2004-14-0Relevant articles and documents
Metal-induced reductive cleavage reactions: An experimental and theoretical (MNDO) study on the stereochemical puzzle of birch and vinylogous birch processes
Saa, Jose M.,Ballester, Pablo,Deya, Pere M.,Capo, Magdalena,Garcias, Xavier
, p. 1035 - 1046 (1996)
The stereochemical puzzle posed by the lithium-promoted Birch and vinylogous Birch reductive cleavage of unsaturated benzyl ethers (BICLE; takes place with retention of configuration of the sensitive 2Δ double bond) and the corresponding cinnamyl analogs (VIBICLE; gives rise to ca. 2.5:1 E:Z mixtures) has been approached by experimental and theoretical means. NMR experiments indicate that the π-type organolithium compounds resulting from these reactions do not form observable mixed aggregates with the lithium silyloxide species generated alongside in the reaction and do not undergo observable isomerization at the temperature of operation. A simplified model for contact, solvent-separated, and isolated ion pairs has allowed us to evaluate these complex reactions in great detail from a theoretical viewpoint, using the MNDO semiempirical method. Relevant features that come out from these comprehensive studies, for which we have employed lithium naphthalenide (LiNaph) or lithium benzenide (LiBenz) as promoters, are as follows: (1) the lowest energy routes for cleavage are those involving contact ion pairs (CIPs) in which the lithium counterion plays a key role by acting as a handle (Lewis acid) to which the leaving group -OR adheres prior to detachment; (2) the different haptomeric structures which reside (local minima) in the potential hypersurface of either the so-called radical anion or the dianion routes show that haptomeric activation is key to understanding cleavage of the C-O bond which, eventually, takes place as a syn β elimination of LiOR; and (3) reductive cleavage of unsaturated benzyl ethers (BICLE) involves transient cation/anion radicals which undergo cleavage and subsequent reduction to the final organolithium with retention of configuration, in accordance with experiment, whereas that of vinylogous cinnamyl ethers (VIBICLE) involve transient dianion/dication species resulting from long-lived cation/anion radicals. In good qualitative agreement with experiment, MNDO finds two diastereomeric routes (ΔΔG* = 0.2 kcal/mol) for cleavage of (appropriately substituted) cinnamyl ethers, but only one for cleavage of the unsaturated benzyl analogs.
Reactivity of the triple ion and separated ion pair of tris(trimethylsilyl) methyllithium with aldehydes: A RINMR study
Jones, Amanda C.,Sanders, Aaron W.,Sikorski, William H.,Jansen, Kristin L.,Reich, Hans J.
, p. 6060 - 6061 (2008/12/20)
Low-temperature rapid-injection NMR (RINMR) experiments were performed on tris(trimethylsilyl)methyllithium. In THF/Me2O solutions, the separated ion (1S) reacted faster than can be measured at -130 °C with MeI and substituted benzaldehydes (k ≥ 2 s-1), whereas the contact ion (1C) dissociated to 1S before reacting. Unexpectedly, the triple ion reacted faster with electron-rich benzaldehydes relative to electron-deficient ones. The addition of HMPA had no effect on the rate of reaction of the triple ion with p-diethylaminobenzaldehyde, and the immediate product of the reaction was the HMPA-solvated separated ion 1S, with the Peterson product forming only slowly. Thus, the aldehyde is catalyzing the dissociation of the triple ion. HMPA greatly decelerated the reaction of 1S (-10), providing an estimate of the Lewis acid activating effect of a THF-solvated lithium cation in an organolithium addition to an aldehyde. Copyright
REACTION OF BUTYLLITHIUM WITH BIS(TRIMETHYLSILYL) PEROXIDE
Baryshnikov, Yu. N.,Kurskii, Yu. A.,Kaloshina, N. N.,Vesnovskaya, G. I.
, p. 2037 - 2040 (2007/10/02)
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