Nathalie Huthera, Andrew F. Parsonsa and P. Terry McGrailb a Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK b Cytecfiberite Research Support Group, PO Box 90, Wilton, Middlesbrough, Cleveland, TS90 8JE, UK

Abstract: The most common method of initiation in free radical reactions involves the reaction of halide precursors with tributyltin hydride but this is far from ideal and the toxicity and difficulty of removing tin-containing by-products. The aim of this project is the development of alternative and more versatile free radical initiators and, in particular the use of dimanganese decacarbonyl [Mn₂(CO)₁₀] as a catalyst in synthesis.

Introduction: Free radical reactions, particularly cyclisations have been extensively studied over the past 20 years. Although the most common method of initiation still involves the reaction of organohalide precursors with tin hydride, the toxicity and the difficulties to remove the tin by-products has led to the development of alternative reagents for radical generation. One example is dimanganese decacarbonyl [Mn₂(CO)₁₀] that has been proved to be of great interest. Recent work within our group¹ has shown that dimanganese decacarbonyl can be used to produce radicals from organohalides, which can then undergo dimerisation or cyclisation reactions. Under photolysis, Mn₂(CO)₁₀ generates the radical •Mn(CO)₅, which reacts efficiently with organohalides abstracting the halogen atom and leading to the formation of XMn(CO)₅ as by-product. This by-product can usually easily be removed by reaction with an excess of DBU (this forms a polar manganese complex) or by oxidation of the manganese complex in air. However, for polar organic compounds, removal of excess DBU and the manganese-DBU complex can be problematic and for air sensitive compounds, the aerial oxidation is not possible. Alternative methods for the removal of XMn(CO)₅ are required in these situations, research has therefore involved the development of solid-supported dimanganese complexes and also the use of ionic liquids although the project has mainly involved the use of biphasic systems.

Results: Previous work by Gibson et al. ² has explored the phase-transfer catalysed reactions of metal carbonyl halides [like Mn(CO)₅Br]. This allowed the separation of Mn(CO)₅Br from organic products by forming a water-soluble complex on reaction with hydroxide ions. Moreover, the in situ formation of Mn₂(CO)₁₀ was also detected. A similar system was then investigated using various radical reactions ³ by, for example, carrying out the reaction in the presence of an aqueous solution of sodium hydroxide and the phase transfer catalyst benzyltriethylammonium chloride (BTAC). Under these reactions, it was anticipated that the manganese halide byproducts would react with hydroxide to form water-soluble manganese complexes [e.g. Et₃(Bn)N⁺MN₂(CO)₉Br¯, Et₃(Bn)N⁺Mn(CO)₅¯]. Initial experiments were centred on the preparation of bibenzyl (2) from benzyl bromide (1) (Scheme 1). Hence photolysis of (1) with Mn₂(CO)₁₀ in dichloromethane in the presence of aqueous sodium hydroxide and BTAC gave the desired dimer (2) in 36% yield. This reaction also produced benzylmanganese decacarbonyl (3) in 43% yield, which was unexpected as this compound was not formed when the same reaction was carried out solely in dichloromethane.^1a The mildness of the conditions used could lead to an attractive synthetic method to various alkylmanganese complexes as compound (3) is traditionally prepared from reaction of Mn₂(CO)₁₀ with Na/Hg and benzyl bromide (or chloride) in anhydrous diethyl ether.⁴

Scheme 1

The most likely route to compound (3) involves nucleophilic substitution of bromide (1) by the manganese pentacarbonyl anion ¯Mn(CO)₅. Benzylmanganese pentacarbonyl (3) could then react with bromide (1) to form (2), in a second nucleophilic substitution reaction, or alternatively, the manganese radical •Mn(CO)₅ could abstract a bromine atom from (1) to form the benzyl radical which dimerises: the formation of the benzylic radical was confirmed by carrying out a photolysis experiment in the presence of TEMPO. The use of diphenyl diselenide in place of TEMPO has also been investigated. The ¯Mn(CO)₅ anion could arise from reaction of Mn₂(CO)₁₀ and/or BrMn(CO)₅ with hydroxide.^2,5 Mn(CO)₅Br has also been shown, for the first time, to initiate biphasic radical and/or ionic reactions, the formation of Mn₂(CO)₁₀ being detected in situ.

Scheme 2

As the formation of dimanganese decacarbonyl has been detected in situ, the amount of complex used has been successfully reduced and this has led to a catalytic system. Moreover, the range of Mn₂(CO)₁₀-mediated transformation has been extended. For example, homocoupling of secondary and tertiary benzylic bromides has been successfully achieved, which was not possible using standard monophasic conditions (Scheme 2).

Radical cyclisation reactions can also be carried out using biphasic conditions, an example is given in Scheme 3.

Scheme 3

• This work has shown, for the first time, that manganese carbonyl-mediated carbon-carbon bond forming reactions can be carried out in biphasic conditions.
• This allows the effective removal of Mn(CO)₅Br from organic products.
• The formation of alternative products to those observed when using Mn₂(CO)₁₀ in dichloromethane is of particular interest; the use of such mild conditions for the formation of organomanganese adducts offers a flexible approach to these types of compounds, which have been shown to undergo synthetically important ionic and/or radical reactions.
• The amount of dimanganese decacarbonyl necessary to perform radical reactions has been significantly reduced,which has led to a catalytic system.
• Solid supported manganese decacarbonyl complexes have also been used in radical reactions.

• Immediate work will concentrate on the synthesis of a range of cyclic sulfonyl functionalised products and investigate subsequent transformations.
• Future work will involve the investigation of further manganese-mediated radical reactions including, for example, polymerisation reactions.
• Cyclisation onto C=C bonds have been extensively studied and the application of these conditions to cyclisation onto other bonds, (e.g. C=N bond) also needs to be explored.
• The dimanganese decacarbonyl complex will also be modified by replacing carbonyl ligands (i.e. the use of phosphine or amine groups) and the effects on the reactivity of these compounds will be investigated.
• Investigation of stereoselective reactions using modified dimanganese complexes bearing chiral ligands.
• The use of other metal carbonyls including Re₂(CO)₁₀ would also be of great interest.
• Finally, reactions in aqueous media will also be investigated.

1. (a) Gilbert, B. C.; Kalz, W.; Lindsay, C. I.; McGrail, P. T.; Parsons, A. F.; Whittaker, D. T. E. J. Chem. Soc., Perkin Trans. 1, 2000, 1187. (b) Gilbert, B. C.; Kalz, W.; Lindsay, C. I.; McGrail, P. T.; Parsons, A. F.; Whittaker, D. T. E. Tetrahedron Lett., 1999, 40, 6095. (c) Gilbert, B. C.; Lindsay, C. I.; McGrail, P. T.; Parsons, A. F.; Whittaker, D. T. E. Synth. Commun., 1999, 29, 2711.
2. Gibson, D. H.; Hsu, W.-L.; Ahmed, F. U. J. Organometal. Chem., 1981, 215, 379.
3. Huther, N.; McGrail, P. T.; Parsons, A. F. Tetrahedron Lett., 2002, 43, 2535.
4. Gismondi, T. E.; Rausch, M. D. J. Organomet. Chem. 1985, 284, 59-71.
5. Pascal, P. Nouveau Traité de Chimie Minérale; Masson et Cie Editeurs: Paris, 1960.

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