Syn-Selective Synthesis of ß-Branched α-Amino Acids by Alkylation of Glycine-Derived Imines with Secondary Sulfonates.

A syn-selective synthesis of ß-branched α-amino acids has been developed based on the alkylation of glycine imine esters with secondary sulfonates. The potassium counterion for the enolate, the solvent, and the leaving group on the electrophile were key levers to maximize the diasteroselectivity of the alkylation. The optimized conditions enabled a straightforward preparation of a number of ß-branched α-amino acids that can be challenging to obtain.

Mechanistic significance of the si-o-pd bond in the palladium-catalyzed cross-coupling reactions of alkenylsilanolates.

Through the combination of reaction kinetics (both catalytic and stoichiometric) and solid-state characterization of arylpalladium(II) alkenylsilanolate complexes, the intermediacy of covalent adducts containing Si-O-Pd linkages in the cross-coupling reactions of organosilanolates has been unambiguously established. Two mechanistically distinct pathways have been demonstrated: (1) transmetalation via a neutral 8-Si-4 intermediate that dominates in the cross-coupling of potassium alkenylsilanolates, and (2) transmetalation via an anionic 10-Si-5 intermediate that dominates in the cross-coupling of cesium alkenylsilanolates. Arylpalladium(II) alkenylsilanolate complexes bearing various phosphine ligands (both bidentate and monodentate) have been isolated, fully characterized, and evaluated for their kinetic competence under thermal (stoichiometric) and anionic (catalytic) conditions. Comparison of the rates for thermal and anionic activation demonstrates that intermediates containing the Si-O-Pd linkage are involved in the cross-coupling process.

Mechanistic significance of the si-o-pd bond in the palladium-catalyzed cross-coupling reactions of arylsilanolates.

Through the combination of reaction kinetics (both stoichiometric and catalytic), solution- and solid-state characterization of arylpalladium(II) arylsilanolates, and computational analysis, the intermediacy of covalent adducts containing Si-O-Pd linkages in the cross-coupling reactions of arylsilanolates has been unambiguously established. Two mechanistically distinct pathways have been demonstrated: (1) transmetalation via a neutral 8-Si-4 intermediate that dominates in the absence of free silanolate (i.e., stoichiometric reactions of arylpalladium(II) arylsilanolate complexes), and (2) transmetalation via an anionic 10-Si-5 intermediate that dominates in the cross-coupling under catalytic conditions (i.e., in the presence of free silanolate). Arylpalladium(II) arylsilanolate complexes bearing various phosphine ligands have been isolated, fully characterized, and evaluated for their kinetic competence under thermal (stoichiometric) and anionic (catalytic) conditions. Comparison of the rates for thermal and anionic activation suggested, but did not prove, that intermediates containing the Si-O-Pd linkage were involved in the cross-coupling process. The isolation of a coordinatively unsaturated, T-shaped arylpalladium(II) arylsilanolate complex ligated with t-Bu3P allowed the unambiguous demonstration of the operation of both pathways involving 8-Si-4 and 10-Si-5 intermediates. Three kinetic regimes were identified: (1) with 0.5-1.0 equiv of added silanolate (with respect to arylpalladium bromide), thermal transmetalation via a neutral 8-Si-4 intermediate; (2) with 1.0-5.0 equiv of added silanolate, activated transmetalation via an anionic 10-Si-5 intermediate; and (3) with >5.0 equiv of added silanolate, concentration-independent (saturation) activated transmetalation via an anionic 10-Si-5 intermediate. Transition states for the intramolecular transmetalation of neutral (8-Si-4) and anionic (10-Si-5) intermediates have been located computationally, and the anionic pathway is favored by 1.8 kcal/mol. The energies of all intermediates and transition states are highly dependent on the configuration around the palladium atom.

Phosphine Oxides as Stabilizing Ligands for the Palladium-Catalyzed Cross-Coupling of Potassium Aryldimethylsilanolates.

The palladium-catalyzed cross-coupling reaction of potassium (4-methoxyphenyl)dimethylsilanolate (K(+)1(-)) with aryl bromides has been demonstrated using triphenylphosphine oxide as a stabilizing ligand. Unsymmetrical biaryls can be prepared from a variety of aryl bromides in good yield with short reaction times. Qualitative kinetics studies compared effects of different phosphine oxides on the rate of cross-coupling and established the beneficial effect of these ligands in the reaction of electron-rich arylsilanolates. The improved yield and reproducibility of the cross-coupling of several bromides was demonstrated by direct comparison of reactions performed with and without triphenylphosphine oxide under non-rigorous exclusion of oxygen.

A qualitative examination of the effects of silicon substituents on the efficiency of cross-coupling reactions.

A qualitative study of the effects of various substituents on the silicon atom in cross-coupling reactions of alkenylsilanes has been carried out. In intermolecular competition experiments, the influence of carbon-based groups (methyl, ethyl, isopropyl, tert-butyl, phenyl, and 3,3,3-trifluoropropyl) and alkoxy groups (monoethoxydimethyl-, diethoxymethyl-, and triethoxy) on the silicon have been evaluated under activation by two different methods, fluoride (TBAF) and silanolate (TMSOK). The influence of the substituents was highly dependent on the method of activation. In the presence of TBAF, there was only a modest steric effect (except for tert-butyl substituents), and the efficiency decreased slightly with increasing numbers of alkoxy groups. In the presence of TMSOK, a significant steric effect was noted, but the number of alkoxy groups had almost no influence. These trends were interpreted in terms of the divergent mechanisms for the cross-coupling process.

Sequential cross-coupling of 1,4-bissilylbutadienes: synthesis of unsymmetrical 1,4-disubstituted 1,3-butadienes.

A mild and general and stereospecific cross-coupling reaction of unsymmetrical 1,4-bissilyl-1,3-butadienes has been accomplished. By the use of either a benzyldimethylsilyl or 2-thienyldimethylsilyl unit at one end of the dienylsilanol, a selective cross-coupling could be effected under mildly basic conditions (KOTMS) to afford 4-aryl-1,3-dienylsilanes in excellent yield for a wide range of aryl and alkenyl coupling partners. The second cross-coupling could be effected cleanly by the action of TBAF with electron-rich or electron-poor halides. The sequential process could be telescoped into a "one pot" procedure with overall excellent yields of the unsymmetrical 1,4-diaryl-1,3-butadienes.

Cross-coupling of alkynylsilanols with aryl halides promoted by potassium trimethylsilanolate.

The palladium-catalyzed cross-coupling of aliphatic alkynylsilanols with aryl iodides has been demonstrated with potassium trimethylsilanolate as the coupling promoter and copper(I) iodide as a cocatalyst. The cross-coupling proceeds at room temperature in good to excellent yield with a range of aryl iodides. A comparison of the reactivity of alkynylsilanols, trimethylsilylalkynes, and terminal alkynes under fluoride and fluoride-free conditions was performed to elucidate the role of silicon in the Sonogashira reaction.