Enantiomerically Pure N Chirally Substituted 1,3-Benzazaphospholes:
Synthesis, Reactivity toward <i>t</i>BuLi, and Conversion
to Functionalized Benzazaphospholes and Catalytically Useful Dihydrobenzazaphospholes
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Abstract
Catalytic C–P coupling of
chiral <i>o</i>-bromoanilines <b>1a</b>–<b>c</b> to the corresponding <i>o</i>-phosphonoanilines <b>2a</b>–<b>c</b>, reduction
to the phosphines <b>3a</b>–<b>c</b>, and final
acid-catalyzed cyclocondensation represents a convenient access to
the title compounds <b>4a</b>–<b>c</b>. Reaction
of <b>4a</b>,<b>b</b> with <i>t</i>BuLi allows
solvent-dependent directed lithiation leading either to 2-lithiobenzazaphospholes
with a −PCLi–NR– substructure (in Et<sub>2</sub>O/KO<i>t</i>Bu), in the case of anisyl substitution
accompanied by partial additional lithiation in <i>o</i>-position of the MeO-group, or to regiospecific “normal”
addition with formation of −P(<i>t</i>Bu)–CHLi–NR–
species. These were trapped by ClSiMe<sub>3</sub>, CO<sub>2</sub>,
or MeOH to give the corresponding substitution products <b>7b</b>, <b>8b</b>, <b>10b</b>, <b>11a</b>,<b>b</b> and <b>12a</b>,<b>b</b>, respectively. <b>12a</b>,<b>b</b>, containing the P–C–COOH structural
unit, forms with Ni(COD)<sub>2</sub> in THF very efficient ethylene
oligomerization catalysts with high selectivity for linear α-olefins.
The structure elucidation of the products is based on conclusive solution
NMR data and crystal structure analyses of the 1-(1<i>S</i>)-anisylethyl compounds <b>3b</b> and <b>4b</b>