- Metal-induced reductive cleavage reactions: An experimental and theoretical (MNDO) study on the stereochemical puzzle of birch and vinylogous birch processes
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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.
- Saa, Jose M.,Ballester, Pablo,Deya, Pere M.,Capo, Magdalena,Garcias, Xavier
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- Pivaloylmetals (tBu-COM: M=Li, MgX, K) as equilibrium components
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Short-lived pivaloylmetals, (H3C)3C-COM, were established as the reactive intermediates arising through thermal heterolytic expulsion of O=CtBu2 from the overcrowded metal alkoxides tBuC(=O)-C(-OM)tBu2 (M=MgX, Li, K). In all three cases, this fission step is counteracted by a faster return process, as shown through the trapping of tBu-COM by O=C(tBu)-C(CD3)3 with formation of the deuterated starting alkoxides. If generated in the absence of trapping agents, all three tBu-COM species "dimerize" to give the enediolates MO-C(tBu)=C(tBu)-OM along with O=CtBu2 (2 equiv). A common-component rate depression by surplus O=CtBu2 proves the existence of some free tBu-COM (separated from O=CtBu2); but companion intermediates with the traits of an undissociated complex such as tBu-COM & O=CtBu2 had to be postulated. The slow fission step generating tBu-COMgX in THF levels the overall rates of dimerization, ketone addition, and deuterium incorporation. Formed by much faster fission steps, both tBu-COLi and tBu-COK add very rapidly to ketones and dimerize somewhat slower (but still fairly fast, as shown through trapping of the emerging O=CtBu2 by H3CLi or PhCH2K, respectively). At first sight surprisingly, the rapid fission, return, and dimerization steps combine to very slow overall decay rates of the precursor Li and K alkoxides in the absence of trapping agents: A detailed study revealed that the fast fission step, generating tBu-COLi in THF, is followed by a kinetic partitioning that is heavily biased toward return and against the product-forming dimerization. Both tBu-COLi and tBu-COK form tBu-CH=O with HN(SiMe3)3, but only tBu-COK is basic enough for being protonated by the precursor acyloin tBuC(=O)-C(-OH)tBu2. Copyright
- Knorr,Boehrer,Schubert,Boehrer
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supporting information; experimental part
p. 7506 - 7515
(2012/07/27)
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- Reactivity of the triple ion and separated ion pair of tris(trimethylsilyl) methyllithium with aldehydes: A RINMR study
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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
- Jones, Amanda C.,Sanders, Aaron W.,Sikorski, William H.,Jansen, Kristin L.,Reich, Hans J.
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p. 6060 - 6061
(2008/12/20)
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- On the regioselectivity of alkylation of the (trimethylsilyl) tetramethylcyclopentadienide anion. A new approach to the synthesis of 1,2,3,4-tetramethylfulvene
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The regioselectivity of alkylation of lithium (trimethylsilyl) tetramethylcyclopentadienide C5Me4SiMe3 -Li+ was studied by 1H and 13C NMR spectroscopy using its reactions with MeI, MeOTs, ClCH2CH 2Br, and ClCH2CH2I in different solvents as representative examples. Sterically non-hindered MeI and MeOTs presumably attack the C atom bonded to the silyl group giving 1,2,3,4,5- pentamethylcyclopentadienylsilane. For bulkier alkyl halides, such as ClCH 2CH2Br and ClCH2CH2I, the regioselectivity of alkylation changes to form preferentially gem-dialkyl-substituted cyclopentadienes. The reaction of C5Me 4SiMe3 -Li+ with formaldehyde affords 1,2,3,4-tetramethylfulvene in a high yield, providing an alternative synthetic approach to a number of ω-functionalized peralkylated cyclopentadienes. The quantum-chemical calculations of the C5Me 4SiMe3 - anion by the RHF and DFT (RMPW1PW91) methods in the valence-split 6-311+G(d,p) basis set are in good agreement with the experimental data.
- Krut'ko,Borzov,Veksler
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p. 2182 - 2186
(2007/10/03)
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- Acyl- and Alkylidenephosphines, XV. 2,2-Dimethylpropylidynephosphine, a Stable Compound with a Phosphorus Atom of Coordination Number 1
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In the presence of small amounts of solid sodium hydroxide, -trimethylsilylphosphine reacts at +20 deg C to give hexamethyldisiloxane and 2,2-dimethylpropylidynephosphine (2).In contrast to similar alkylidynephosphines this compound is stable at room temperature.The IR and mass spectrum are discussed, and 1H, 13C and 31P NMR spectral data are given. - Keywords: Alkylidynephosphines, 2,2-Dimethylpropylidynephosphine, P-C Triple Bond
- Becker, Gerd,Gresser, Gudrun,Uhl, Werner
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