aryl boroxines using a TADDOL-derived phosphoramidite as the ligand provides the corresponding coupling products with good to excellent enantioselectivities. Structural studies support that the formation of configurationally labile oxidative addition palladacycles is the key for the success of the methodology.

Lewis basic, metal‐free pyridyloxazolines catalyze the reduction of prochiral aromatic ketones and ketimines with Cl3SiH in good enantioselectivity (up to 94 % ee). Arene–arene interactions between the substrate and the catalyst are likely to play a role in the enantiodifferentiation process.

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The synthesis of commercial EDDHA produces o,o-EDDHA as the main reaction product, together with a mixture of regioisomers (o,p-EDDHA and p,p-EDDHA) and other unknown byproducts also able to complex Fe3+. These compounds have been obtained by direct synthesis, and their structures have been determined by ESI-MS analysis as oligomeric EDDHA-like products, formed by polysubstitution in the phenolic rings. Short-term experiments show that the iron complexes of samples enriched in these oligomeric byproducts have adequate stability in solution, but a significant amount of them is lost after interaction with soils and soil materials. Mildly chlorotic cucumber plants are able to reduce iron better from o,p-EDDHA/Fe3+ than from the iron complexes of the oligomeric byproducts. In hydroponics, the chlorotic soybean susceptible plants have a lower potential for Fe absorption from these byproducts than from o,o-EDDHA/Fe3+ and from o,p-EDDHA/Fe3+. In the studied conditions, the iron chelates of EDDHA byproducts do not have the long-lasting effect shown by o,o-EDDHA/Fe3+ and present a less efficient fast-action effect than the o,p-EDDHA/Fe3+.

ABSTRACT:The Pd-catalyzed enantioselective C-P cross-coupling between racemic, configurationally stable heterobiaryl triflates and trialkylsilyldiaryl(dialkyl)phosphines has been used for the synthesis of several families of enantiomerically enriched heterobiaryl phosphines including QUINAP, PINAP, and QUINAZOLINAP analogues, which can be performed with good yields and enantioselectivities using JOSIPHOS-type bidentate phosphines. The strategy relies on two key assumptions: (I) the N-atom of the heterocycle is a better ligand than triflate and, upon oxidative addition, it incorporates into the coordination sphere of the Pd II center to form cationic cyclic intermediates, and (II) the geometry of the palladacycle results in a widening of the angles involved in the stabilization of the stereogenic axis, facilitating a fast interconversion of diastereomeric structures and, hence, a dynamic kinetic C-P cross-coupling reaction. These starting hypotheses are supported by experimental and computational studies.

The very low reduction potential of the chelate Fe(III)-EDDHA (EDDHA = ethylenediamine N,N'-bis(2-hydroxy)phenylacetic acid) makes it unreactive in photochemically or chemically induced electron transfer processes. The lack of reactivity of this complex toward light invalidates photodegradation as an alternative mechanism for environmental elimination. However, in spite of its low reduction potential, the biological reduction of Fe(III)-EDDHA is very effective. Based on electrochemical measurements, it is proposed that Fe(III)-EDDHA itself is not the substrate of the enzyme ferric chelate reductase. Likely, at the more acidic pH in the vicinity of the roots, the ferric chelate in a closed form (FeL-) could generate a vacant coordination site that leads to an open hexacoordinate species (FeHL) where the reduction of the metal by the enzyme takes place.

The Pd 0 -catalyzed coupling of racemic heterobiaryl bromides, triflates or nonaflates with aryl/alkyl primary amines using QUINAP as the ligand provides the corresponding axially chiral heterobiaryl amines with excellent yields and enantioselectivities. Reactivity and structural studies of neutral and cationic oxidative addition intermediates support a dynamic kinetic asymmetric amination mechanism based on the labilization of the stereogenic axis in the latter, and suggest that coordination of the amine to the Pd center is the stereodetermining step.In recent years, significant advances have been achieved in the field of asymmetric cross-coupling, in particular for the synthesis of axially chiral biaryls.1 In sharp contrast, the direct asymmetric heteroaryl-aryl cross-coupling remains as an unmet challenge, 2 limiting the access to functionalized heterobiaryls with appealing structures for their use as ligands in asymmetric catalysis. As a remarkable example, the use Isoquinoline-Amino Naphthalene (IAN) and related derivatives, which can be seen as N(sp 2 ),N(sp 3 ) analogues of QUINAP, have been scarcely investigated.3 A plausible explanation is the poor availability: there are no commercially available representatives and their synthesis still requires chromatographic separation of diastereomeric mixtures (Scheme 1, eq. 1), 4 while the lack of a general and practical method of synthesis has also limited the structural diversity of known ligands of this type. Recently, we have reported a novel strategy for the synthesis of functionalized heterobiaryls based on dynamic kinetic asymmetric C-C 5 and C-P 6 bond formations starting from heterobiaryl triflates to ensure the formations of cationic oxidative addition intermediates (Scheme 1, eq. 2). Stimulated by the growing potential of related axially chiral heterobidentate ligands, 7 we decided to focus on the development of dynamic kinetic Buchwald-Hartwig (DYKAT: Dynamic Kinetic Asymmetric Transformation) amination of heterobiaryl electrophiles for the asymmetric synthesis of axially chiral IAN-type diamines (Scheme 1, eq. 3).The unprecedented asymmetric amination of heterobiaryls is a particularly challenging goal due to the specific conditions required. First, a strong base is generally needed to achieve good reactivities, so that compatibility issues might arise with the heteroScheme 1. Synthetic approaches to IAN amines biaryl triflates used in previous DYKAT processes. Second, the racemization barriers for IAN amines are significantly lower than those of arylated products I or QUINAP-type products II, 8 making necessary to work under exceptionally mild conditions. 9 In order to minimize the hydrolysis of the starting material, we started using the coupling between nonaflate (±)-1A 10 and aniline 5a as a model reaction for the synthesis of IAN 6Aa, using NaOtBu as the base, dry toluene as the solvent at 60 °C and 10 mol% Pd(dba)2/12 mol% ligand as the catalyst system (Scheme 2).Ligands that showed a good performance in related processes were select...