Allyl alkali-metal compounds

Simple allyl alkali metal compounds have only a small capability for discriminating between diastereotopic faces of carbonyl compounds. Although a matter of simple diastereoselectivity, this can be concluded from the reaction of conformationally locked 4-/erf-butylcyclohexanone... 

Whatever the best explanation may be, an indication that allylic alkali metal compounds or allylic carbanions do in fact form the less stable of the two possible acids on neutralization is found in the results of the reduction of aromatic compounds by dissolving metals.376The detection of a paramagnetic intermediate in a similar system and polaro-graphic evidence indicate a one electron transfer in the rate and potential determining step.878 376 The mechanism therefore involves ions (or organometallic intermediates) like the following ... 

Hence measurements have been made on unsubstituted allyl alkali-metal compounds, and also on neopentylallyl (I, 5,5-dimethylhexen-2-) and neopentylmethallyl (II, 2,5,5-trimethyl-hexen-2-) alkali-metal compounds which are models of the polymerizing chain end in the anionic polymerization of butadiene and isoprene respectively. 

The allyl alkali-metal compounds give 2 line C-13 nmr spectra. The terminal positions are equivalent either because of rapid equilibrium between two covalent structures, or because the structure is a delocalized symmetrical ion. The chemical shifts and C-C coupling constants are recorded in table I. Reasonable agreement with literature 6 values are found (8, 2). 

If the allyl alkali metal compounds are ionic compounds, then if the allyl ion is delocalized the terminal carbons would be equivalent and contain most of the charge, and all three... 

In the present study, the Jg g values in the allyl alkali metal compounds are near 60 hz for the K and Cs compounds, and a little lower for the Li compound, table I. Allyl mercury is not a delocalized system, and if any exchange occurs between the two ends of the allyl radical it is slow on the NMR time scale. Consequently and Jg Y are measurable and are h2 and 69 hz... 

Free energies AG have been reported for the rotational barriers in allyl alkali metal compounds (set CPI, Table 10) . As the only variable is the alkali metal ion, the AG values were correlated with equation 45 ... 

Table 14. Calculated charges on Allylic Carbon Atoms of (I) NeopentylallyP- and (II) Neopentyl-methylallylb-Alkali Metal Compounds 139,1961...

Ion-pair effects, as in the case of the unsubstituted allyl alkali metal compounds 23a-d, do not markedly influence the rotational barriers of the 1,3-diphenylallyl-lithium species 27a-f, although the reason is different 27a-f are solvent-separated ion pairs 36). Addition of HMPT to the THF solution of 27a raises the AG3 c value by 0.9 kcal/mol which corresponds to a rate retardation of 5-6 times. In the case of the methyl-substituted allyl anion 27b HMPT slows down the rate by a factor of only 2-3. With the 2-cyano anion 27c the AG values of the Li+ compound in THF and the Li +, Na+ andK+ species in dimethyl sulfoxide (DMSO) are the same18). In the case of 27e and 27f the rate of the rearrangement is not affected if HMPT or TMEDA are added to the THF solutions. The observations of some solvent dependence in the case of the sterically less hindered 27a and b, but of no effect with the more crowded 27e and f are in line with the general observation that solvent-separated ion pairs are favored with respect to contact ions pairs by increasing steric hindrance 25). Hence, these experimental results could be interpreted to mean that in the case of 27a and b contact ion pairs participate in the allyl anion rearrangement reaction. 

In previous chapters the influence of the gegenion and the solvent on rearrangements of carbanions mostly concerned the rate of a certain reaction (and thus kinetic aspects), as, e.g., in the case of the cyclopropyl-allyl anion rearrangement (Sect. 2.1), the ally anion isomerization (Sect. 2.2), the cyclization of 5-hexenyl alkali metal compounds (Sect. 4) or the configurational isomerization of a-substituted vinyl-lithium compounds (Sect. 5). 

Table 9. Calculated Charges on Allyl Carbon Atoms of Neopentylallyl- and Neopentylmethylallyl-alkali Metal Compound ...

Table 10 Calculated charges on allyl carbon atoms of (6) neopentylallyl- and (7) neopentylmethylallyl-alkali metal 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... 

Calculations of alkali metal allyl derivatives involving all alkali metals (Li-Cs) indicate a preferred geometry with the metal symmetrically bound in a predominantly electrostatic manner to all three carbon atoms.143 Solution studies of allyllithium in ether indicate the compounds to be highly aggregated in THF complex dynamic behavior is observed. 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

All the allylic compounds were prepared by reacting the appropriate mercury allyl compounds with either a film of the alkali metal, or finely divided lithium, in an evacuated apparatus fitted with breakseals, a glass filter, and a nmr tube. The solvent was THF except where noted. 

All unsaturated hydrocarbons that possess removable allylic hydrogens or activated double bonds readily undergo oligomerization in the presence of basic reagents.30,31 Alkali metals, organoalkali compounds, and alkali alcoholates are most frequently applied. 

Temperature variations of 1H- and 13C-NMR spectra of allyl and pentadienyl compounds of the alkali metals have given information about barriers to rotation about the C—C bonds. The endo and exo isomers of the allyl anions [Eq. (1)] are formed stereospecifically at low temperatures from Z-and E-alkenes, respectively (75,76). 

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). 

Pentadienyl carbanions are analogous to allyl anions with an extended delocalization of charge. Reaction of 1,3- or 1,4-pentadienes and alkali metals in THF in the presence of a base, such as NMes or TMEDA, affords crystalline pentadienylalkali metal complexes. A contact ion pair structure is predicted for these compounds by theoretical calculations and is consistent with solution structural data obtained by NMR. The pentadienyl anion usually interacts with the cation as an rj - or ) -ligand depending on the structural orientation of the backbone carbon atoms of the pentadienyl anion (W-, S-, or U-shaped skeletal structures). A contact ion pair structure having a W-shaped pentadienyl ligand is shown (16). 2,4-Disubstituted... 

The Tsuji-Trost reaction involves Pd-catalyzed allylation of stabilized enolates and related compounds, such as malonate and acetoacetate anions. Since most of the counterions have been alkali metals, such as Li and Na, this vast topic is mostly outside the scope of this... 

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