Nucleophile-directed selectivity towards linear carbonates in the niobium pentaethoxide-catalysed cycloaddition of CO₂ and propylene oxide

Abstract

The reaction between CO2 and propylene oxide in the presence of Nb(EtO)5 and 4-dimethylaminopyridine leads to the formation of an acyclic carbonate through an unprecedented pathway.

Cycloadditions to Epoxides Catalyzed by Group III–V Transition‐Metal Complexes

“…[41] For Nb(EtO) 5 /DMAP only al inear carbonateb earing an ethyl ester functionality was formed (IV,S cheme 9), as identified in the in situ IR spectrum of the reaction by the appearance of ab and at 1747 rather than 1810 cm À1 for PC (see Figure 3, right). [41] In contrast, Nb(EtO) 5 / NBu 4 Br initially yielded PC, but the reactionr ate dropped progressivelyt oz ero with the formation of IV,h inting at simultaneous catalyst decomposition (Figure 3, left).…”

“…[41] For Nb(EtO) 5 /DMAP only al inear carbonateb earing an ethyl ester functionality was formed (IV,S cheme 9), as identified in the in situ IR spectrum of the reaction by the appearance of ab and at 1747 rather than 1810 cm À1 for PC (see Figure 3, right). [41] In contrast, Nb(EtO) 5 / NBu 4 Br initially yielded PC, but the reactionr ate dropped progressivelyt oz ero with the formation of IV,h inting at simultaneous catalyst decomposition (Figure 3, left). An in situ IR investigation carried out on the cycloaddition reaction catalyzed by Nb(EtO) 5 /DMAP( see Figure 3a nd Scheme 9f or further details) confirmed the formation of all the expected reaction intermediates (Scheme3)u pto hemicarbonate species III.H owever,t his species was apparently unable to release the cyclic carbonate product and underwent the attack of an ethoxy ligand to yield linear carbonate IV.This behavior was attributed to the intramolecular attack of an ethoxide anion on the carbonyl moiety being faster than ring closure to yield PC.…”

“…It can be dramatically modulated by tuning these parameters through the judicious choice of the co-catalyst and of the substituents at the Nb center.F or instance, an increase in the nucleophilicity of the co-catalytic component pursued by employing stronger pyridine-based nucleophiles, [44] such as TCAP (16,9 -azajulodine, Scheme 8), led to am arked decreaseo ft he catalytic efficiency (Table 5, entry 3). [41] On the other hand, aw eaker Lewis acid such as Nb(EtO) 5 ,u sed in the presence of NBu 4 Br,a ppeared to be am ore efficient catalytic pair than NbCl 5 /NBu 4 Br by displaying an initial rate of carbonate product formation that doubled that of the latterp air (Scheme 8, Ta ble 5, entries 2a nd 5). [41] However,t he reaction catalyzedb yN b(EtO) 5 /NBu 4 Br was found to also produce al inear ester byproduct by meanso fa n unprecedented reactionm echanism that led to progressive catalystd ecomposition and deactivation (for the mechanistic implications, see below).I nas tudy targetings pecifically the improvement of the co-catalytic component, Cokoja, Kühn, and co-workerse mployed av arietyo fi midazolium bromides such as 17-19,( Scheme 8) and studied the influence on catalytic efficiency of the side chains on the imidazole scaffold.…”

“…[41] On the other hand, aw eaker Lewis acid such as Nb(EtO) 5 ,u sed in the presence of NBu 4 Br,a ppeared to be am ore efficient catalytic pair than NbCl 5 /NBu 4 Br by displaying an initial rate of carbonate product formation that doubled that of the latterp air (Scheme 8, Ta ble 5, entries 2a nd 5). [41] However,t he reaction catalyzedb yN b(EtO) 5 /NBu 4 Br was found to also produce al inear ester byproduct by meanso fa n unprecedented reactionm echanism that led to progressive catalystd ecomposition and deactivation (for the mechanistic implications, see below).I nas tudy targetings pecifically the improvement of the co-catalytic component, Cokoja, Kühn, and co-workerse mployed av arietyo fi midazolium bromides such as 17-19,( Scheme 8) and studied the influence on catalytic efficiency of the side chains on the imidazole scaffold. [42] Higher catalytic activity was observed by weakening the interaction between thei onic components of the co-catalytic pair (imidazolium cation and bromide anion)t hus resulting in am ore nucleophilic bromide anion.T his was achieved by alkylating the imidazole backbone at the C2-position.…”

“…[c] initial rate of propylene carbonate formation as measured by in situ IR spectroscopy according to Refs [41,43].…”