Thesis
Novel N-heterocyclic activations mediated by magnesium reagents having sterically hindered ligands
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2012
- Thesis identifier
- T13326
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Building on recent advances on the synthesis of homometallic Mg compounds bearing highly sterically demanding ligands, but going significantly beyond the state-of-the-art, this thesis report focuses on the synthesis of new sodium magnesiates and magnesium reagents supported by bulky N-chelating ligands {Ph2Si(NAr*)2}2- and {Ar*N=C(Me)CH(Me)NAr*}- (nacnac) as well as their exploitation within the areas of deprotonative metallation, heterocyclic activation and catalytic application in hydroamination reactions of organic heterocumulenes. Firstly, a series of novel sodium magnesiates bearing the sterically demanding bis(amido)silyl ligand {Ph2Si(NAr*)2}2- (Ar*= 2,6-i-Pr2-C6H3) has been prepared. Thus alkyl derivatives [{Na(THF)6}+{(Ph2Si(NAr*)2)Mg(R)(THF)}-] (R= Bu, 2; CH2SiMe3, 5) have been synthesised and isolated as crystalline solids by reacting bis(amine)silyl [Ph2Si(NHAr*)2] 1 with the relevant sodium magnesiate NaMg(Bu)R2 (prepared in situ by co-complexation of BuNa with the relevant magnesium alkyl MgR2). In addition, compound 5 has been used as a precursor for the synthesis of a new series of amido sodium magnesiates [{Na(THF)6}+{(Ph2Si(NAr*)2)Mg(NR2)(THF)x}-] (NR2 = HMDS, x=1, 6; NR2= NPh2, x=1, 7; NR2 = NiPr2, x=1, 8; NR2 = DMP, x=1, 9; NR2 = TMP, x=0, 10). The reactions of NH2Ar* and pyrrole with complex 5 led to the formation of mixed-metal complexes [{Na(THF)6}+{(Ph2Si(NAr*)(NHAr*))Mg(NHAr*)2(THF)}-] 11 and [{(Ph2Si(NAr*)(NHAr*))Mg(NC4H4)2(THF)Na(THF)2}] 12 respectively. Reactivity studies disclosed that TMP derivative 10 was able to carry out the α-magnesiation of thiophene to yield [{Na(THF)6}+{Ph2Si(NAr*)2)Mg(C4H3S)(THF)}-] 13. In moving to reactivity studies with 1,3 benzoazoles complex 2 promotes the chemeoselective magnesiation of methylbenzimidazole (MeBIm) to yield [{Na(THF)5}2+{(Ph2Si(NAr*)2)Mg(MeBIm*)}2-] 14 (MeBIm* = methylbenzimidazolyl), analysis of bond parameters, NMR data and DFT studies suggest that N-methylbenzimidazolyl ligand displays a metal carbene character.;The reaction of 2 with benzothiazole (Btz) promotes a unique activation process of this heterocycle initiating an unprecedented cascade of reactions, where the initial magnesiation of Btz is followed by an intricate sequence of C-C coupling, ring opening, benzothiazolyl insertion into a C=N bond and intramolecular deprotonation leading to the ring opening and functionalisation of three Btz molecules resulting in novel sodium magnesiate [L2Mg2Na2(THF)5] 15. Extending these studies to the diazine molecule quinoxaline allows the synergic entrapment of radical anion (Qox) in the form of dimer [{Na(THF)6}2+{(Ph2Si(NAr*)2)Mg(Qox)}2-] 20 resulting from the homoleptic cleavage of Mg-C bond in the alkyl precursor 2. EPR studies show that 20 is diamagnetic in the solid-state dimeric structure. Homometallic magnesium species [(Dippnacnac)Mg(Bu)(THF)] 22 and [(Dippnacnac)Mg(TMP)] 23 were prepared and structurally defined and their reactivity towards 1,3-benzoazoles has been assessed. Reactions of 22 and 23 with benzoxazole allowed the isolation in both cases of [(Dippnacnac)Mg{O(o-C6H4)NC}(THF)] 24 resulting from ring cleavage of benzoxazolyl anion. Benzothiazole (Btz) was magnesiated at the C2 position by the amido derivative to form [{(Dippnacnac)Mg(Btz*)}2] 25 (Btz*= 2-benzothiazolyl), however when 22 was reacted with Btz a novel activation of Btz occurred in a cascade reaction involving magnesiation, C-C coupling and ring opening resulting in [(Dippnacnac)Mg{(Btz*)C(H)=N(2-C6H4-1-S)}] 26. Complex 23 when reacted with nitrogen derivative methylbenzimidazole (MeBIm) promotes the magnesiation at the C2 position to yield [{(Dippnacnac)Mg(MeBIm*)}2] 27. In contrast alkyl derivative 22 only coordinated to MeBIm through the lone pair on the N to yield [(Dippnacnac)Mg(Bu)(MeBIm)] 28. Studies assessing the catalytic ability of sodium magnesiates to promote the hydroamination of isocyanates and carbodiimides, show that homoleptic [NaMg(CH2SiMe3)2] 32 can effectively catalyse at room temperature the reactions of HNPh2 with several aliphatic isocyanates to yield the relevant ureas [(NHR)C(=O)(NPh2)] (R = tBu, 29, R = Cy, 39, R = Et, 43) in yields ranging from 99 to 100%. Similarly tris(amido) complex [NaMg(NPh2)3] 30 catalyses the trimerisation of aryl isocyanates under mild reaction conditions. The isolation of key intermediates of the stoichiometric reactions provided important information to propose a possible catalytic cycle. Guanidines [(NHR)C(=NR)NPh2] (R =Cy, 48, R = iPr, 49) could be prepared in good-moderate yields (64-65%) by reacting NHPh2 with RN=C=NR using 30 as a catalyst.
- Resource Type
- DOI
- Date Created
- 2012
- Former identifier
- 967005