Diastereoselective and enantioselective silylation of 2-arylcyclohexanols.

The silylation-based kinetic resolution of trans 2-arylcyclohexanols was accomplished by employing a triaryl silyl chloride as the derivatizing reagent with a commercially available isothiourea catalyst. The methodology is selective for the trans diastereomer over the cis, which provides an opportunity to selectively derivatize one stereoisomer out of a mixture of four. By employing this technology, a facile, convenient method to form a highly enantiomerically enriched silylated alcohol was accomplished through a one-pot reduction-silylation sequence that started with a 2-aryl-substituted ketone.

Linear free-energy relationship and rate study on a silylation-based kinetic resolution: mechanistic insights.

The substituent effect of different p-substituted triphenylsilyl chlorides on silylation-based kinetic resolutions was explored. Electron-donating groups slow down the reaction rate and improve the selectivity, while electron-withdrawing groups increase the reaction rate and decrease the selectivity. Linear free-energy relationships were found correlating both selectivity factors and initial rates to the σ(para) Hammett parameters. A weak correlation of selectivity factors to Charton values was also observed when just alkyl substituents were employed but was nonexistent when substituents with more electronic effects were incorporated. The rate data suggest that a significant redistribution of charge occurs in the transition state, with an overall decrease in positive charge. The linear free-energy relationship derived from selectivity factors is best understood by the Hammond postulate. Early and late transition states describe the amount of substrate participation in the transition state and therefore the difference in energy between the diastereomeric transition states of the two enantiomers. This work highlights our efforts toward understanding the mechanism and origin of selectivity in our silylation-based kinetic resolution.

Halogenated catechols from cycloaddition reactions of η-(2-ethoxyvinylketene)iron(0) complexes with 1-haloalkynes.

1-chloroalkynes and 1-bromohexyne undergo cycloaddition reactions with ethoxyvinylketeneiron(0) complexes to form chloro and bromocatechols. With most substituents, the halogen is incorporated ortho to the phenolic hydroxyl group regioselectively. With chloroethyne, chlorohexyne, and methyl chloropropiolate, the reverse regioselection is observed. Ab initio calculations reveal that the products are, in most cases, nearly isoenergetic, which indicates that the intermediate ketene-alkyne adduct geometry must be important in determining the product distribution.

Incorporation of hexafluorobutyne into furans or phenols via reaction with iron(0) carbene or vinylketene complexes.

Reaction of iron(0) carbene complexes with hexafluorobut-2-yne produces 3,4-bis(trifluoromethylated)furans in a process that is favored for electron rich carbenes. No traces of furannulation or benzannulation are observed with Group 6 Fischer carbenes. Reaction of hexafluorobut-2-yne with a vinylketene iron(0) complex gives a 2,3-bis-trifluoromethylated phenol.