Abstract
The study of N-heterocyclic carbenes (NHCs) as organocatalysts has proliferated in recent years, and they have been found to be useful in a variety of reactions. In an attempt to further expand their utility and to study their recyclability, we designed and synthesized a series of self-supported NHCs in which the catalytic carbene groups form part of a densely functionalized polymer backbone, and studied them as organocatalysts. Of the self-Supported NHCs examined, a benzimidazole derived polymer with flexible linkers connecting the catalytic groups was found to be the most efficient organocatalyst in a model benzoin condensation reaction, and thus it was used in a variety of such reactions, including some involving catalyst recycling. Furthermore, it was also used to catalyze a set of redox esterification reactions involving conjugated unsaturated aldehydes. In all of these reactions the catalyst afforded good yield of the desired product and its polymeric nature facilitated product purification.
Highlights
IntroductionA1.sInretrsoedaurcchtiorengarding N-hereocyclic carbenes (NHCs) has increased in recent years, it has become clear that they are highly useful organocatalysts for a wide range of molecular transformations due tboecthomAeiesr raceblseielaairtrychthtoaretrgeatahrcdetyinwgairtNeh‐hahelidrgeheohlcyyycduliescsefcauanlrbdeonrggeeasnne(oNrcaHatteaClsyt)hstehsacsoforirnrecrasepaowsenidddeiinngrarenhcgoeemntooyfeenamorsloa,lteietcuhrleaaasrctive intermtraendsifaotrems a[1ti–o1n2s].dIunelitgohtthoefirthaebirilgitryeattourteilaictyt awnitdhspaledceihaylidzeesd arenadctgiveniteyr,aetfefotrhtes hcaovrreesbpeoenndmingade to simplhifoymtoheeniroluastee arenadctfiavceiliinttaetremthedeiiartreesc[y1c–l1in2]g.,Iannldigohnt eofsttrhaetierggyretaotaucthiliietyveatnhdessepeaciimalsizheadsrbeaecetnivtiotya,ttach NHCesfftoortas hpaovleymbeeern smuapdpeotortstimhaptliafyllothweisr fuosre tahnedmfactiolitbaeteetahseiilryrerceymcloinvge,danadt othnee setnradteogfyatoreacahciteioven and potentthieaslelyarimeussheads.bFeoernetoxaamttapclhe,NimHCidsatzooalipuomlymanedr srueplaptoerdt tihoanticalllioqwusidfosrhtahveme btoeebne aetatsailcyhreedmtoovvedarious inert aptothlyemenedr coaf rarirerasct[io1n3–a1n7d],paontednttihalelyserecuasneds.eFrvore eaxsamprpoleto, inmaitdeadzoplriuemcuarsnodrrsetlaotetdheiocnoicrrleiqsupiodns ding NHChoarvgeanbeoecnataatltyascthse[d18to–2v4a]r.iHouoswineevretr,paoldyrmaewrbcaacrkrieorfst[h1i3s–s1t7r]a,taengdy itshethseatctahnesuersveeoaf sanprionteorntapteodlymer suppoprret cluorwsoerrssttohtehelocaodrriensgpolenvdeilngofNthHeCcoartgaalynsotc,aetavleynstsm[1o8r–e2s4o].iHf aowlienvkeerr, agdrorauwpbiascuksoefdthfiosrsatrtattaecghyment, and inisctrheaatstehsetuhseeaomf aonuinnteortfpsoollyvmenert srueqpupoirretdlofwoerrrsetahcetiloonadsi(nFgigleuvreel 1oaf )t.hAe cnateamlyesrt,geivnegnsmtroarteegsyo itfoawork aroun(liFdnikgtheurirsegi1rsoas)uu.peAtinhsaeutmspeeodrgtefinongrtiaasttlrtlaaycterhegmdyeutnocte,wsaotnhrdke aianrmocruoenuadsnettshoitfshdeisesaaumdeotwuhanetitgpohoftt,esenosltvipaeelnlcytiarrleeldyquufcioreresditmhfeoidraamrzeooaulcitnuiotmnosfNHC precudresaodrswtehiagthrt,eeqsupierceiaslulybfsotritiumeindtaszoolniubmotNhHnCitrporgeecunrsaotorsmthsa(tvriedqeuinirferasu),bisstittouesnetlsf-osnupbpotohrtnitthroegceantalyst by linaktoinmgs t(hviedecaintfarlay),tiics gtoroseulpf‐ssuipnpsourct hthae mcaatanlynsetrbsyolitnhkaitntghtehye acaretalloytciactgerdouinpsthine smucahinapmoalnynmeer rsochain, rathetrhtahtathneayttaarcehloecdatteodiitn(Fthigeumraein1bp)o. lymer chain, rather than attached to it (Figure 1b)
With our target polymeric salts in hand, we examined their utility as N-hereocyclic carbenes (NHCs) precursors in the Wbeintzhooinucrotnadregnestatpioonlyomf beerniczasladlethsyidneh(1a3nad),towperoedxuacme itnheedalpthhea‐irhyudtriolixty kaestoNneHpCropdruecctu1r4sao. rTshein the benzobiansecDoBnUdewnassautisoedn inofDbMeSnOzafoldr edhepyrdoeto(n1a3tiao)ntoof tphreopdoulycme etrhsetoaglepnheara-hteytdheroNxHyCkceattoanlyeticpgroroduupcst, 14a
We have prepared a series of self-supported imidazolium and benzimidazolium polymeric salts (4–6) that can function as NHC precursors by attaching the linker groups as nitrogen substituents
Summary
A1.sInretrsoedaurcchtiorengarding N-hereocyclic carbenes (NHCs) has increased in recent years, it has become clear that they are highly useful organocatalysts for a wide range of molecular transformations due tboecthomAeiesr raceblseielaairtrychthtoaretrgeatahrcdetyinwgairtNeh‐hahelidrgeheohlcyyycduliescsefcauanlrbdeonrggeeasnne(oNrcaHatteaClsyt)hstehsacsoforirnrecrasepaowsenidddeiinngrarenhcgoeemntooyfeenamorsloa,lteietcuhrleaaasrctive intermtraendsifaotrems a[1ti–o1n2s].dIunelitgohtthoefirthaebirilgitryeattourteilaictyt awnitdhspaledceihaylidzeesd arenadctgiveniteyr,aetfefotrhtes hcaovrreesbpeoenndmingade to simplhifoymtoheeniroluastee arenadctfiavceiliinttaetremthedeiiartreesc[y1c–l1in2]g.,Iannldigohnt eofsttrhaetierggyretaotaucthiliietyveatnhdessepeaciimalsizheadsrbeaecetnivtiotya,ttach NHCesfftoortas hpaovleymbeeern smuapdpeotortstimhaptliafyllothweisr fuosre tahnedmfactiolitbaeteetahseiilryrerceymcloinvge,danadt othnee setnradteogfyatoreacahciteioven and potentthieaslelyarimeussheads.bFeoernetoxaamttapclhe,NimHCidsatzooalipuomlymanedr srueplaptoerdt tihoanticalllioqwusidfosrhtahveme btoeebne aetatsailcyhreedmtoovvedarious inert aptothlyemenedr coaf rarirerasct[io1n3–a1n7d],paontednttihalelyserecuasneds.eFrvore eaxsamprpoleto, inmaitdeadzoplriuemcuarsnodrrsetlaotetdheiocnoicrrleiqsupiodns ding NHChoarvgeanbeoecnataatltyascthse[d18to–2v4a]r.iHouoswineevretr,paoldyrmaewrbcaacrkrieorfst[h1i3s–s1t7r]a,taengdy itshethseatctahnesuersveeoaf sanprionteorntapteodlymer suppoprret cluorwsoerrssttohtehelocaodrriensgpolenvdeilngofNthHeCcoartgaalynsotc,aetavleynstsm[1o8r–e2s4o].iHf aowlienvkeerr, agdrorauwpbiascuksoefdthfiosrsatrtattaecghyment, and inisctrheaatstehsetuhseeaomf aonuinnteortfpsoollyvmenert srueqpupoirretdlofwoerrrsetahcetiloonadsi(nFgigleuvreel 1oaf )t.hAe cnateamlyesrt,geivnegnsmtroarteegsyo itfoawork aroun(liFdnikgtheurirsegi1rsoas)uu.peAtinhsaeutmspeeodrgtefinongrtiaasttlrtlaaycterhegmdyeutnocte,wsaotnhrdke aianrmocruoenuadsnettshoitfshdeisesaaumdeotwuhanetitgpohoftt,esenosltvipaeelnlcytiarrleeldyquufcioreresditmhfeoidraamrzeooaulcitnuiotmnosfNHC precudresaodrswtehiagthrt,eeqsupierceiaslulybfsotritiumeindtaszoolniubmotNhHnCitrporgeecunrsaotorsmthsa(tvriedqeuinirferasu),bisstittouesnetlsf-osnupbpotohrtnitthroegceantalyst by linaktoinmgs t(hviedecaintfarlay),tiics gtoroseulpf‐ssuipnpsourct hthae mcaatanlynsetrbsyolitnhkaitntghtehye acaretalloytciactgerdouinpsthine smucahinapmoalnynmeer rsochain, rathetrhtahtathneayttaarcehloecdatteodiitn(Fthigeumraein1bp)o. lymer chain, rather than attached to it (Figure 1b). The solid product that precipitated from the solution and was filtered, washed with additional DMF (50 mL) and Et2O (2 × 50 mL), and dried under vacuum at 70 ◦C for 24 h. Polymer 4 was obtained in 75% yield as a white powder It was completely insoluble in all solvents examined and was not characterized by NMR analysis. Polymer 5 was obtained as light yellow powder with 86% yield. Bromoethane (5.4 g, 50.0 mmol) was added to the reaction mixture and the resulting solution was heated at 70 ◦C for 6 h more. The slightly yellow solid product that precipitated from the solution was filtered, washed with diethyl ether (2 × 25 mL), and dried under vacuum at 70 ◦C for 24 h to afford 15 (95% yield).
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