| dc.description.abstract | A wide variety of mono- and disubstituted derivatives of the 1,3,2-diazaboracyclohexane ring system were prepared as possible precursors to linear poly(iminoboranes). These "condensation monomers" were primarily synthesized by a nucleophilic substitution reaction to form products in high yield (eq 1). The 2-phenyl-1,3-diazaboracyclohexane ring system was first investigated.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)Two new diborylamines, where E = H and E$\sp\prime$ = B(NMe$\sb2$), B(Ph)NMe${\sb2,}$ were prepared as polymer precursors. Preliminary thermolysis studies showed that each precursor, when heated at 200$\sp\circ$C for 7 days, eliminated dimethylamine to form yellow oily residues. Size exclusion chromatography of these residues indicated molecular weights of ca. 6,000, corresponding to approximately 26 repeat units. However, their silylated analogues (E = SiMe$\sb3$) were found to be thermally stable under the same pyrolysis conditions. Other derivatives (e.g., E, E$\sp\prime$ = SiMe$\sb3$) were prepared that underwent additional derivative chemistry by Si-N bond cleavage reactions with PhBCl$\sb2$ to form the unstable silyl/chloro analogues. Also, a bis (bis(dimethylamino)boryl) compound (E = E$\sp\prime$ = B(NMe$\sb2)\sb2$), containing a long acyclic B-N skeleton, was prepared and structurally characterized. It may serve as a model for a future boron-nitrogen polymer. Its structure was corroborated by a dynamic NMR study and a B-NMe$\sb2$ rotational barrier of $\sim$13 kcal/mol was estimated. Other derivatives were also studied by variable temperature NMR spectroscopy and their rotational barriers correspond to those reported for related compounds. Other efforts included the synthesis of a novel electrophile containing the B-OCH$\sb2$CF$\sb3$ function, ClB(Ph)OCH$\sb2$CF$\sb3$. However, the reactions of this chloroborane with the R = Ph ring system led to unstable products. It was concluded that there were many problems associated with the R = Ph ring system: (1) the inability to monosubstitute with certain electrophiles (e.g., HMe$\sb2$SiCl); (2) the instability of those precursors containing trifluoroethoxy or chloro functionalities; and (3) the extremely high boiling points ($>$100$\sp\circ$C) of some of the B-phenyl precursors sometimes led to decomposition during distillation. Two approaches were then undertaken in order to circumvent these problems. First, the R group on the ring boron was changed both electronically and sterically to the $\sp{\rm i}$Pr and $\sp{\rm t}$Bu substituents. The second approach involved the use of a less reactive silyl substituent, namely the dimethyl(tert-butyl)silyl moiety. | |
