Two formylation procedures , Hive Novel Discourse


		Kinetic (Hive Bee) 07-04-04 10:15 No 517404				Two formylation procedures (Rated as: excellent)
						There are numerous procedures for the formylation of aromatic systems in the literature. The Duff reaction is a well known procedure for the formylation of highly activated aromatics such as phenols and anilines. The yields are often terrible, and the procedure does not work at all on many substrates. Using trifluoroacetic acid¹ as solvent overcomes this problem, with the reaction working on less activated nuclei. Aldehyde formation has also been accomplished by the Bouveault aldehyde synthesis,² in which a Grignard reagent is treated with an N,N-disubstituted formamide such as DMF, followed by acidic workup. Although the procedure was first reported 100 years ago, it was only relatively recently³ that a high-yielding general procedure appeared in the literature. Indane⁴ seemed a valid substrate to test both of the above reactions out: it does not react well under standard Vilsmeier conditions,⁵ and the alternative, Friedel-Crafts formylation, uses the carcinogenic reagent dichloromethyl methyl ether. The first method tried was the modified Duff formylation with hexamine in trifluoroacetic acid, giving the aldehyde directly. The same product was then made by a two step method. Indane was first brominated to give 5-bromoindane,⁶ and treated with Mg to give the Grignard reagent 5-indanylmagnesiumbromide. Addition of DMF followed by acidic workup furnished the corresponding carbonyl compound. Graphical abstract Molecule: Formylation of indane ("c2ccc1CCCc1c2>>C(=O)c2ccc1CCCc1c2") Experimental Indane-5-carboxaldehyde via Duff reaction 100mmol indane 100mmol hexamine Trifluoroacetic acid A stirred solution of indane and hexamine in 75ml^a TFA was heated to 95^oC for 12 hours. The deep red-brown solution was then removed from the heat and diluted with 250ml water. The resulting mixture was vigorously stirred for 30 minutes, then was made basic by the cautious addition of sodium carbonate decahydrate.^b The dark oil which precipitated was extracted with 2x50ml ether and the combined extracts were dried over MgSO₄. The solvent was removed and the product distilled at 127-129^oC to give the title product as a clear, colourless, pleasant (marzipan!) smelling oil. Yield: 8.8g (60mmol, 60%) ^a This is half the amount of acid used in the original article and still gives comparable results. However, 100ml TFA is suggested for a reaction at this scale, using a slightly lower temperature. ^b The anhydrous carbonate should work equally as well, as should potassium carbonate. 5-Bromoindane 200mmol indane 220mmol bromine 220mmol ZnCl₂ Glacial acetic acid A flask containing a solution of indane and ZnCl₂ in 120ml acetic acid, wrapped in foil to exclude the contents from light, was cooled to 5^oC. A solution of bromine in 20ml acetic acid was added over 1 3/4h, keeping the temperature around 5^oC. The solution was stirred for a further 4 hours at 5^oC, and was then poured into 400ml water. The product was extracted with 2x50ml DCM and the combined extracts were washed with 100ml water, 100ml sodium bicarbonate solution, 100ml 0.5M sodium metabisulfite solution,^a 100ml sodium bicarbonate solution and 100ml brine. After drying over MgSO₄, the solvent was removed and the product distilled at 112-116^oC to give the title product as a clear, colourless, pleasant smelling oil. Yield: 28.4g (144mmol, 72%) ^a This removes all of the colour. Thanks to Barium for this tip. Indane-5-carboxaldehyde via Grignard addition to DMF 105mmol 5-bromoindane 107mmol Mg turnings 105mmol DMF 5mmol ethylmagnesiumbromide THF To a stirred solution of 5mmol ethylmagnesiumbromide^a in 100ml anhydrous THF containing magnesium turnings was added 5-bromoindane dropwise. The exothermic reaction initiated spontaneously and the remaining 5-bromoindane was added, taking care to keep the reaction from boiling over. When the reaction had returned to room temperature, the flask was cooled to 0^oC. A solution of DMF in 75ml anhydrous THF was added over 15 minutes, causing a mild temperature rise. The flask was removed from the ice bath and allowed to stir for a further 2 hours. The colour changed from purple-brown to a light pea-green over this period. The reaction was then quenched by the addition of approximately 80ml 3M HCl. The biphasic mixture was added to 500ml water, and the product extracted with 2x50ml ether. The extracts were combined and washed with 100ml water, 100ml sodium bicarbonate solution and 50ml brine. Removal of the solvent and distillation under vacuum gave the title product as a clear, colourless oil boiling at 132-137^oC,^b with an identical smell to the product obtained by the first method. Yield: 10.1g (69mmol, 69%) ^a This trick⁷ enabled the otherwise difficult initiation of the Grignard reaction to occur extremely easily. The ethylmagnesiumbromide was formed in-situ but it should be equally possible to add the pre-made reagent. It could be a good way to initiate other tricky Grignard reactions, such as the formation of 2,5-dimethoxyphenylmagnesiumbromide. ^b There was a slight forerun which had the smell of indane. This can be formed by quenching too early (Grignard degradation). The reaction may be slower in THF than in the ether used by the authors in reference 3; stirring for a further hour may increase the yield. References ¹ Formylation of Aromatic Compounds with Hexamethylenetetramine and Trifluoroacetic Acid (../rhodium /formylation.duff-hmta-tfa.html) ² Bouveault Aldehyde Synthesis (http://themerckindex.chemfinder.com/TheMerckIndex/NameReactions/ONR55.htm) ³ Post 512847 (Kinetic: "Grignard formylation article", Novel Discourse) ⁴ Post 100325 (assholium: "Re: Grignard reagent/methyl group swap", Chemistry Discourse) ⁵ Synthesis of Indanylamphetamine (IAP) (../rhodium /iap.html) ⁶ The bromination gives a mixture of 4- and 5-bromoindane, with 5-bromoindane being the predominant isomer. Using low temperatures and a Lewis acid catalyst increases the selectivity for the desired 5-bromoindane. See Journal of the Americal Chemical Society, 63, 301-303 (giving a 2:1 ratio of 5- to 4-bromoindane with no Lewis acid catalyst), Journal of Organic Chemistry, 53, 4531-4534 (giving a 9:1 ratio of 5- to 4-bromoindane with 10mol% Fe turnings), and Organic Syntheses, CV 3, 138 (http://www.orgsyn.org/orgsyn/prep.asp?prep=cv3p0138) ⁷ This was used by Nichols in the cyclisation of 1,4-bis(2-chloroethoxy)2,5-dibromobenzene: see Post 364601 (Barium: "Another way of inserting the aminopropyl", Serious Chemistry).



		starlight (Hive Bee) 07-22-04 11:16 No 520841				Tips for making the workup easier
						The modified Duff is excellent. Consistent yields seem easy to obtain (I say this because its possible to get close to Kinetic's yield on the first attempt). However the neutralization step is a bit time consuming / foamy. If you wish to cut down on the time taken then you can neutralize a lot of the TFA with 33.5% NaOH solution (use 100ml of this solution to neutralize a reaction using 150ml of TFA (double the scale above). And then finish off the neutralization with sodium carbonate. This does not harm the product in any way and the yields are still good. You can also use about half as much water in the hydrolysis step if you wish, and let the reaction mixture cool to 30C before adding the water (not that water is expensive, but it allows you to conduct an entire 200mmol reaction in a 500ml flask).