Unusual methylation reaction of gem-bromofluorospiropentanes with methyllithium
Graphical abstract
Introduction
Previously we have reported an unusual skeletal rearrangement of gem-dibromospiropentanes under treatment with methyllithium at low temperatures (Scheme 1).1, 1(a), 1(b) In general, the reaction of dibromides I with methyllithium gives corresponding allenes III as major products (Scheme 1, route A). This transformation, so-called Doering–Scattebol–Moore reaction, is well documented.2, 2(a), 2(b), 2(c), 2(d), 2(e) We have found that lowering the reaction temperature to −55 °C favors the formation of rearranged products IV–VII and disfavors the formation of allenes III.1, 1(a), 1(b) We have demonstrated the general character of this process and found that the formation of ‘monomeric’ (IV, V; Scheme 1, route B) or ‘dimeric’ (VI, VII; Scheme 1, route C) products was possible depending on the substituents in the dibromocyclopropane fragment.3, 3(a), 3(b), 3(c) In several cases we have also observed the products of type VIII arised from the insertion of some intermediate into C–H bond of diethyl ether used as a solvent.4, 4(a), 4(b)
The general mechanistic scheme was supposed to include the nucleophilic attack of neighboring three-membered C–C bond at intermediate Li–carbenoid II with Br as a leaving group.3c Studying the reaction mechanism, we were interested in the influence of the halogen atom nature on the direction of the dihalogenospiropentane reactions with MeLi. Since no data concerning this problem was available,5 in this paper we have synthesized a series of model gem-bromofluorospiropentanes 11–20 and investigated their reaction with methyllithium.
Section snippets
Results and discussion
Bromofluorospiropentanes 11–20 were synthesized in good yields by the cycloaddition of bromofluorocarbene6, 6(a), 6(b) to corresponding methylenecyclopropanes 1–10 (Table 1). The compounds 14–16 and 19 are obtained as mixtures of two diastereomeres (of four possible ones for 14 and 15). Indeed, in NMR spectra of compounds 14 and 15 we can see only two sets of signals, corresponding just two diastereomeres (A and B). Thus, addition of bromofluorocarbene to alkenes 4, 5 is in a sense a
General
NMR spectra were recorded on a Bruker DPX-400 spectrometer (400.13 and 100.62 MHz for 1H and 13C, respectively) at room temperature; chemical shifts δ were measured with reference to the solvent (1H: CDCl3, δ=7.24 ppm; 13C: CDCl3, δ=77.13 ppm). Mass spectra were taken on a Finnigan MAT 95 XL spectrometer (70 eV) using electron impact ionization (EI) and GC–MS coupling. MALDI-TOF spectra were recorded in positive mode using Bruker Ultraflex mass spectrometer with dithranol as a matrix. Accurate mass
Acknowledgements
We thank the Division of Chemistry and Materials Science RAS (Program N 1) and the President's grant ‘Support of Leading Scientific School’ N 5538.2008.3 (academician N.S.Z.) and the Russian Foundation for Basic Research (Project 09-03-00717-a) for financial support of this work.
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