High regio-and stereocontrol in the dehydroxy — fluorination of propargylic alcohols and the corresponding Cobalt-carbonyl complexes

Reaction of Alkynyl- and Alkenyltrifluoroborates with Propargyldicobalt Cations: Alkynylation, Alkenylation, and Cyclopropanation Product Pathways

The Lewis acid-mediated Nicholas reactions of propargyl acetate-Co₂(CO)₆ complexes with a series of potassium alkynyltrifluoroborates and potassium alkenyltrifluoroborates are described. Alkynyltrifluoroborates directly alkynylate the intermediate propargyldicobalt cations. In contrast, alkenyltrifluoroborates proceed through one of the three modes of dominant reactivity: C-2-substituted alkenyltrifluorobrates directly alkenylate, predominantly with the retention of stereochemistry. C-1-substituted alkenyltrifluoroborates alkenylate at C-2. Potassium vinyltrifluoroborate incorporates a cyclopropane at the site propargyl to alkynedicobalt. Computational analysis of these systems explains the differential modes of reactivity of alkenyltrifluoroborates and outlines the probable mechanisms for the formation of each product.

Introduction of fluorine and fluorine-containing functional groups

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

Synthesis of propargylic fluorides from allenylsilanes

Allenylsilanes reacted at room temperature in acetonitrile with Selectfluor, an electrophilic fluorinating reagent, to give secondary propargylic fluorides in moderate to good yields; mechanistically, a side-product resulting from a 1,2-silyl shift testifies to the presence of a cationic intermediate.

The Solid-Phase Nicholas Reaction: Scope and Limitations

Two libraries of alpha-substituted alkynes has been prepared on solid phase using a sequential Sonogashira/Nicholas reaction approach. The scope of nucleophiles in the Nicholas reaction on solid phase has been investigated, including carbon, oxygen, nitrogen, sulfur, fluoride, and hydride nucleophiles. The conditions for the reaction sequence have been optimized in terms of Lewis acid, catalyst for the Sonogashira step, temperature, reaction time, and decomplexation method, enabling the five-step sequence to be performed in 1 day.

Tethered 2 + 2 + 2 cycloaddition reactions of cobalt–cycloheptyne complexes

Cycloheptyne-dicobalt hexacarbonyl complexes, substituted by propargylic ether functions, undergo 2 + 2 + 2 cycloaddition reactions with alkynes to give tricyclic benzocycloheptanes; an all-intramolecular version of this transformation is also possible.

The propargylic route as a short and versatile entry to optically active monofluorinated compounds

Using selected models and appropriate NMR techniques, it has been demonstrated that dehydroxyfluorination in the propargylic position can be highly regio- and stereoselective. The corresponding propargylic fluorides are very useful intermediates for short preparations of stereodefined unsaturated or polyunsaturated compounds with a single fluorine atom in allylic or propargylic position. This strategy offers good means for the synthesis of chiral, nonracemic monofluorinated analogues of natural products.

Synthesis of new optically active propargylic fluorides and application to the enantioselective synthesis of monofluorinated analogues of fatty acid metabolites

A new approach to obtain optically active unsaturated or polyunsaturated systems with a single fluorine atom in an allylic or propargylic position is reported. Central to this strategy is the high regio- and stereocontrol observed during the fluorination of propargylic alcohols allowing a short and efficient synthesis of 1. Further, simple functional group transformations gave the enals 2 and 3. These three key intermediates were used for the preparation of optically active monofluorinated analogues of fatty acid metabolites.