Allylic lithium compounds

Two main structural types have been identified for allyl alkali metal species solvated ions in the form of CIPs where a delocalized anion with metal coordination is perpendicular to the ligand plane,130-134 or unsolvated allylic lithium compounds displaying localized ligand systems with NMR spectra closely resembling those of alkenes.135-138... 

We asked the question if it is possible to replace the phenyl residues which are parts of the reagents of Scheme 3 by organoelement groups. On these groups we make the following demands They should be electron withdrawing and resistant to the attack of 2-azaallyl-lithium, of allyl-lithium compounds, of water, and of oxygen as well. Moreover they should be easily removable after the cycloaddition. 

Such a system could be the indicated bond rotation in an allylic lithium compound 2 (equation 18). 

Rotation around the CD—CHAHB bond of the allylic lithium compound in equation 18 is an example of a degenerate reorganization, which we abbreviate as in equation 23. 

We use the spin product representation, so that a spin product for the methylene hydrogens in the allylic lithium compound in equation 18 is written as ab, where the order of spins follows the numbering of the sites and ab, a1 b1 are two spin states connected by a Amz = — 1 transition. The effect of rotation around the C2—C3 bond is to exchange the environments of the methylene hydrogens and to permute the order of their spins in the spin products (equation 24). 

With few exceptions most allylic lithium compounds dissolve in and coordinate with ethers and tertiary amines29. As of this writing there have been no reports of 13C—7Li or 13C—6Li spin coupling in any allylic lithium compounds aside from some internally solvated species described below. 

Other allylic lithium compounds which have been investigated include 26 to 32 omitting solvation around lithium. 

TABLE 7. Barriers to rotation in allylic lithium compounds... 

At low temperature the 13C NMR spectra of these geminal methyls consisted of clean 1 1 doublets for each of the compounds. Diastereotopic methyls established the chiral characters of these three compounds. Above 170 K, with increasing temperature there was progressive averaging of these doublets. Phenomenologically, this implied the operation of transfer of coordinated TMEDA between faces of the allyl plane which is, overall, inversion. The process is first order in the allylic lithium compound. The activation parameters are typically A// of 5 to 7 kcal mol 1 with a large negative AX of ca —25 5 eu... 

Internally solvated allylic lithium compounds undergo three fast equilibrium reorganization processes—inversion at lithium bound carbon, bimolecular C—Li exchange and lithium 1,3-sigmatropic shifts49. 

TABLE 12. Selected X-ray structural and NMR parameters of internally solvated allylic lithium compounds (M = monomer, D = dimer, P = polymer). For numbering see structure 6749d... 

The second dynamic process involves C—Li exchange. Around and below 230 K, most of the internally solvated allylic lithium compounds exhibit one bond spin coupling between 13C and 7Li (/ = 3/2) and to 6Li (/ = 1). The 13C NMR of lithium bound carbon consists of equally spaced equal multiplets, an equal triplet for coupling to 6Li and an equal quartet for coupling to 7Li. The separation between adjacent lines are the coupling constants. 

TABLE 13. Quantitative and qualitative dynamic behavior of internally solvated allylic lithium compounds in diethyl ether-dio solution... 

Bimolecular exchange implies a dimeric transition state shown in partial form as 81, most likely preceded by a dimeric intermediate. Note that compounds 68 and 75 are dimers in the ground state. Thus, dimeric structures may be energetically accessible from ground state monomeric internally solvated allylic lithium compounds. Further, it is interesting... 

The electropositive nature of alkali metals allows for insertions into alkali metal-carbon bonds. Thus, the reaction of an organoaUcali metal reagent with an unsaturated organic molecule produces a more complex organoaUcali metal compound. In particular, organolithium compounds will react in this way with conjugated dienes, even under mild conditions. For example, 1,3-butadiene reacts with ferr-butyl lithium to give an allyl lithium compound (equation 15). 

In the course of the preparation of maytansinoid model compoimds Goodwin et al. [116] used the regioselective a-alkylation of an allylic sulfide attached to an imidazole imit (one member of the G family depicted in Scheme 44), as already proposed by Evans [106] (Scheme 47). Metalation of 174 led to an allylic lithium compound with the metal chelated by the imidazole unit. This compound readily reacted with iodide 175 dehvering exclu-... 

Extensive use of vicinal proton-proton couplings has been made by Fraenkel and Qiu in their studies on the structure and dynamic behaviour of several methyl substituted allylic lithium compounds, by Shestakova et who studied the structure of the complex between lanthanum(III) nitrate and 1,9 diaza-18-crown-6, and by Goto et whose subjects of interest have been square-planar ternary platinum(II) complexes with A -ethyl- or A -benzyl-1,2-ethanediamine and 2,2 -bipyridine or 1,10-phenanthroline. 

The dynamics of bimolecular C-Li exchange in several allylic lithium compounds with ligands tethered at C2 has been investigated by Fraenkel et alF by the use of the Li-C couplings yielding the thermodynamic data of this process. 

Boranes are easily prepared from alkenes or alkynes by hydroboration (Section 6.4) borates are made from aryl or allyl lithium compounds and trimethyl borate, among other routes. 

As shown in Scheme 3.1 [63], organolithium compounds in hydrocarbon solvent exist largely in the form of tetramers or hexamers. Initiation apparently occurs when a monomolecular species in equilibrium with the associated species adds to the 1,4- of butadiene to give an allylic lithium compound, which propagates by repeated 1,4-addition until all butadiene is consumed. The intermediate allylic lithium compounds are stable enough to remain active at room temperature almost indefinitely [63]. 

Jiuc couplings have been observed by Fraenkel et al in four allylic lithium compounds prepared with a tethered ligand, (CH30CH2CH2)2 NCH2C(CH3)2-L, attached to a terminal allyl carbon. These were equimolar equilibrium mixtures of 3-enrfo-L-allyllitium with 3-exu-L-allyllitium, and l-evo-TMS-3-e do-L-allyllitium with l-exo-TMS-3-exu-L-allyllitium where Jciu of 5.2, 9.1, 3.5 and 6.8 Hz, respectively, have been observed for C-1 the couplings between the lithium and C-2 nuclei were considerably smaller, of ca. 2 Hz or less. 

The transformation of allylic lithium compounds into their sodium and potassium analogs seems to occur with acceptable completeness. Extensive washing... 

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