Olefination McMurry

A new synthesis of sterically hindered o-substituted tetraphenylethenes via McMurry olefination of the corresponding 2,2 -disubstituted benzophenones exploits electronic effects that dominate over steric considerations.187... 

Then Li et al. reported on a concise total synthesis of ( )-sarcophytol A (3) from the derivative of , -farnesol (91) by a low-valent titanium-mediated intramolecular McMurry olefination strategy (Scheme 6-4). ... 

Stereoselectivity. See Asymmetric induction Axial/equatorial-, Cis/trans-, Enantio-, Endo/exo- or Erythro/threo-Selectivity Inversion Retention definition (e.e.), 107 footnote Steric hindrance, overcoming of in acylations, 145 in aldol type reactions, 55-56 in corrin synthesis, 261-262 in Diels-Alder cyclizations, 86 in Michael type additions, 90 in oiefinations Barton olefination, 34-35 McMurry olefination, 41 Peterson olefination, 33 in syntheses of ce-hydrdoxy ketones, 52 Steric strain, due to bridges (Bredt s rule) effect on enolization, 276, 277, 296, 299 effect on f3-lactam stability, 311-315 —, due to crowding, release of in chlorophyll synthesis, 258-259 in metc-cyclophane rearrangement, 38, 338 in dodecahedrane synthesis, 336-337 in prismane synthesis, 330 in tetrahedrane synthesis, 330 —, due to small angles, release of, 79-80, 330-333, 337... 

A useful relative of the acyloin condensation is the McMurry olefination reaction. l in this reaction, ketones or aldehydes are treated with Ti(0) (TiCls + LiAlH4 TiCls + K TiCl3 + Li)2i5 to give alkenes.2 6 Reductive coupling is possible with a wide variety of ketones and aldehydes. i 3,217 Retinal (276), for example, was coupled with this reagent to give P-carotene (277) in 85% yield. ... 

McMurry olefin synthesis. Lenoir has published a variation of the McMurry reaction for reductive coupling of ketones (6, 589). The actual reagent presumably is also TiCl2, but is produced by reduction of TiCU in THF or dioxane with zinc. This reagent in the presence of pyridine converts ketones into tetrasubstituted alkenes. The reaction is most satisfactory with symmetrical ketones mixtures of (E)- and (Z)-isomers obtain from unsymmetrical ketones, with the latter predominating. Strongly hindered ketones are reduced by the reagent to the secondary alcohol. 

Promoted McMurry olefination reactions The beneficial effect of pyridine on the TiCls-Zn reagent has long been recognized [216]. Thus, with this system, bis([3j-ferrocenophane-l-ylidene) 49 and its cis isomer could be prepared in 70% yield only after the addition of two equivalents of pyridine (py) in the absence of the aromatic base, a pinacol rearrangement product was obtained in 78% yield, which can be attributed to the Lewis acidity of the ZnCl2 that is formed during the reaction [67] (Scheme 6.8). 

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