title: 9-Acylation of 1-acyldipyrromethanes containing a dialkylboron mask for the alpha-acylpyrrole motif.
abstract: 1,9-Diacyldipyrromethanes are important precursors to porphyrins, yet synthetic access remains limited owing to (1) poor conversion in the 9-acylation of 1-acyldipyrromethanes and (2) handling difficulties because acyldipyrromethanes typically streak upon chromatography and give amorphous powders upon attempted crystallization. A reliable means for converting a dipyrromethane to a 1-acyldipyrromethane-dialkylboron complex was recently developed, where the dialkylboron (BR(2)) unit renders the complex hydrophobic and thereby facilitates isolation. Herein a refined preparation of 1,9-diacyldipyrromethanes is presented that employs the 1-acyldipyrromethane-BR(2) complex as a substrate for 9-acylation. The dialkylboron unit provides protection for the alpha-acylpyrrole unit. 9-Acylation requires formation of the pyrrolyl-MgBr reagent and the presence of 1 equiv of a nonnucleophilic base to quench the proton liberated upon alpha-acylation. Reaction of the 1-acyldipyrromethane-BR(2) complex (1 equiv) with mesitylmagnesium bromide (2 equiv) followed by the addition of an acylating agent (S-2-pyridyl thioate or acid chloride, 1.1 equiv) gives the corresponding 1,9-diacyldipyrromethane-BR(2) complex. The acylation method afforded 1,9-diacyldipyrromethane-BR(2) complexes with limited or no chromatography in yields of 64-92%. The 1,9-diacyldipyrromethane-BR(2) complexes are stable to routine handling, are readily soluble in common organic solvents, crystallize readily, and can now be prepared in multigram quantities through use of stoichiometric quantities of reagents.
authors.affiliation: Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA.
authors.forename: Syeda Huma H
authors.initials: SH
authors.lastname: Zaidi
authors.forename: Kannan
authors.initials: K
authors.lastname: Muthukumaran
authors.forename: Shun-ichi
authors.initials: S
authors.lastname: Tamaru
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Lindsey
mesh_terms: D000215:Acylation; D001896:Boron Compounds / Q000737:chemistry*; D008697:Methane / Q000031:analogs & derivatives* / Q000138:chemical synthesis*; D008958:Models, Molecular; D015394:Molecular Structure; D011758:Pyrroles / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D001896:Boron Compounds; D011758:Pyrroles; D008697:Methane