FULL PAPER Upon the α-methylenation of methyl propanoate via catalytic dehydrogenation of methanol Patrizia Lorusso,[a] Jacorien Coetzee,[a] Graham R. Eastham[b] and David J. Cole-Hamilton*[a] Abstract: A one-pot system for the conversion of methyl propanoate afford methyl 3-hydroxy-2-methylpropanoate as an intermediate, (MeP) to methyl methacrylate (MMA) has been investigated. In which, in turn, would dehydrate to MMA (Scheme 1). particular, this study is focused on the possibility of performing catalytic dehydrogenation of methanol for the in situ production of anhydrous formaldehyde, which is then consumed in a one-pot base-catalysed condensation with MeP to afford methyl 3-hydroxy-2- methylpropanoate, which spontaneously dehydrogenates to MMA, some of which is subsequently hydrogenated to methyl 2- methypropanoate (MiBu). Scheme 1. Proposed one-pot formation of methyl methacrylate (MMA) from methyl propanote (MeP) and methanol Introduction Alpha methylenation of simple esters can be achieved in low Formaldehyde is a chemical used widely in several industrial yield using condensation of the ester with formaldehyde processes including the manufacture of building materials. A catalysed by caesium oxide on silica at high temperature[3], or by remarkable example in this area is represented by the innovative a complex series of reactions using Meldrum’s acid two-step Alpha technology developed by Lucite for the large Eschemoser’s iodide salt (dimethylmethyleneimmonium scale production of methyl methacrylate (MMA), the essential iodide).[4] We are not aware that it has been achieved using building block of all acrylic-based products.[1] This successful metal complex catalysed reactions. However, the reactions technology involves methoxycarbonylation of ethene to methyl involved (Scheme 1) are very similar to those involved in propanoate (MeP), in the presence of a palladium based hydrogen borrowing reactions,[5] acceptorless dehydrogenation[6] complex,[2] followed by condensation with gaseous and hydrogen autotransfer.[7] Generally, the hydrogen from the formaldehyde on a fixed bed catalyst (caesium oxide on silica), dehydrogenation of the alcohol is transferred back into the affording MMA as the final product.[1b, 3] The formaldehyde product leading to alkylation rather than alkylenation. In addition, involved in this second stage is initially produced as formalin in a the substrates that are alkylated are exclusively ketones rather separate process and then dehydrated to afford anhydrous than esters. formaldehyde, which is essential for ensuring high selectivity to Methanol has only rarely been used as an in situ source of MMA . Another drawback of this procedure is represented by the formaldehyde. Formaldeyhde is presumably formed in the easy polymerisation of paraformaldehyde giving insoluble thermal[8] or photochemical[9] production of hydrogen from polyoxymethylenes which can lead to severe fouling of transfer methanol, and subsequent reactions of the formed formaldehyde lines.[3] The development of an alternative process where can lead for example to dimethoxymethane,[9b, 10] to CO and anhydrous formaldehyde is produced in situ would provide a H ,[9a], to CO or carbonate in the presence of hydroxide simplification over the current process. As an alternative second 2 2 bases,[8b] to formylamines on reaction with primary or secondary step, the possibility of performing the one-pot α-methylenation of amines,[11] or it can be added to a metal allyl formed from an methyl propanoate has not been investigated so far. In an ideal allene to give 3-hydroxylethyl-1-alkenes in a reaction which is system, anhydrous formaldehyde would be generated in situ by formally the addition of a C-H of methanol across a double catalytic dehydrogenation of methanol and would subsequently bond.[12] In early work, some of us showed that, using methanol undergo base-catalysed condensation with methyl propanoate to as the hydrogen source in hydrocarbonylation reactions of 1- hexene to heptanol catalysed by Rh/PEt3 complexes, the formed formaldehyde was converted into methyl formate (Scheme 2). [a] Ms. P. Lorusso, Dr. J. Coetzee and Prof. D. J. Cole-Hamilton EaStCHEM School of Chemistry Interestingly, in these reactions, methanol proved to be a better The University of St Andrews source of hydrogen than ethanol, 1-butanol or 2-propanol.[13] Purdie Building, North Haugh, St Andrews, Fife, KY16 9ST, UK E-mail: [email protected] [b] Dr. G. R. Eastham Lucite International, Technology Centre P.O. Box 90, Wilton, Middlesborough, Cleveland, TS6 8JE, UK Scheme 2. The use of methanol as a source of hydrogen in the Supporting information for this article is given via a link at the end of hydrocarbonylation pf 1-hexene (R = C4H9). Methyl heptanoate, methyl 2- the document. methyhexanoate and hexylhexanoate are also significant products. Methyl [13] formate is formed from methanol. FULL PAPER Results and Discussion Preliminary studies in which methanol, methyl propanoate and sodium methoxide were heated in the presence of catalyst 1 Minor amounts of MMA were detected after heating MeP showed the presence of small amounts of both MMA and MiBu. together with Na2CO3 at 170 °C in a stainless steel batch Scheme 4 depicts a plausible mechanism for the formation of autoclave in the absence of any added formaldehyde. In addition MMA based on a published mechanism for the dehydrogenation to MMA, the GC-MS spectrum of the crude product revealed the of methanol, ethanol or propan-2-ol catalysed by related presence of the hydrogenated product, methyl 2- ruthenium complexes.[21] In the initial stage a methoxide ion is methylpropanoate (MiBu) along with 3-pentanone and significant generated by deprotonation of methanol in the presence of the amounts of methanol, suggesting that a carbon alkylating agent added base. This species is then believed to enter the cycle by and hydrogen source must have been present at some stage displacing PPh3 from [RuH2(CO)(PPh3)3] (1), I, generating an during the reaction. Methanol, possibly from hydrolysis of methyl anionic methoxy intermediate II; (b) loss of PPh3 followed by β- propanoate was believed to be the most likely source of the hydrogen abstraction allows the formation of coordinated methylene group in MMA (see Scheme 3). Based on this formaldehyde, III, onto which (c) deprotonated MeP attacks; (d) observation, the in situ generation of anhydrous formaldehyde protonation and displacement of methyl 3-hydroxy-2- and hydrogen gas by metal-catalysed dehydrogenation of methylpropanoate by PPh3 produces the anionic trihydrido methanol has been investigated. The role of the base in this carbonyl complex IV; (e) the trihydrido complex, by abstracting a system is twofold. It is involved in the deprotonation of methanol proton from a molecule of methanol, could regenerate the affording the alkoxide ion, which is the active species in methoxide ion and afford molecular hydrogen (which remains methanol dehydrogenation, and it is essential for the α- associated with the metal complex as a coordinated dihydrogen deprotonation of MeP which can then undergo condensation molecule) IV; (f) the liberation of molecular hydrogen in the final with the newly formed formaldehyde affording an intermediate step regenerates the catalyst I; (g) the methyl 3-hydroxy-2- species, methyl 3-hydroxy-2-methylpropanoate. The latter methylpropanoate produced in stage d spontaneously provides the desired product, MMA, via spontaneous water dehydrates to give MMA as the final product; (h) MiBu is then elimination (Scheme 3). Since H2 is produced in the reaction undesirably formed by catalytic hydrogenation of MMA. The system and we wish not to hydrogenate MMA, suitable catalysts nucleophilic attack of deprotonated MeP is shown as occurring should be chemoselective for the dehydrogenation of methanol to coordinated formaldehyde (c) on the basis of a precedent of leading to formaldehyde without promoting the hydrogenation of attack of an amine[11] and because coordinated formaldehyde the C=C bond in MMA to give methyl 2-methylpropanoate will be a better electrophile. It could also occur onto free (MiBu). formaldehyde. Scheme 3. Route to methyl methacrylate involving attack of deprotonated MeP on formaldehyde formed by the in situ dehydrogenation of methanol. Some of us reported in the late 80’s that catalytic dehydrogenation of alcohols can be achieved from a wide range of alcoholic substrates using Rh and Ru catalysts.[14] Among these catalysts, one of the best was [RuH2(CO)(PPh3)3] (1). Similar complexes, such as [RuHCl(CO)(PPh3)3 have been reported to give some preference for hydrogenation of C=O over C=C bonds, e. g. in cinnamaldehyde.[15] Therefore, based on the principle of microscopic reversibility, it was examined as a suitable dehydrogenation catalyst in these preliminary studies together with other hydrogenating catalysts. The initial catalyst screening involved four Ru species which exhibited only minor activity, Shvo’s catalyst[16] (2), [RuCl(CO) (η5-Ph C )][17] (3), 2 5 5 Scheme 4. Proposed mechanism for ruthenium catalysed dehydrogenation of [18] [19] [Ru(OAc)2(TriPhos)] (4), [RuH2(CO)(TriPhos)] (5), and a Rh MeOH leading to the in situ production of anhydrous formaldehyde and [20] catalyst [RhH(CO)(PEt3)3] (6) which showed some activity but subsequent condensation with MeP to afford MMA via water elimination and no selectivity to MMA (MMA yield 0%; MiBu yield 6 %). MiBu by hydrogenation of MMA. (P = PPh3) FULL PAPER The low yields of MMA and MiBu could arise because of poor Similarly, upon heating a mixture of MeP, methanol, NaOMe and deprotonation of MeP or low rates of formation of formaldehyde. 0.13% of 1 in toluene, in a Hastelloy™ autoclave at 170 °C for 3 Hence, a set of deuterium labelling experiments was conducted hours, MMA was produced together with the hydrogenated by- with the aim of determining the extent to which a variety of product MiBu, but once again, a substantial amount of methyl bases affect the α-deprotonation of MeP. Three parallel propanoate was formed.