Cathodic carbonylation of 1-halogenopentanes

摘要

J. CHEM.SOC. PERKIN TRANS. 1 1992 Cathodic Carbonylation of 1-Halogenopentanes Kunihisa Yoshida," Masaru Kobayashi and Sei-ichi Amano Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka,Osaka 560, Japan Reaction conditions for electroreductive conversion of 1-halogenopentane into hexanal using Fe(CO), have been investigated in detail. The use of acetonitrile as solvent containing tetraalkylammonium halides or toluene-p-sulfonate and a divided cell gave good results. Under the conditions, the best results were obtained with the bromide as starting halide. Since acyl anions are not commonly accessible, nucleophilic acylation is generally achieved with a variety of reagents, among them acylmetallic compounds which are synthetically equivalent to acyl anions.Organometallic acyl-anion transfer reagents can be produced by the reaction of carbanion donors with carbon monoxide or metal carbonyls. For example, carbon monoxide reacts with organolithium compounds at low temperature to give nucleo- Table 1 Cathodic carbonylation of 1-bromopentane" Anodic Hex an a 1 Solvent Electrolyte additive Conversion () yield' ("A) AN Et,NBr None 88 71 (80) AN Et,NOTs b 83 68 (83) AN Bu,NClO, b 83 57 (68) AN Bu,NBF, b 86 54 (63) Et,NOTs b 92 33 (35) and Grignard reagents' '-' react with carbonylnickel or pentacarbonyliron to give acylmetallic compounds; the reaction of organic halides with carbonyl metal anions such as disodium tetracarbonyliron also give such compounds.Acylmetallic intermediates are also formed in phase-transfer carbonyl- ation.19s20 Electroreduction of alkyl halides gives carbanions by way of radical anions and/or radicals;2 similar reduction of metal carbonyls gave the corresponding anions (e.g. pentacarbonyl-iron gives tetracarbonyliron diani~n).~~.~, In our work we have examined the electroreduction of 1-halogenopentanes with a metal carbonyl, our efforts initially being directed towards finding a suitable solvent-electrolyte system. Here we describe results for a variety of reaction condition^.^ Results and Discussion In the present work, we selected co-ordinated carbon monoxide as a C1 source in the electroreaction (1) where RX is 1-bromopentane. RX + Fe(CO), 5RCOFe(CO),J-RCHO (1) Initially, in a search for optimum reaction conditions, various polar organic solvents containing tetraalkylammonium or alkali metal salts as supporting electrolytes were examined by using a divided cell.Whilst acetonitrile (AN), N,N-dimethyl-formamide (DMF), and N-methylpyrrolidinone (NMP)pro-duced good results, dimethyl sulfoxide, hexamethylphosphoric triamide (HMPA), nitromethane and methanol were found to be unsuitable as solvents. Aliphatic quaternary ammonium ions proved useful as supporting-electrolyte cations, whereas alkali metal ions were not. When nonhalide anions such as toluene-p- sulfonate (OTs-) were used as counterions, sparingly soluble compounds formed coatings on the anode surface and led to a progressive increase in the cell voltage.The presence of Bu,NI in the anodic compartment was effective in avoiding this. Table 1 shows the influence of different electrolyte media on the yields of hexanal brought about by protolytic work-up with aqueous 2 mol dm-3 HCl. Some 5 of hexanal was recovered from the anolyte after hydrolytic work-up; reported yields take account of these values. Table 2 summarizes the results carried out with different cell DMFphilic acylating reagentsI4 and organolithium cornpo~nds~-~~ NMP Et,NOTs b 69 14 (21) " C,H, 'Br = Fe(CO), = 0.1 mol dm-3; divided cell; SScathode; Pt anode; current, 50 mA; electricity, 2.0 F mol-'; proton donor used in work-up, aqueous 2 mol dm-3 HCI. Bu,NI = 0.102 rnol dm-3.'Based on RBr used. The value in parentheses represents the yield based on RBr consumed. Table 2 Influence of cell modes on the yields" Cell Electrolyte Conversion () Yield* () Divided Divided Undivided Undivided Et,NOTs Et,NBr Et,NBr Et,NOTs 82 88 42 37 67 (82) 71 (80) 23 (54) 20 (53) "Solvent, AN; RBr = Fe(CO), = 0.1 mol dm-3; Pt electrodes; current, 0.05 A; electricity,2.0 F mol-'. Based on RBr used. The value in parentheses represents the yield based on RBr consumed. Table 3 Relation between electrical consumption and conversion a 2.0 83 68 (83) 2.1 86 64(75)2.2 99 63 (63) Divided cell; solvent, AN; electrolyte, Et,NOTs; RBr = Fe(CO), = 0.1 mol dm-3; SS cathode; Pt anode; current, 0.05 A.Based on RBr used. The value in parentheses represents the yield based on RBr unrecovered. Table4 Influence of cathode materials" Cathode Conversion () Hexanal yield '() ss 83 68 (83) Pt 82 67 (82) cu 82 66 (80) Ni 79 65 (83) Pb 88 63 (72) 74 58 (78) CHf 76 52 (69) " Divided cell; solvent, AN; electrolyte, Et,NOTs; RBr = Fe(CO), = 0.1 rnol dm-3; current, 50 mA; electricity, 2.0 F mol-'. bVitreous carbon. 'Based on RBr used. The value in parentheses represents the yield based on RBr consumed. 1128 J. CHEM. SOC. PERKIN TRANS. I 1992 Table 5 Effect of changing halide atoms of RX on the nature and yield of products" Yield () X of RX Solvent Electrolyte Anodic additive Conversion () RCHO R,CO I AN Et,NOTs b 96 61 1 I AN Bu,NI None 99 58 3 I NMP Bu,NI None 99 40 7 I NMP Et,NOTs b 97 22 7 Br AN Et,NBr None 88 71 None Br AN Et,NOTs b 83 68 None Br NMP Et,NOTs b 69 14 None c1 AN Et,NOTs b 67 31 Nonec1 AN Et,NCI None 57 16 None a Divided cell; SS cathode; Pt anode; RX = Fe(CO), = 0.1 mol dm-3; current, 50 mA; electricity, 2.0 F molt'. Bu,NI = 0.102 mol dm-3.'Based on RX used. types. The use of an undivided cell depressed both yield and percentage conversion. Table 3 presents the effect of electrical consumption on conversion. Complete consumption of the bromide requires 2.2 F mol-' and the data listed show that the yield rather decreased with increasing electrical consumption. The role of the electrode materials was also examined with different cathodes.As can be seen in Table 4, cathode materials have little effect on the yield of product. Table 5 presents the effect of changing halide atoms of RX on the nature and yield of products. The data listed in Table 5 show that the order of hexanal yield is RBr RI RCl. Alkyl iodide gave a small amount of ketone by-product, a second alkylation during the electroreaction giving the symmetrical compound. Conclusions The best reaction medium for the desired cathodic carbonylation was acetonitrile containing tetraalkylammonium halides (or OTs-) in a divided cell. With OTs- ion as a counterion, tetraalkylammonium halide must also be used as a depolarizer. Although cathode materials had little effect on the yield of product, use of an undivided cell lowered the percentage conversion.The best results for conversion of l-halogenopen- tanes into hexanal were obtained with the bromide. Experimental Materials.-Commercial acetonitrile was stirred for 2 days with CaH, (10 g dm-3) until no further hydrogen was evolved; it was then decanted and fractionally distilled from P205 (5 g dm-3). This product was refluxed over CaH, (5 g dmP3) for several hours, slowly fractionally and then stored in a Schlenk tube under a nitrogen atmosphere. N-Methyl- pyrrolidone was, after storage over CaH, for a few days, thrice distilled under reduced pressure, stored under a nitrogen atmosphere and used within a few days of the di~tillation.~' DMF was dried for 3 days over type 4 A molecular sieves, during this time several changes of sieves were made, and then fractionally distilled under reduced pressure; nitrogen was passed through the apparatus during the di~tillation.~~.~~ Et,NBr and Et,NCl were purchased.The following supporting electrolytes were prepared according to the literature procedure: E~,NOTS,~' Bu,NBF,,~' Bu,C~O,~ and Bu,NI~~.1-Bromo-, 1-chloro-, and 1-iodo-pentane and Fe- (CO),, obtained commercially, were purified by distillation. Hexanal and undecan-amp;one as reference materials were commercial samples. Electroreaction.-A standard procedure is illustrated. The reaction was carried out in a divided cell with a stainless steel (SS) plate cathode (18 cm2) and a similar Pt plate anode.The catholyte was composed of acetonitrile (50 cm3) l-bromo- pentane (5 mmol), Fe(CO), (5 mmol), and Et,NBr (2.4 g, 11.4 mmol). The anolyte was the same medium in the absence of the substrates. The cathode and anode compartments, kept under a nitrogen atmosphere, were stirred magnetically. The reaction was performed at 50 mA of constant current by using a direct- current power supply at room temperature until 2.0 F mol-' of added substrate had passed through the solution, which took ca. 5 h. After completion of the reduction, the catholyte was treated with aqueous 2 mol dm-3 HCI and extracted with pentane. The pentane extract was washed with dilute aqueous sodium hydrogencarbonate. The anolyte was similarly treated with aqueous HCl.The extract was washed successively with dilute aqueous sodium thiosulfate and brine. 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