Experimental and theoretical insight into the mechanism of CO2 cycloaddition to epoxides catalyzed by ammonium ferrates

Ammonium zincates as suitable catalyst for the room temperature cycloaddition of CO2 to epoxides

“…It is worth to note that, as known from the literature, the activity of simple quaternary ammonium halides, i.e., TBAX, in this reaction is not negligible (Caló et al, 2002;Wang et al, 2012;Montoya et al, 2015). However, we have already shown that under the same reaction conditions, both TBACl and TBABr are underperforming, albeit maintaining very high selectivity (Panza et al, 2022). We repeated the reaction under the same catalytic conditions by using TBAI that performed comparably to the others ammonium halides and gave only a 42% conversion with 95% selectivity in 2a (See Supplementary Material).…”

“…The purity was confirmed by elemental analyses, whilst accurate high resolution mass spectra were carried out in CH 3 CN with the double aim to assess the proposed structure and to study the equilibria between the undissociated dianion [ZnX 4 ] 2and the solvated forms ([ZnX 3 ] -+ X − ) and (ZnX 2 + 2X − ). This kind of equilibria between "ate" and neutral salts and its relevance to the nucleophilic ring opening of epoxides has been already disclosed by us in the case of the tetrabutylammonium ferrates (Panza et al, 2022), and it was already predicted by Capacchione and coworkers in the case of [FeBr 4 ]anion (Della Monica et al, 2019b).…”

“…To our delight, all the zincate catalysts tested gave instead quantitative conversions of the starting epoxide in 4 h reaction time, with excellent selectivity in the case of the bromide and iodide salts (94 and 95% respectively, entries 2 and 3, Table 2). We should emphasize, however, that in this case 0.5 mol% of the catalyst contains the double of equivalents of the ammonium cation with respect to the monoanionic ferrates analogues, but we already showed that its role is limited essentially to a rigid shift of all energies towards lower values (Panza et al, 2022). When we reduced the reaction time at 2h, we still observed a quantitative conversion, except for [TBA] 2 [ZnCl 4 ] as catalyst, with selectivity in the range 91-98% (entries 4-6, Table 2).…”

“…Since the optimized conditions with the recently reported "ferrate" catalysts were found to be 0.5 mol% catalyst loading at 100 °C and under 0.8 MPa of CO 2 , initially we set there the starting point for the optimization using tetrachloro-, tetrabromo-and tetraiodo-zincates. Under the previously described conditions, when using [TBA][FeCl 3 Br] as the catalyst, 83% of SO was converted into styrene carbonate (SC), 2a, in 4 h with 95% selectivity (Panza et al, 2022). To our delight, all the zincate catalysts tested gave instead quantitative conversions of the starting epoxide in 4 h reaction time, with excellent selectivity in the case of the bromide and iodide salts (94 and 95% respectively, entries 2 and 3, Table 2).…”

“…We have recently exploited the reactivity of tetrabutylammonium ferrates of the general formulae [TBA] [FeX 3 Y] (TBA = n Bu 4 N, X, Y = Cl, Br), that can be obtained from inexpensive chemicals such as tetrabutylammonium halides and ferric salts (Wyrzykowski et al, 2006(Wyrzykowski et al, , 2007, as stand-alone catalysts in the CO 2 cycloaddition to epoxides (Panza et al, 2022). The effect of the experimental factors (reaction temperature, CO 2 pressure, type of nucleophile and recycling of the catalyst), together with a full set of theoretical calculations, were studied in depth.…”