No abstract

Simple additions are the most atom economic way to effect alkylations. The ability to effect the hydrocarbonation of allenes asymmetrically then becomes a highly efficient alkylation protocol. The first example of such a protocol involves the ability of a palladium(0) catalyst derived from palladium trifluoroacetate dimer and the bis-2-diphenylphosphinobenzamide of trans-1,2-diamininocyclohexane to catalyze additions to benzyloxyalkene. Various substituted Meldrum's acids including hydroxy Meldrum's acid react well in the presence of 1 mol % trifluoroacetic acid to give one regioisomer with ee's ranging from 82 to 99%. Switching to azlactones to access unusual quarternary amino acids requires somewhat more basic conditions. Thus, use of 2 mol % potassium alpha-butoxide and 20 mol % hippuric acid leads to a smooth reaction to produce a simple regiosomer. This nucleophile raises the question of facial selectivity with respect to both the nucleophile and the electrophile. Excellent diastereoselectivity (dr 13-20:1) and enantioselectivity (85-94% ee) are obtained. Thus, a new approach for asymmetric allylic alkylations of carbon pronucleophiles by simple additions provides a very efficient, more atom economic strategy for asymmetric C-C bond formation.

In the present study we describe the efficient synthesis of various Au(I) complexes supported by NHC ligands. Some of these ligands have a pendant pyridine arm that is linked with various tethers (CH 2 ) n to the NHC backbone (n = 0-2). The chloride in the Au(I) complexes is easily and cleanly replaced by an aryl group upon reaction with an aryl-Grignard reagent. The thus obtained aryl Au(I) complexes are cleanly oxidized to the corresponding Au(III) complexes with phenyliodoso dichloride, as are the corresponding halide Au(I) complexes. The attempted salt metathesis with the parent Au(III) complex led to the oxidative coupling of the aryl residues with formation of the Au(I) complex. Some of the complexes are promising catalysts in the cycloisomerization of an ω-alkynylfuran to isobenzofuranol in the presence of a silver salt. For those precursors with pendant pyridine arms, a cationic dimeric Au complex was isolated and characterized, which represents a catalyst resting state and forms under reaction conditions.

Enhancing atom economy of the metal-catalyzed asymmetric allylic alkylation (AAA) shifts from the usual nucleophilic displacement of a leaving group to an addition of a pronucleophile to a double bond. Using 1-alkoxyallenes as proelectrophiles, the palladium-catalyzed AAA proceeds with 1,3-dicarbonyl compounds as pronucleophiles with excellent regioselectivity and enantiomeric excess under optimized conditions. The pH of the medium proved crucial for reactivity/selectivity. By using the more acidic Meldrum's acids, the reactions required a co-catalytic amount of Brønsted acid, such as trifluoroacetic acid. Single regioisomeric products of 82-99 % ee were obtained. On the other hand, the less acidic 1,3-diketones failed to react under such conditions. The fact that a less acidic acid like benzoic acid sufficed, suggested the need for general base catalysis as well. Thus, a mixture of triethylamine and benzoic acid proved optimal (ee's 93-99). Employment of the (R,R)-phenyl Trost ligand gave a product with S configuration. A model to rationalize the results has been developed.

The important enantioselective hydroformylation catalyst [(R,S)-Binaphos](CO)2RhH has been reexamained by low-temperature NMR spectroscopy. Both 1H and 31P NMR spectroscopy at −90 °C allow direct observation of a mixture of two apical−equatorial chelates. The major chelate, 1 eq,ap , was shown to have an equatorial phosphine and an apical phosphite. Its structure was unambiguously assigned using low-temperature 31P NMR spectroscopy with selective decoupling of aromatic hydrogens but not of the rhodium hydride, which showed a 225 Hz trans phosphite to RhH coupling. The equilibrium constant for [1 eq,ap ]/[1 ap,eq ] was determined over a wide temperature range.