<i>N</i>-Methylated Diphenylguanidines:  Conformations, Propeller-Type Molecular Chirality, and Construction of Water-Soluble Oligomers with Multilayered Aromatic Structures

The crystal structures of <i>N,N</i>‘-diphenylguanidine (<b>1</b>) and its <i>N</i>-methylated derivatives were investigated, and the conformational properties of these molecules were utilized to construct water-soluble oligomers with multilayered aromatic structures. <i>N,N</i>‘-Diphenylguanidine (<b>1</b>) afforded two types of crystals, chiral (<i>P</i>2₁2₁2₁) and racemic (<i>P</i>2₁/<i>c</i>), upon recrystallization from EtOH. In both crystals, <b>1</b> exists in the (<i>E</i>,<i>Z</i>) conformation, in which one C−N bond (length:  1.28−1.30 Å) attached to a phenyl ring shows double-bond character. In contrast, <i>N,N</i>‘-dimethyl-<i>N,N</i>‘-diphenylguanidine (<b>4a</b>) exists in the (<i>Z</i>,<i>Z</i>) conformation with the two aromatic rings facing each other. As judged from the crystal structures of several <i>N</i>-methylated compounds, the conformational preferences of diphenylguanidines appear to be related to those of aromatic anilides. <i>N,N,N</i>‘,<i>N</i>‘‘-Tetramethyl-<i>N</i>‘,<i>N</i>‘‘-diphenylguanidinium iodide (<b>6</b>) afforded chiral crystals, like <b>1</b> and <i>N</i>-methyl-<i>N,N</i>‘-diphenylguanidine (<b>2</b>). The absolute structure of each enantiomeric propeller conformation of <b>6</b> was determined by X-ray analysis using the Bijvoet difference method. The <i>Z</i>-conformational preference of <b>4</b> allowed us to synthesize oligomeric di- or tetraguanidines (<b>9</b>−<b>12</b>) which have multilayered aromatic structures both as a crystal and in organic and aqueous solvents.