Bromine-lithium exchange preparation

Condensation of the alkoxynaphthyldihydrooxazoles with 2-methoxy-l-naphthyllithium, prepared by bromine-lithium exchange, did not lead to the same product as the corresponding reactions with the magnesium compound, but, unexpectedly, to a rearranged isomeric 1 -bi-naphthyl derivative with high enantiomeric excess28. 

The mixed 1,2-bimetallic Zn/Si-reagent 47 is a versatile species which reacts with aldehydes in high diastereoselectivity . It is prepared by a bromine/lithium exchange reaction starting from 48 followed by a transmetalation with ZnCl2 (Scheme 21). The... 

Indole synthesis. Fuhrer and Gschwend3 have converted N-pivaloylaniline into the dilithio derivative a by reaction with 2 equivalents of n-bulyllithium and have reported that orr/io-substituted derivatives are obtained in good yield by reaction of I Wilh an electrophile. The same species can be prepared somewhat more efficiently from iin o-bromo-N-pivaloylaniline (1) by bromine lithium exchange and N-deprotonation with methyllithium and r-butyllithium (equation I). This dilithium riegcnl can be used for synthesis of indoles.4 Thus it reacts with a bisclcctrophile... 

The thiepines 180 were prepared via a bromine/lithium exchange for /7/%-lithiation of 179 (Equation 26). Lithiation was achieved using /-BuLi (2equiv for each bromine) and sulfur diimidazole was used as the sulfur electrophile. Thiepines 180a-d were obtained in 12-38% yield. 

The sequence of bromine-lithium exchange and insertion of tellurium into the carbon-lithium bond was repeated to obtain dilithium 1,2-dietheneditellurolates as starting materials for the preparation of tetratellurafulvalenes5. This four-step procedure is carried out in one pot without isolation of any of the intermediates. 

Dialkyloxyvinyllithiums 574 are another type of acyllithium equivalent which bear an alkoxy group at the /3-position. They were initially prepared by bromine-lithium exchange from l-bromo-l,2-diethoxyethene with n-BuLi in ether at — 35 °C849 and from l-bromo-l,2-dimethoxyethene and n-BuLi in ether at — 78 °C850 to provide intermediates 575 and 576, respectively as mixtures of Z/ -isomers. 

Exposure of the substrate 217 to excess /-butyllithium induces bromine-lithium exchange and metalation ortho to the fluorine substituent, thus generating the benzyne intermediate 218, and subsequent treatment of the second intermediate species 219 with an electrophile eventually leads to the 4-substituted indole 220, illustrating an interesting entry into the indole nucleus (Scheme 24) <1999TL1049, 2002CEJ2034>. This chemistry has also been extended to preparation of carbazole derivatives <2002CEJ2034>. 

In a few cases, interesting synthetic intermediates can be successfully prepared by bromine-lithium exchange under mild conditions [202], (compare also Ref. [233]). 

The majority of the metallated hetero-aromatic compounds are obtained by direct metallation [1, 2, 5]. The hetero-atom effect (inductive, coordinative and polarizability influence) in most cases directs the metallation to the position a- to the hetero atom and gives rise to an easier deprotonation compared with that of benzene [1]. In some cases, bromine-lithium exchange offers the possibility of introducing the metal in a position that is not accessible by direct deprotonation. In incidental cases, Grignard compounds have been prepared from halogenated hetero aromates and magnesium. 

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