Organolithium compounds dimerization

The 1 1 zincate reagent is believed to be dimeric. At higher ratios of organolithium compounds, 2 1 and 3 1 species can be formed.174... 

Wyman, Allen and Altares (20) reported that the carbonation of poly-(styryl)lithium in benzene with gaseous carbon dioxide produced only a 60% yield of carboxylic acid the acid was contaminated with significant amounts of the corresponding ketone (dimer) and tertiary alcohol (trimer) as shown in eq. 6. A recent, careful, detailed investigation of the carbonation of polymeric organolithium compounds has... 

No carboxylic acid functionality was detected either by thin-layer chromatographic analyses or by end-group titration. Therefore, procedures are now available to control the carbonation of polymeric organolithium compounds to efficiently produce either the carbox-ylated chain ends or the corresponding ketone dimer. 

There are multiple systems for naming organolithium compounds. In one, CeHsLi is named phenyl lithium and w-C4H9Li is w-butyl lithium. In another, these species are named Uthiobenzene and 1-lithiobutane, respectively, when the lithium atom is regarded as a substituent on the hydrocarbon parent. A third nomenclature approach assumes these species are ionic salts, e.g. the above two compounds are called lithium phenylide and lithium butylide. We will bypass any questions of aggregation by referring to these compounds by their monomeric names (e.g. phenyl lithium and not dimeric phenyl lithium, phenyl lithium dimer nor diphenyl dilithium), and where monomeric species are actually meant, we will make this explicit. 

Organolithium compounds tend to associate into dimers, higher oligomers and polymers of two types Complexes where the Li atoms are linked to each other by a chain of one or more atoms of other elements (C, N, O etc.), and complexes where the Li and other metallic atoms are close to each other, forming clusters. Section V presents examples of application of instrumental methods—mainly NMR and XRD—to structural elucidation of these associated species. 

A study of the state of association of the functionalized organolithium compounds 204a-d was carried out by multinuclear ( H, Li, Li, C, N and P) NMR spectroscopy, using Li- and N-enriched species. Spectral evidence, supported in part by XRD crystallographic evidence, points to compounds 204a-c being dimerically associated in etheric solutions in three different forms (205-207). The interconversion among these three... 

The dimeric complex 74 reacts with phenylacetylene or ferrocenylacetylene to yield the tetrameric complexes 75a and 75b, respectively, according to equation 26. These complexes are stable in CDCI3 solution in the absence of air and can be characterized by H and NMR spectroscopies. The low solubility of 75a in unreactive organic solvents precludes detailed studies of the solution structure in reactive solvents it decomposes to a dimeric complex, 76, according to equation 27 3. j jjg association behavior of these complexes resembles that of analogous organolithium compounds - 303... 

In general, simple alkyllithiums exist predominantly as either hexamers (for sterically unhindered RLi) or tetramers (for sterically hindered RLi) in hydrocarbon solvents and as tetramers in basic solvents (9-12). Polymeric organolithium compounds such as poly(styryl)lithium exist as dimers in hydrocarbon solution and are unassociated in basic solvents such as tetrahydrofuran (13-15). The state of association of poly-(dienyl)lithiums in hydrocarbon solution is a subject of current... 

This supposition is supported by results for linking reactions of polymeric organolithium compounds which indicate that the steric requirements of a poly(styryl) chain end are larger than those for a poly(dienyl) chain end ( 4,25). Since a larger sensitivity to base steric requirements is exhibited by poly-(isoprenyl)lithium and it is known that the coordination process for poly(styryl)lithium involves coordination to give the unassociated species (eq 1), it is concluded that tetrahydrofuran coordination with poly(isoprenyl)lithium must involve interaction with an associated species (presumably the dimer) to explain the large sensitivity to the steric requirements of the base. 

The crystal structures of numerous organolithium compounds and complexes with 0—Li bonds are now available (2-5). Table IV lists a number of these species, as well as two derivatives of heavier alkali metals. As with the C—Li derivatives just discussed (Tables II and III), clustered (ROLi) tetramers and hexamers, as well as ring dimers, are prevalent. Note that (OLi)2,3 ring systems also are pseudoplanar (Fig. 21a). However, extensive stacking leading to polymers will only occur if the substituents on 0 are small and if polar ligands are absent. Otherwise, limited (double) stacks or unstacked rings form. 

The validity of the viscosity measurements regarding the reported52) influence of anisole and diphenyl ether on the association of the poly(styryl)lithium dimers has, though, been questioned 78-160>161>. Suffice it to note that the fallacies in the data provided 160,161) have been commented upon elsewhere 162). Even though it is well-known that ethereal solvents can interact with organolithium compounds, no explanation was given 78,160,161 as to why aromatic ethers should be completely exempt from this general behavior. 

Any rigorous study of the oxidation of polymeric organolithium compounds should consider these products and their variation in yield with reaction conditions. To date, few of these reaction products have been considered, let alone identified and analyzed. However, the presence of the macroperoxide has been identified recently among the products of the oxidation of poly(styryl)lithium 352). Lithium aluminium hydride reduction followed by SEC analysis of the dimer fraction before and after reduction... 

Acyl anions (RC(=0)M) are unstable, and quickly dimerize at temperatures >-100 °C (Section 5.4.7). These intermediates are best generated by reaction of organolithium compounds or cuprates with carbon monoxide at -110 °C and should be trapped immediately by an electrophile [344—347]. Metalated formic acid esters (R0C(=0)M) have been generated as intermediates by treatment of alcoholates with carbon monoxide, and can either be protonated to yield formic acid esters, or left to rearrange to carboxylates (R0C(=0)M —> RC02M) (Scheme 5.38) [348]. Related intermediates are presumably also formed by treatment of alcohols with formamide acetals (Scheme 5.38) [349]. More stable than acyl lithium compounds are acyl silanes or transition metal acyl complexes, which can also be used to perform nucleophilic acylations [350],... 

The preferred site of deprotonation of di- or polysubstituted arenes is not easy to predict. In 1,3-disubstituted benzenes in which both substituents facilitate ortho-metalation, deprotonation will usually occur between these two groups [181, 365, 408, 416-419] (Scheme 5.45). Dialkylamino groups, however, can sometimes deactivate ortho positions (fourth reaction, Scheme 5.45), but this does not always happen [181, 420], 3-Chloroanisole [411] and 3-fluoroanisole [421] are deprotonated by organolithium compounds between the two functional groups, but the lithiated arenes dimerize readily at -78 °C, presumably via intermediate aryne formation (last example, Scheme 5.45). 

Scheme5.69. Dimerization of organolithium compounds and imidazolones[401,508, 511, 515], Ar=4-(MeO)C6H4.

Organolithium compounds occur in solution as dimeric, tetrameric, or hexameric aggregates held together by electron-deficient bridge bonds (14). The actual degree of association depends on the alkyl group involved and the solvent. The nature of the association in these derivatives permits two types of exchange ... 

The preferential deaggregation of organolithium compounds at lower temperatures accounts for the fact that the latter can typically be reacted at -78°C ( dry ice temperature ) in many cases the monomeric organolithium compound is reactive, whereas the corresponding dimers, tetramers and/or hexamers are unreactive. Under these conditions a low reaction temperature not only maximizes the concentration of the monomeric organolithium compound, but the reaction rate as well ( ). 

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