Propargyl radical chemistry: renaissance instigated by metal coordination.

Over the last two decades, radical chemistry of propargyl systems was developed into a potent synthetic field providing access to classes of organic compounds that are otherwise hardly accessible. The levels of diastereoselection thus achieved (up to 100%) are unprecedented for free propargyl radicals, as well as for organic radicals π-bonded to transition metals. These advances were enabled by the coordination of the triple bond to a Co2(CO)6 core that precluded an acetylene-allene rearrangement, stabilized requisite propargyl cations, created conformational constraints at the carbon-carbon bond formation site, configurationally altered the acetylenic moiety allowing for 1,3-steric induction upon the newly formed stereocenters, increased bulkiness of propargyl triads thus controlling the spatial orientation of converging radicals, and allowed for α-to-γ projection of the reaction site and alteration of the transiency of radical intermediates.

Impact of reducing agent, temperature, and substrate topology on diastereoselectivity of the intermolecular coupling reactions, or how "free" are cobalt-complexed propargyl radicals?

Applicability of the term "free radical" to organometallic radicals was studied by using the stereoselectivity of radical C-C bond formation as a diagnostic tool. Based on diastereoselectivity data, it was concluded that the reduction of π-bonded, Co2(CO)6-complexed propargyl cations with heterogeneous reducing agents (Zn, Mg) generates "free radicals", while homogeneous reductants (Cp2Co, Na-Ph2CO) produce "sequestered radicals", presumably associated with reductant-derived oxidized species. The latter are comparable in molecular volume to the requisite radical species, thus restricting the motion and conformational freedoms of converging, transition metal-complexed propargyl radicals.

Stereoselective synthesis of meso-1,5-cyclodecadiynes.

Reduction of Co(2)(CO)(6)-stabilized bis-propargyl cations with cobaltocene occurs with high meso-diastereoselectivity (up to 97%), affording, upon decomplexation, meso-1,5-cyclodecadiynes, otherwise hardly accessible.

Spontaneous generation and stereoselective coupling of Co2(CO)6-complexed propargyl radicals.

[reaction: see text] The spontaneous generation and stereoselective coupling of Co(2)(CO)(6)-complexed propargyl radicals have been discovered. One- and two-step complementary methods (Method A: (1) HBF(4); (2) CH(2)Cl(2), 20 degrees C; Method B: Tf(2)O, CH(2)Cl(2), 20 degrees C) provided an easy access to synthetically useful d,l-3,4-diaryl-1,5-alkadiynes (de 74-98%).

Thymidine 3',5'-diphosphoric acid derived cations and radicals: ab initio study.

The relative stabilities of thymidine-3',5'-diphosphoric acid (1) derived isomeric cations and radicals were calculated and key geometric parameters were thoroughly analyzed. The most probable sites of a hydride-ion (1', 2', 5-Me) and H-atom (4', 5', 5-Me) abstraction were identified, thus allowing prediction of the regioselectivity of potential damage to the deoxyribose ring and thymine moiety caused by carcinogenic agents of electrophilic and radical nature. [structure: see text]