Benzyl-lithium

Benzylic deprotonation occurs under normal lithiation conditions when both ortho-positions are occupied. Interestingly, Thomas and co-workers were able to deprotonate a benzylic proton in the presence of an ortho proton (Scheme 8.137). Thus, metalation of 2-(2-methylphenyl)oxazoline 424 produced a benzylic lithium species that reacted with a Cbz-protected leucinal 425 to give a modest yield of the iminolactone diastereomers 426 and 427 together with the expected alcohols 428 and 429. The mixture of 426—429 was efficiently hydrolyzed to give the lactones 430 and 431. 

Alkylation with 1-bromopropane was shown to give the S-configurated 1-methylphenyl-butyl diisopropylcarbamate preferentially. This result is remarkable as alkylation, in this case, occurred by a metalloretentive mode, in contrast to that generally observed on alkylation of benzylic lithium compounds. 

The present synthesis of 4,6-dimethyl-l-hepten-4-ol is an example of the preparation and use of allyllithium, hitherto unknown as a pure reagent. The same general procedure may be used to prepare vinyllithium from ether solutions of any of the compounds (CH2=CH) Sn(CeH,5)4 ( = 1-4), and benzyl-lithium from any of the (CeH6CH2)nSn(C6H5)4 n(w = 1-4) compounds.10... 

A number of cycloalkyl-, vinyl-, aryl-, and benzyllithium compounds (predominantly benzyl-lithiums) are converted into fluoro derivatives in good to excellent yields, e.g. formation of 4, 5, and 6 19 however, when this method was applied to the synthesisof 3-fluorobenzocyclobutene from the lithium salt a violent explosion occurred when the reaction mixture was warmed from — 70 C to room temperature.20 Various fluoro-substituted thiophenes 7 are obtained when the starting compounds (thiophene, 2-methylthiophene, etc.) arc transformed with al-kyllithium compounds to the corresponding lithium derivatives then fluorinated with perchloryl fluoride at 0 C.21 Potassium tricyanomethanidc is converted at —15 C in triglyme into tricyanofluoromethane in 81 % yield.22... 

The allylic- and benzylic-lithium active centers, which can be characterized as having polarized covalent carbon-lithium bonds in hydrocarbon solvents, have been extensively studied with regard to their structure, their kinetic behavior in the propagation event, and their association states. These latter two topics are the subject of this... 

Lastly, electron transfer processes can also compete with nucleophilic acyl substitution in the reaction of perfluorodiacyl peroxides with Grignards [305]. Such a process can lead to a coupling of the Grignard with the perfluoroalkyl radical intermediate. (In contrast, benzyl lithium gives no perfluoroalkylation under the same conditions.)... 

The limited configurational stability of a-alkylthio organolithiums does not extend to those with benzylic lithium-bearing carbon atoms.112 A Hoffmann test reaction of 233 with 6, for example, gives a 40 60 ratio of stereoisomers 234 whether enantiomerically pure or racemic 6 is used.6 Dynamic NMR experiments2 quantified the barrier to racemisation in 233 as 9.95 kcal mol-1 at 213 K, the temperature at which the diastereotopic CH2 group coalesces. 

Manganese(m) acetate oxidation (cf. Vol. 3, p. 34) of camphene gives (186) as a 95 5 mixture by carboxymethyl radical insertion no rearranged products were obtained, in contrast to /3-pinene which gave Wagner-Meerwein products only, and no free-radical insertion.279 The E- and Z-isomers of (187) probably result from a non-concerted biradical intermediate formed by benzyne addition to camphene.280 Benzyl-lithium adds to the aminocamphor (188) exclusively from the exo-side whereas only the competing enolization reaction occurs with more sterically hindered organometallics.281... 

Rotation around the ring-benzyl bond of 4 brings about the first-order interconversion of two benzylic lithium compounds, 4 and 5 (equation 29). Considering just the NMR of the ring protons, the conversion e n-> (e4(ae)) , involves the two same pair of spin products but different Hamiltonians (equations 30 and 31). 

A variety of tertiary benzylic lithium compounds have been prepared by addition of f-butyllithium in isooctane with TMEDA to substituted styrenes 34 at 243 K10 (equation 96). 

TABLE 8. Activation Parameters for rotation in tertiary benzylic lithiums TMEDA in isooctane10... 

Among benzylic lithium compounds which exhibit tetrahedral structure at lithium bound Ca, experiments have been undertaken to investigate the dynamics of inversion at the latter carbanionic carbon. For example, Peoples and Grutzner36 reported NMR data for 7-phenyl-7-metallonorbomanes. With M = K or Cs, all methylene carbons were magnetically equivalent. 

In further studies of ion-pairing, a variety of sec-a-silyl benzylic lithium compounds 39, 40, 41 and 42, were prepared, both externally and internally solvated, the latter by means of a potential ligand attached to the carbanionic moiety. Ion-paired carbanide salts tend to assemble into several arrangements which differ in aggregation, solvation and in the proximity of anion to cation. Many of these species interconvert rapidly relative to the NMR time scale even at quite low temperatures. An internally solvated ion-pair carbanide salt is more likely to assume a single molecular structure, to undergo the latter exchange processes more slowly and thus be more amenable to NMR spectroscopic studies of structure and dynamic behavior. 

The high yyw-selectivity seems to be independent of the stereochemistry of the starting material, since the use of 39Z also resulted in the preferential formation of the svn-isomer in a similar ratio. To explain this, the authors proposed 5-membered cyclic benzyllithium species having a. sy 2-like carbon to which two lithium atoms coordinate from both upper and lower sites as shown in 43 (Scheme 18). Such a dilithiated species would selectively react with electrophiles from the opposite site of the O—Li substituent. Another intermediate, 44, in which the benzylic lithium is coordinated with the heteroatom, may also be considered53. Both intermediates are likely, since in each of them the steric hindrance between the phenyl group and the alkyl group is minimal. Assuming that the reaction with... 

A remarkable example of tandem conjugate addition-aldol reaction has been recently reported by Tomioka and coworkers. The transient lithium enolate, generated by 1,4-addition of benzyl lithium thiolate onto the corresponding a,-unsaturated ester182,595, is followed by an intramolecular aldol tandem cyclization, resulting in a five-membered... 

One of the M-Si bonds in l,l-bis(trimethylsilyl)metalloles 17 (M = Si, Ge) is cleaved by the reaction with benzyl lithium to give the monoanions 18 which are further converted to the respective hafnocenes 2 (Figure 3)... 

The reaction of phenyl and benzyl lithium with 14 results in 1,4-addition (path b, equation 99) and gives rise to the ketene anion 331a. Hydrolysis of this intermediate... 

The results reported here describe an investigation of the optimum conditions of preparation of the tertiary diamine phenyllithium, benzyl-lithium, allyllithium, and lithiated TMEDA complexes. These reagents were allowed to react with some common reagents to help delineate the synthetic usefulness of the complexes. 

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. 

The possibility of using the solid, golden yellow complex benzyl-lithium-TED (1 1) as an alternative for some special applications prompted a study of the best conditions for its preparation. The benzyl-... 

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. 

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