Applications of NHCs and isothioureas as Lewis base organocatalysts

Abstract

This thesis details investigations into two modes of organocatalytic enolate generation and reactivity. The first is the addition of chiral NHCs to a ketene to form an azolium enolate. Investigations into three different electrophiles within this system will be discussed. The second catalytic system investigated is the formation of a C1-ammonium enolate through the addition of chiral isothioureas to activated carboxylic acids. Studies on one electrophile within this system will be discussed. Chapter 2 investigates the use of bespoke C₂-symmetric NHCs in the asymmetric Staudinger reaction: the formal [2+2] cycloaddition between ketenes and imines. A chiral relay mechanism was proposed and experimentally validated with β-lactams prepared in good yields (up to 85%) and moderate enantioselectivity (up to 61% ee). Chapter 3 describes the asymmetric formal [4+2] cycloaddition between alkylarylketenes and γ-substituted-β,γ-unsaturated α-ketocarboxylates. A substrate dependant switch in diastereoselectivity was observed with γ-aryl α-ketocarboxylates giving preferentially the syn-dihydropyranones (up to 70:30 dr syn:anti, up to 98% ee syn) and γ-alkyl α-ketocarboxylates giving preferentially the anti-dihydropyranones (up to 18:82 dr syn:anti, up to 75% ee anti, 97% ee syn). Chapter 4 illustrates the extension of the methodology described in Chapter 3 to include γ-substituted-β,γ-unsaturated α-ketophosphonates. Once more, a substrate dependant switch in diastereoselectivity was observed with γ-aryl α-ketophosphonates giving preferentially the syn-dihydropyranone-phosphonates (up to 72:28 dr syn:anti, up to 98% ee syn) and γ-methyl α-ketophosphonate giving preferentially the anti-dihydropyranone-phosphonates (20:80 dr syn:anti, 71% ee anti, 90% ee syn). Within this system it is also possible to generate the alkylarylketenes in situ with no loss in stereoselectivity but with typically improved yield when compared with the corresponding two-step procedure. Chapter 5 describes the activation of arylacetic acids via the formation of a mixed anhydride followed by C1-ammonium enolate generation with a chiral isothiourea. Asymmetric Michael addition and lactonisation with γ-substituted-β,γ-unsaturated α-ketophosphonates gave the corresponding anti-dihydropyranone-phosphonates (up to 89:11 dr anti:syn, up to 97% ee anti). More conveniently however, in situ ring opening with MeOH gave di-esters with excellent stereocontrol (up to >98:<2 dr anti:syn, up to 99% ee anti) and which can be readily derivatised.