anticipated pressures on fossil fuel resources in the coming decades mean that raw materials for the chemical industry must be found that can reduce our reliance on fossil - fuel - based feedstocks .
hence , the development of new production processes for chemical syntheses from renewable resources is a challenge that should be a priority for all major economies .
research into / application of carbon dioxide utilization ( cdu ) technologies has recently gained increased interest .
cdu offers an attractive alternative to the well - established carbon dioxide storage ( ccs ) processes , however , scale of production means that cdu is unlikely to significantly reduce anthropogenic carbon dioxide emissions , but waste carbon dioxide under cdu protocols can be converted into useful chemicals or fuels .
carbon dioxide is abundant , cheap and non - toxic when compared with other c1 building blocks , such as phosgene , and has been used in the manufacture of salicylic acid , urea and cyclic carbonates for 50100 years .
however , because of the relative inertness of carbon dioxide these processes are significantly energy demanding with reactions taking place at high temperatures and pressures . therefore , if cdu technologies are to become viable processes reliance on very little energy input is required , hence processes at mild temperatures ( < 100 c ) and atmospheric pressures should be targeted .
the insertion of carbon dioxide into epoxides to afford either five - membered ring cyclic carbonates or the corresponding polycarbonate has attracted considerable attention due to the industrial application of the products arising from these reactions . for example , north and co - workers have employed bimetallic aluminium salen complexes incorporating an ammonium bromide co - catalyst to prepare cyclic carbonates in good yield from not only highly purified carbon dioxide but also directly from a fossil fuel power station stack by use of a supported salen catalyst and flow reactor .
interestingly , structurally related salen - based catalysts have been successful in the corresponding polycarbonate process , for example darensbourg and co - workers have reported several studies employing a chromium - based salen complex to afford a range of polycarbonates ; as have lee and co - workers who have employed salen  cobalt catalysts .
other catalytic systems have also been successfully employed by both coates and more recently williams , who has developed a series of highly active bimetallic catalysts that are able to perform the polymerisation reactions with atmospheric pressure carbon dioxide .
decortes and kleij have also reported atmosperice pressure co2 insertion to form five - membered ring carbonates using zinc  salophen catalysts .
far less exposure has been given to carbon dioxide insertion into other heterocyclic ring systems , mainly due to the requirement for a high - energy substrate such as an epoxide for the insertion to be favourable .
one possible product from carbon dioxide insertion into the challenging four - membered ring analogue of epoxides , oxetanes , would be the synthesis of six - membered ring cyclic carbonates or polycarbonates .
trimethylene carbonate ( tmc ) 1 is a useful monomer for ring - opening polymerization , as complete retention of carbon dioxide is observed when polymerized to 2 using a metal or non - metal catalyst ( scheme 1 ) .
these polymers are of significant value since they are biodegradable and can be used in a range of medical devices .
traditional approaches to tmc 1 and poly(tmc ) 2 . traditionally tmc 1 has been prepared from 1,3-propanediol and the highly poisonous phosgene gas , however , more recently several greener alternatives have arisen , for example transesterification employing 1,3-propanediol and a dialkylcarbonate ( scheme 1 ) .
an attractive approach would be to use carbon dioxide , which has previously been employed to prepare five - membered ring cyclic carbonates from epoxides , however , selective formation of tmc 1 using carbon dioxide is a non - trivial process as it is thermodynamically less stable than its corresponding co - polymer 2 , thus selective production of the monomer presents a significant challenge .
baba and co - workers were the first to report the ring expansion of oxetanes with carbon dioxide to afford six - membered cyclic carbonates , they employed stibonium iodide or organotin iodide / nbu3po catalysts at typically 100 c and 49 atmospheres of carbon dioxide .
recently darensbourg has shown that trimethylene carbonate 1 can be obtained from trimethylene oxide 3 , vo(acac)2/nbu4nbr at 60 c under 35 atmospheres of carbon dioxide ( scheme 2 ) .
excellent conversion and selectivity was observed after 8 h , but reducing the pressure below 10 atmospheres of carbon dioxide severely reduced the reactivity of the catalyst .
we have recently shown that electrosynthesis can be a powerful tool for a range of important reactions and can be employed in the catalyst - free synthesis of five - membered ring cyclic carbonates under atmospheric pressure carbon dioxide and mild temperatures .
the importance of six - membered ring cyclic carbonates and their inherent thermodynamic instability prompted us to investigate this important carbon dioxide insertion reaction .
we believed that it should be possible to develop an electrochemical system that is easy to set up , cheap , reliable , requires no expensive catalysts , runs under atmospheric co2 pressures and at ambient temperatures .
importantly , employing an electrosynthetic system is cost neutral in terms of energy consumption if combined with a suitable renewable energy source , for example , solar .
we started our investigation by applying our previously reported optimized conditions for the synthesis of five - membered ring cyclic carbonates from carbon dioxide and epoxides .
gratifyingly , this resulted in the selective synthesis of tmc 1 from trimethylene oxide 3 , and under these reaction conditions we did not observe the thermodynamically more stable co - polymer 2 .
however , compound 1 was only formed in a disappointing 40 % yield ( table 1 , entry 12 ) , a range of electrode materials were then screened in an attempt to enhance the isolated yield observed but this process proved fruitless .
optimization studies for the production of tmc 1 from trimethylene oxide 3[a ] general conditions : cu cathode , mg anode , co2 ( 1 atm , balloon ) , supporting electrolyte ( 1.0 equiv . ) , mecn , single compartment cell , 60 ma , 6 h. ratio calculated from the crude reaction h nmr spectrum by integration of cyclic carbonate vs. co - polymer signals . isolated yield of tmc 1 after chromatography
we then turned our attention to the supporting electrolyte and a screen employing tetrabutylammonium chloride , bromide , iodide and hexafluorophosphate identified the iodide as the most promising candidate , with 50 % yield of tmc 1 being formed at 25 c with > 99:1 selectivity for the monomer ( table 1 , entry 2 ) .
of the supporting electrolyte was required for good yield to be achieved ( 50 % , table 1 , entry 7 ) and an increase in yield to 60 % could be achieved through a slight increase in the applied current ( 60 to 90 ma , table 1 , entry 9 ) .
a series of experiments to look at the effect of temperature on the reaction were also carried out and finally we were able to obtain tmc 1 in 96 % yield , although the reaction was not quite 100 % selective for the monomer ( table 1 , entry 18 ) .
kleij and co - workers recently reported the synthesis of five - membered cyclic carbonates in excellent yield with either an iron- or aluminium - based catalyst at 102 atm co2 .
they also reported high yields and selectivity of oxetane to tmc 1 with the same systems but observed that addition of steric bulk to the oxetane substrate at the 3,3-position ( two methyl groups , 4 ) dramatically decreased the yield and selectivity of the co2 incorporation reaction ( typically 928 % yield , table 2 entries 1 and 2 ) .
darensbourg has also reported that 3,3-substituted oxetanes are slow substrates for co2 insertion at 35 atm but has not yet disclosed the yields when employing his vanadium - catalysed conditions ( table 2 , entry 3 ) .
when employing ( salen)cr chloride complexes , darensbourg has reported a product distribution of 88 % in favour of copolymer 6 , when exploring the equilibrium distribution of a range of oxetanes .
a comparison of processes for 5,5-dimethyl-1,3-dioxan-2-one 5 synthesis[a ] conditions : [ fe(tphoa)]2 ( 0.5 mol-% ) , bu4ni ( 5 mol-% ) , mek , co2 ( 102 atm ) , 85 c . conditions : al triphenolate catalyst ( 0.5 mol-% ) , bu4ni ( 2.5 mol-% ) , co2 ( 102 atm ) , 70 c .
vo(acac)2 ( 5 mol-% ) , bunbr ( 5 mol-% ) , 60 c , co2 ( 35 atm ) .
( salen)cr chloride complex , bu4nn3 ( 2 equiv . ) , co2 ( 35 atm ) , toluene , 110 c . conditions : cu cathode , mg anode , co2 ( 1 atm , balloon ) , bu4ni ( 1.0 equiv . ) , mecn , single compartment cell , 90 ma , 50 c . under our conditions
the formation of 5,5-dimethyl-1,3-dioxan-2-one 5 proceeded relatively smoothly with an unoptimised 70 % yield being obtained at 50 c when employing tetrabutylammonium iodide and 60 ma ( table 2 , entry 4 ) .
a proposed mechanism is described in scheme 3 in which magnesium iodide is prepared in situ , following the reports of north and co - workers tributylamine is released under the reaction conditions[4 g ] and also activates co2 towards eventual incorporation to tmc , with magnesium iodide regeneration .
we have developed , to the best of our knowledge , the first selective procedure for the formation of the tmc monomer 1 at atmospheric pressure carbon dioxide from oxetane .
this process is one of only a handful of approaches that allow the conversion of oxetane to the cyclic carbonate rather than the thermodynamically more stable polymer 2 .
the equipment required to perform this co2 incorporation reaction is cheap and should be readily available in any undergraduate teaching facility , that is , copper wire , magnesium ribbon and a power supply .
the protocol described affords the highest reported yield for the 3,3-dimethyl - substituted oxetane and we are currently exploring the co2 insertion into these challenging substituted oxetanes , for which there are currently very few commercially available .
representative procedure for the formation of cyclic carbonates : trimethylcarbonate ( tmc ) ( 2 ) : tetrabutylammonium iodide ( 0.74 g , 2.0 mmol ) was dissolved in acetonitrile ( 145 ml ) , the resulting solution was flushed with co2 for 1 h at room temperature and trimethylene oxide ( 0.12 g , 2.0 mmol ) added as a solution in acetonitrile ( 5 ml ) .
the reactor was heated to 75 c and potential was then applied to the system ( constant current : 90 ma ) for 6 h in a single compartment cell [ mg anode and copper(0 ) cathode ] . on completion
the reaction mixture was washed with aqueous hcl ( 0.1 m , 50 ml ) followed by extraction with et2o ( 3  35 ml ) .
the combined organic extracts were then dried with mgso4 and evaporated under reduced pressure to afford an amber oil , which was suspended in etoac ( 100 ml ) .
after 1 h the precipitated bu4ni (  70 % ) was removed by filtration and the solvent evaporated under reduced pressure to afford an amber oil .
this crude material was purified by column chromatography on silica gel eluting with ethyl acetate / light petroleum to give a colourless solid ( 0.196 g , 96 % ) .
h nmr ( 400 mhz , cdcl3 , me4si ) :  = 2.122.18 ( m , 2 h ) , 4.454.48 ( m , 4 h ) ppm .
c nmr ( 100 mhz , cdcl3 , me4si ) :  = 21.8 , 68.0 , 148.5 ppm .
ir ( ch2cl2 ) :   = 1252 ( c  o ) , 1740 ( c = o ) cm .
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 carbon dioxide utilisation ( cdu ) is currently gaining increased interest due to the abundance of co2 and its possible application as a c1 building block . 
 we herein report the first example of atmospheric pressure carbon dioxide incorporation into oxetane to selectively form trimethylene carbonate ( tmc ) , which is a significant challenge as tmc is thermodynamically less favoured than its corresponding co - polymer . 
Introduction
Results and Discussion
Conclusions
Experimental Section
Supporting Information