Benzyllithium complexes

Chromium tricarbonyl complexed to benzaldehyde acetals can be reduced using lithium 4,4 -di(t-butyl)biphenyl to give benzyllithium complexes that can be further functionahzed using electrophiles (Scheme 94). 

Compounds of Group 1. - (6Li, 15N) and (6Li, 13C) couplings were observed for mixed complexes formed between LiCH2CN and chiral lithium amides (1H, 6Li, 13C, 15N data).1 7Li and 31P H) HMQC experiments were used to assign the structures of benzyllithium complexes of /V-methyl-/V-ben-zylphosphinamide, e.g. (I).2 111 and 13C NMR and 13C-111 correlation spectra were used to confirm the presence of a C-Si-Ni-Li 4-membered heterocycle in [benzylbis(dimethylamino)-methylsilyl-K2-C,7V](7V, N, N, N -tetramethylenedia-mine-K2-iV,/V)lithium(I).3... 

Top Crystalline benzyllithium complexes. Bottom Crystalline diphenylmethyllithium complexes. 

Optimum Conditions for Preparing Benzyllithium from Toluene. Both the TMEDA and TED complexes of benzyllithium were investigated. Toluene metalation proceeds much faster than does benzene metalation under similar conditions. The benzyllithium complexes were more soluble in hydrocarbon solvents than were the corresponding phenyllithium complexes. This method of preparation of benzyllithium is the most convenient of the few literature procedures available. Other procedures described are the cleavage of benzyl methyl ether with lithium... 

The easiest access to most benzyllithium, -sodium, or -potassium derivatives consists of the deprotonation of the corresponding carbon acids. Hydrocarbons, such as toluene, exhibit a remarkably low kinetic acidity. Excess toluene (without further solvent) is converted into benzyllithium by the action of butyllithium in the presence of complexing diamines such as A. Af.Af.jV -tetramethylethylenediamine (TMEDA) or l,4-diazabicyclo[2.2.2]octane (DABCO) at elevated temperatures1 a procedure is published in reference 2. 

Another system that has been investigated by C CP/MAS NMR spectroscopy as a function of different ligands is a-(dimethylamino)benzyllithium (2, Scheme 1) . The DEE complex was proven to exist in the solid state as an rf coordinated dimer . All the studied complexes are of an tf type according to comparison to solution NMR data. However, the actual structure varies as reflected by the shift difference between the two orf/zo-carbons. This difference ranges from 4.4 ppm for the N, N, N, N, N"-pentamethyldiethylenetriamine (PMDTA) complex to 20.3 ppm for the TMEDA complex. 

Standard organolithium reagents such as butyllithium, ec-butyllithium or tert-butyllithium deprotonate rapidly, if not instantaneously, the relatively acidic hydrocarbons of the 1,4-diene, diaryhnethane, triarylmethane, fluorene, indene and cyclopentadiene families and all terminal acetylenes (1-alkynes) as well. Butyllithium alone is ineffective toward toluene but its coordination complex with A/ ,A/ ,iV, iV-tetramethylethylenediamine does produce benzyllithium in high yield when heated to 80 To introduce metal into less reactive hydrocarbons one has either to rely on neighboring group-assistance or to employ so-called superbases. 

Beak and coworkers found the (—)-sparteine-complex of iV-Boc-Af-(p-methoxyphe-nyl)benzyllithium 244, obtained from 243 by deprotonation with n-BuLi/(—)-sparteine (11) in toluene, to be configurationally stable (equation 57) . On trapping 244 with different electrophiles, the substitution products 245 are formed with high ee. Efficient addition reactions with imines and aldehydes have also been reported. The p-methoxyphenyl residue is conveniently removed by treatment with cerinm ammoninm nitrate (CAN). 

Since carbohthiations usually proceed as syn additions, 458 is expected to be formed first. Due to the configurationally labile benzylic centre it epimerizes to the trani-substitu-ted chelate complex epi-45S. The substitution of epi-458 is assumed to occur with inversion at the benzylic centre. Sterically more demanding reagents (t-BuLi) or the well-stabilized benzyllithium do not add. The reaction works with the same efficiency when other complexing cinnamyl derivatives, such as ethers and primary, secondary, or tertiary amines, are used as substrates . A substoichiometric amount (5 mol%) of (—)-sparteine (11) serves equally well. The appropriate (Z)-cinnamyl derivatives give rise to ewf-459, since the opposite enantiotopic face of the double bond is attacked . 

Remarkably, the closely related benzyllithium 304 is configurationally unstable even at -78 °C.138 Transmetallation of 303 (88% ee) at -78 °C gave an organolithium which reacted to give racemic product 305 in the presence or absence of TMEDA. Furthermore, the reaction of the 304-(-)-sparteine complex with each of racemic or enantiomerically pure 2 in a Hoffmann test gave the same 1 1.6 ratio of diastereoisomers. It is not yet clear whether this unexpected difference between 301 and 304 is due to an electronic difference between the naphthyl and phenyl systems, or whether it arises from the difference in steric hindrance, and therefore the dihedral angle between the ring and the amide, in the two compounds. 

In THF, the alkyllithium compounds are aggregated [157] and the situation is reminiscent of the conditions in hydrocarbon solutions. At high concentrations, the association number (i. e. the number of molecules in the aggregate) decreases. This anomaly is explained by the existence of aggregate—solvent complexes, for example (MeLi)4 8THF Benzyllithium and its polymeric analogue polystyryllithium are not associated. Phenyllithium is mostly present as a dimer or monomer. Both forms are in equilibrium and are solvated. Only the monomeric form of the initiator is active. In practice, benzyllithium reacts only in the form of an ion pair. The fraction of the free benzyl anion must be very small [151c]. 

The benzophenonedilithium compound 50, formed by reduction of benzophenone with lithium metal, crystallizes as a dimer (69). The four lithium atoms in the structure are divided into two different sets. The two benzophenone moieties are bridged, through the carbonyl oxygen atoms, by two symmetry-equivalent lithium atoms. Each of the two other lithiums is bonded to one phenyl ring and the ketone functionality reminiscent of that observed in benzyllithium (29), dilithiodibenzyl ketone (42), and dilithiodibenzylacetylene (49). The two different types of lithium atoms are complexed further to THE and TMEDA. 

Equilibration of Configurationally Labile Organo-lithium Reagents. The equilibration of diastereomeric pairs of alkyllithium-(—)-sparteine complexes and trapping by achiral electrophiles gives enantioenriched products. Examples are a-(A/,JV-diisopropylcarbamoyloxy)benzyllithium in ether, not in THF, l-phenylethyllithium, and the dilithium salt of A/-methyl-3-phenylpropanoic acid amide (eq 2). ... 

A mixed Li /Mg+ aggregate corresponding to (213) is formed with either phenyl or methyl carbanions. An unusual lithium/magnesium acetylide is formed with stoichiometry Li2[(PhCsC)3Mg(TMEDA)]2 and is depicted as (214). The same authors also report the ion pair characterized as the mixed benzyllithium/magnesium TMEDA complex (215). ° A different mixed lithium/magnesium aggregate depicted as (216) is found for the THF-solvated anion of tris(trimethylsi-lyl)methyl carbanion. ... 

If solvent-separated ion pairs or free ions were present, they should produce similar polymer microstructure to that obtained from contact ion pairs since propagation will involve only the allyl anion. There is no evidence for anything other than contact ion pairs in 1 1 lithium complexes with chelating diamines or triamines in hydrocarbon solvents. Only by using excess diamine or the more powerful chelating tetramines can we test the idea. As mentioned previously these are capable of producing some separated ion pairs when the anion is a sufficiently weak nucleophile to be displaced from the lithium by a neutral tertiary amine. With a benzyllithium tetramine complex, both contact and separated ion pair structures were observed spectroscopically. Since allyl and benzyl anions have rather similar charge delocalization, it is reasonable to expect that a tetramine complex of polybutadienyllithium would have similar proportions of contact and separated ion pairs. 

It might be expected that in the presence of TMEDA or other tertiary diamines anomalous reaction products might be obtained with organolithium compounds such as benzyllithium. A number of reports in the literature disclose instances of the expected reaction products from reactions such as carbonation to the carboxylic acid and addition to benzophenone (I, 3, 4, 12). The phenyllithium-TMEDA (1 1) complex in benzene was allowed to react with benzophenone to give a 95% yield of triphenylcarbinol and with cyclohexanone to yield 59% of the 1-phenylcyclohexanol. The reaction with excess trimethylsilyl chloride is apparently quantitative. The main consideration in using these complexes is to use low temperatures for reaction and aqueous washes of ammonium chloride solution in the work-up to remove all of the tertiary diamine (the odor can be detected in low concentrations.)... 

The crystalline yellow 1 1 benzyllithium-TMEDA complex has a lower solubility in toluene than does the (benzyllithium )2-TMEDA complex which tends to form oils. Because of its higher solubility and to eliminate as much of the tertiary diamine as possible, the 2 1 complex was used for synthetic evaluations. It is probably the optimum ratio to use in this benzyllithium system. Solutions of up to 38 wt % benzyl-lithium in toluene or about 59 wt % of the (benzyllithium )2-TMEDA complex have been prepared. At ratios of 4 1 or more in toluene, a dark-red tar or oil also separated from the solution. With hexane as the solvent, yellow crystals of the 1 1 benzyllithium-TMEDA complex precipitated at a ratio of 2 1 or more, a dark-red oil separated instead. 

Synthetic Application of (Benzyllithium —TMED A and Benzyl-lithium—TED. All the synthetic application studies were run with an aged (benzyllithium)2-TMEDA toluene solution that had 0.8% of the total organolithium compounds present as the tolyllithium isomers (only meta present) or with benzyllithium-TED crystalline complex free of ring isomers. 

The solutions of (benzyllithium )2-TMEDA were much more convenient to use than the solid, relatively insoluble benzyllithium-TED complex. The reaction yields vary between these two complexes. Whether this is a function of the different solvent systems used for each, lack of optimization, or the actual tertiary diamine present was not examined, however. 

Benzyllithium-TED. 1-Bromobutane. Added 1-bromobutane to complex dissolved in ether at reflux over 1 hr refluxed overnight hydrolyzed distilled bp 62°C (0.5 mm) (lit. bp 81°C) 80%, n-amylben-zene (36). 

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