Radical coupling diastereoselectivity

Lomolder, R. and Schafer, H.J. (1987) Low-temperature photolysis of peracetylated dodecanoyl peroxides of tartaric acid and D-gluconic acid in the solid state- a diastereoselective radical coupling. Angewandte Chemie, International Edition, 26, 1253-1254. 

In a reaction closely related to the latter, pyranylidene derivatives are obtained by the intermolecular radical coupling of alkynyl- or alkenylcarbene complexes and epoxides. Good diastereoselectivities are observed when cyclic epoxides are used. Moreover, the best results are reached by the generation of the alkyl radical using titanocene monochloride dimer [90] (Scheme 43). 

Sml2-H20-amine mediated diastereoselective intramolecular couplings were reported for the synthesis of dihydrobenzofuran, and a radical mechanism was proposed to account for this reaction <03OBC2423>. A similar approach for the synthesis of pyridine-fused polycyclic amines was also developed by use of AlBN/BUjSnH as a reducing system <03TL2995>. 

Scheme 12.1 Synthesis of gem-difluorocyclopropane through diastereoselective Michael reaction followed by radical coupling.

Merlic et al. showed that radical coupling reactions between unsaturated carbene tungsten or chromium carbene complexes and epoxides in the presence of Cp2TiCl lead to tetrahydro-pyranylidene carbene complexes with high diastereoselectivity <91JA9855,93TL227). 

An example of a diastereoselective reaction is the radical coupling of dinuclear acetylenic cobalt complexes. This coupling leads to the compound of relative configuration (R, / ) (2.52) with a diastereoselectivity of 0.9 ° (Figure 2.71). (Hi) Enzymatic reactions... 

Ghatlia, N. D. and Turro, N. J., Diastereoselective induction in radical coupling reactions photolysis of 2,4-diphenylpentan-3-ones adsorbed on faujasite zeolite, /. Photochem. Photobiol A Chem., 57, 7, 1991. 

Carboxylates, which are chiral in the a-position totally lose their optical activity in mixed Kolbe electrolyses [93, 94]. This racemization supports either a free radical or its fast dynamic desorption-adsorption at the electrode. A clearer distinction can be made by looking at the diastereoselectivity of the coupling reaction. Adsorbed radicals should be stabilized and thus react via a more product like transition state... 

Negishi E, Tan Z (2005) Diastereoselective, Enantioselective, and Regioselective Carbo-alumination Reactions Catalyzed by Zirconocene Derivatives. 8 139-176 Netherton M, Fu GC (2005)Pa]ladium-catalyzed Cross-Coupling Reactions of Unactivated Alkyl Electrophiles with Organometallic Compounds. 14 85-108 Nicolaou KC, King NP, He Y (1998) Ring-Closing Metathesis in the Synthesis of EpothUones and Polyether Natmal Products. 1 73-104 Nishiyama H (2004) Cyclopropanation with Ruthenium Catalysts. 11 81-92 Noels A, Demonceau A, Delaude L (2004) Ruthenium Promoted Catalysed Radical Processes toward Fine Chemistry. 11 155-171... 

Acroleins and u,(i-unsaturated ketones are coupled with aliphatic aldehydes in the Cr-catalyzed diastereoselective coupling in the presence of Mn and MesSiCl [60]. As shown in Scheme 24, the generated radical species undergoes the further one-electron reduction to afford the corresponding al-lylchromium 41, which reacts with the aldehyde to give the corresponding... 

The results show that, in the coupling of anodically generated radicals, good diastereoselectivities can only be achieved when the intermediate radical... 

Oxidation of arylolefins, enolethers, or dienes yields intermolecular homocoupling products in moderate to good yield (see Sect. 13.2.1.4) however, no pronounced diastereoselectivity was observed. This is also due to the fact that the coupling sites do not tolerate substituents that would make up a prostereogenic center. Furthermore, the fairly stable cations of the dimerized radical cation solvolyze stereounselectively. The same holds for the intermolecular coupling of aromatic compounds, in... 

Intermolecular coupling of ketones and alkenes, promoted by SmH, occurs with excellent stereochemical control. In one such reaction, samarium(II) iodide has been used to prepare cyclobutanones and cyclobutanols from chiral, 6-oxohex-2-enoates (equation 137)520. The reaction is performed in THF in the presence of HMPT and occurs in good yield with excellent stereocontrol. If appropriately located carbonyl and alkene moieties are present in a molecule, then Sml2-HMPT can be used to form cyclooctanols by a radical cyclization process in some cases there is a reasonable degree of diastereoselectivity (equation 138)521,522. 

The samarium-lV-bromosuccinimide combination reductively dimerizes carbonyl compounds.163 This pinacol-type coupling gives diols in 60-80% yield, with some diastereoselectivity the by-product from simple reduction (i.e. alcohol) is typically 5-10%. The conditions suggest a single electron transfer to give carbonyl radical anion, which then self-couples. Even congested ketones such as benzophenone and fluorenone worked well. 

Prototypical radical additions were examined under manganese-mediated photolysis conditions with InCp as the Lewis acid, coupling isopropyl iodide with a variety of y-hydrazonoesters 35a-35d (Table 6) bearing varied substitution at the position a to the ester. The a-methyl, a,a-dimethyl, and a-benzyloxy substituents appeared to have little effect on reaction efficiency and selectivity, as all provided the isopropyl adducts with consistently high diastereoselectivities and excellent yields (91-98%). Surprisingly, the selectivity was only slightly... 

---