Low-temperature functionalisation of alkanes and cycloalkanes by 'classical' and 'non-classical' (superacidic) Friedel–Crafts complexes

Abstract

Low-temperature functionalisation of alkanes and cydoalkanes by 'classical' and 'non-classical' (superaddic) Friedel-Crafts complexesI S Akhrem, A V Orlinkov, M E Vol'pin (deceased) Contents I. Introduction 849 II. Functionalisation of isoalkanes and activated cydoalkanes by equimolar complexes RCOX -AlCb and related systems 850 III. Functionalisation of alkanes and cydoalkanes with aprotic organic superaddic complexes RCOX -2A1C1 3 853 IV. Reactions of acylium salts with saturated hydrocarbons in protic superacids 857 V. The nature of the active complexes in acyl halide -aluminium halide systems 858 VI. Conclusion 862 Abstract. The results of studies on direct functionalisation of activated alkanes and cydoalkanes under the action of 'classical' Friedel-Crafts complexes (viz. equimolar complexes of acyl halides with aluminium halides) and related systems containing smaller or somewhat larger amounts of aluminium halide are surveyed. The studies carried out during the last decade on functionalisation of saturated hydrocarbons devoid of tertiary carbon atoms, by aprotic organic superacids RCOX-2AICI3 are summarised. Reactions of alkanes with acylium cations in superacidic media are considered. The published data on the structure of complexes RCOX • AICI3 and RCOX • 2A1C1 3 and on the nature of the active complexes in the reactions of arenes with acylium cations and with complexes RCOX-AICI3 in both acidic and organic media as well as in the reactions of alkanes with acylium salts in protic superacids and with superaddic complexes RCOX -2AICI3 in aprotic solvents are analysed. The prospects for the synthesis of organic compounds from alkanes and cydoalkanes under the action of complexes of acyl halides with aluminium halides are outlined. The bibliography includes 128 references. IS Akhrem, A V Orlinkov Laboratory of complex organometallic catalysts, Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sdences, ul. Vavilova 28,117813 Moscow, Russian Federation. Fax (7-095) 135 50 85 Vol'pin M E Director of the same Institute Received 15 April 1995 Vspekhi Khimii 65 (10) 920-935 (1996); translated by Z P Bobkova

Polyhalomethanes Combined with Lewis Acids in Alkane Chemistry

“…Contrary to the RCOX• n AlX 3 complexes, − whose activity is extremely high at n = 2 and completely absent at n = 1, both 1:1 and 1:2 polyhalomethane• n AlBr 3 systems show high activity . The influence of AlBr 3 content on activity of polyhalomethane is not uniform.…”

“…One might expect that protonation of polyhalomethyl cations or the formation of donor−acceptor complexes of these cations with Lewis acids would enhance the electrophilicity. Such multicharged cations might be responsible for the superelectrophilic properties of polyhalomethanes in the presence of protic and aprotic superacids (Scheme ), as has been proposed for the systems containing acylium salts in the presence of strong protic and Lewis acids. ,,−

113

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“…These results suggest that only the donor−acceptor complexes with a small positive charge on the halogen atom are present in the equimolar systems, whereas in the systems with excess aluminum halide, polar complexes with high positive charges on the halogens are produced. Similarly, the RCOX• n AlX 3 exist in solution as donor−acceptor complexes that are unreactive toward alkanes ( n = 1) or mixtures of such complexes with much more reactive acylium salts ( n = 2). ,, The bidentate complex, which has a large positive charge on the nodal chlorine atom, has been considered 323 as the most likely candidate for the role of a key superelectrophile in the CCl 4 • n AlCl 3 systems. The charge on the tricoordinate chlorine atom amounts to +0.64 ( ab initio ) or +0.54 au (AM1), which is greater than in the CCl 3 + cation by 0.13 ( ab initio ) and 0.21 au (AM1).…”

“…The discovery in the 1980s of the superelectrophilic properties of acylhalide/aluminum halide complexes of the form RCOX•2AlX 3, − as opposed to the classical Friedel−Crafts RCOX•AlX 3 equimolar complexes, − was the high point in electrophilic low-temperature alkane chemistry for that decade. These systems are far superior in their reactivity in alkane reactions to all previously known systems based on either mineral acids or Lewis acids, usually even including the strongest protic superacids. − They have been shown to be extremely active in initiating various transformations of alkanes accompanied by C−C bond cleavage, such as in isomerization, − alkylation, , and oxidative coupling. , In those cases for which cracking reactions can be suppressed, selective single-stage functionalization can be successfully achieved. − ,− …”

“…The discovery in the 1980s of the superelectrophilic properties of acylhalide/aluminum halide complexes of the form RCOX•2AlX 3, [98][99][100] as opposed to the classical Friedel-Crafts RCOX•AlX 3 equimolar complexes, [101][102][103][104][105][106] was the high point in electrophilic low-temperature alkane chemistry for that decade. These systems are far superior in their reactivity in alkane reactions to all previously known systems based on either mineral acids or Lewis acids, usually even including the strongest protic superacids.…”