Some chemistry of orthomanganated ketones

Abstract

The major part of this thesis describes the preparation and reactions of orthomanganated aromatic ketones, such as (1), diagram. Among the new species characterised are substituted acetophenones, analogues of (1), namely: 3,4,5-trimethoxy (3); 4,5,6-trimethoxy (4); 3-t-butyldimethylsiloxy-4,5-dimethoxy (5); 4,5-dimethoxy (6); 5,6-dimethoxy (7); 3-benzyloxy-4,5-dimethoxy (8); and 3-t-butyldimethylsiloxy (9). Hetero aromatic ketones have been used to prepare the corresponding orthomanganated compounds (10), (11), (12), (14) and (15) from the respective ketones 2-acetylthiophene, 3-acetyl-2,5-dimethylthiophene, 2-acetyl-N-methylpyrrole, 3-acetylindole and 2- acetylfuran. The crystal structure of η²-(3-acetyl-2,5-dimethylthien-4-yl)tetracarbonylmanganese (11) has been determined. The 5-manganated derivative (13) of flavanone has also been made. Attempted orthometallations of acetophenones with C₆H₅CH₂Co(CO)₄, C₆H₅CH₂Fe(CO)₂Cp and Pd(OAc)₂ were unsuccessful. Reaction of (4) with Br₂ in carbon tetrachloride gave the orthobromoacetophenone 2'-bromo-3' ,4' ,5'-trimethoxyacetophenone (21) plus [Mn(CO)₄Br]₂. The corresponding orthoiodoacetophenone (28) and [Mn(CO)₄Cl]₂ was obtained using ICl. Other orthohalogenated ketones were obtained similarly. The reactions were sometimes unsuccessful (e.g. for (1)) or complicated (e.g. (5) competitive ring bromination). Oxidation of (1) or (4) with cerium (IV) ammonium nitrate (0.5 mol) in CD₃CO₂D gives the corresponding orthomonodeuterio acetophenones (19) or (20). A by-product from the oxidation of (4), the cyclic hemiketal, (36), was identified by X-ray crystal structure analysis, and one other product, the lactone, (37) was identified spectrally. Determination by ¹H-NMR of the ratio of the 3-deuterio- (1a) to 3-protio- (1) formed in the orthometallation of orthomonodeuterio acetophenone (19) by C₆H₅CH₂Mn(CO)₅ gave a hydrogen isotope effect of 3.6. Similarly, for 2'-deuterio-3' ,4' ,5'-trimethoxyacetophenone (20) the value was 2.9. However, when the orthodeuterioacetophenone (19) was reacted with cis - C₆H₅CH₂Mn(CO)₄P(C₆H₅)₃ the product fac-η²-(2-acetylphenyl) tricarbonyltriphenylphosphinomanganese (16)/(16a) showed a D/H ratio of only 1.1. Implications of these isotope effects for mechanisms are discussed. Reactions of orthomanganted ketones with vinyl reagents in the presence of Pd(II) were studied in two solvents. In methanol, (1) with methyl acrylate (H₂C=CHCOOCH₃) and Li₂PdCl₄ (1 mol) gave methyl E-3-(2-acetylphenyl)acrylate (38). Similarly η²-(2-acetylthien-3-yl)-tetracarbonylmanganese (10) gave methyl E-3-(2-acetylthien-3-yl) acrylate (40). A variety of other products from coupling reactions with vinyl acetate, acrylonitrile, styrene, methyl vinyl ketone, acrolein and allyl alcohol are reported, and possible mechanisms for coupling reactions are discussed. In acetonitrile as solvent, under reflux, with PdCl₂ (1 mol) and excess LiCl, some of the orthomanganated ketones provided new routes to indenes in their reaction with methyl acrylate. Thus (4) gave methyl 3-methyl-5,6,7-trimethoxyindene-2-carboxylate (53), and (2) and (3) gave the corresponding indenes [(51) and (49)]. The thiophene derivative η²-(3-acetyl-2,5-dimethylthien-4-yl)tetracarbonylmanganese (11) also gave the corresponding bicyclic compound, methyl 2,5,6-trimethyl-3H-cyclopenta-[c]-thiophene-4-carboxylate, (54) but the major product was the arylated acrylate ester (55) in this case. In reaction of (2) with HgC1₂ in refluxing methanol, trans-metallation occurs to form chloro(2-acetyl-5-methoxyphenyl)mercury(II) (57). The corresponding mercuric chloride of (1), that is, chloro(2-acetylphenyl)mercury(II) (56), has also been prepared and its crystal structure is reported. In a separate study dodecahydrotriphenylene (C₁₈H₂₄) (58) has been used to prepare the isoelectronic metal arene π-complexes structures η⁶-(C₁₈H₂₄)Cr(CO)₃ (59) and η⁶-(C₁₈H₂₄)Mn(CO)₃⁺ (60). The crystal structures of the former and of the two distinct crystalline modifications of the latter, as BF₄⁻ salts are reported and compared.