Reactivity of 1,3-Disubstituted Indoles with Lithium
Compounds: Substituents and Solvents Effects on Coordination and Reactivity
of Resulting 1,3-Disubstituted-2-Indolyl Lithium Complexes
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Abstract
Reactivity of 1,3-disubstituted
indolyl compounds with lithium reagents was studied to reveal the
substituents and solvent effects on coordination modes and reactivities
resulting in different indolyl lithium complexes. Treatment of 1-alkyl-3-imino
functionalized compounds 1-R-3-(R′NCH)C<sub>8</sub>H<sub>5</sub>N [R = Bn, R′ = Dipp (<b>HL</b><sup><b>1</b></sup>); R = Bn, R′ = <sup><i>t</i></sup>Bu (<b>HL</b><sup><b>2</b></sup>); R = CH<sub>3</sub>OCH<sub>2</sub>, R′ = Dipp (<b>HL</b><sup><b>3</b></sup>); Dipp = <sup><i>i</i></sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>] with Me<sub>3</sub>SiCH<sub>2</sub>Li or <sup><i>n</i></sup>BuLi in hydrocarbon solvents (toluene or <i>n</i>-hexane) produced 1,3-disubstituted-2-indolyl lithium complexes
[η<sup>1</sup>:(μ<sub>2</sub>-η<sup>1</sup>:η<sup>1</sup>)-1-Bn-3-(DippNCH)C<sub>8</sub>H<sub>4</sub>NLi]<sub>2</sub> (<b>1</b>), {[η<sup>1</sup>:(μ<sub>3</sub>-η<sup>1</sup>:η<sup>1</sup>:η<sup>1</sup>)-1-Bn-3-(<sup><i>t</i></sup>BuNCH)C<sub>8</sub>H<sub>4</sub>N][η<sup>2</sup>:η<sup>1</sup>:(μ<sub>2</sub>-η<sup>1</sup>:η<sup>1</sup>)-1-Bn-3-(<sup><i>t</i></sup>BuNCH)C<sub>8</sub>H<sub>4</sub>N][η<sup>1</sup>:(μ<sub>2</sub>-η<sup>1</sup>:η<sup>1</sup>)-1-Bn-3-(<sup><i>t</i></sup>BuNCH)C<sub>8</sub>H<sub>4</sub>N]Li<sub>3</sub>} (<b>2</b>), and [η<sup>1</sup>:η<sup>1</sup>:(μ<sub>2</sub>-η<sup>1</sup>:η<sup>1</sup>)-1-CH<sub>3</sub>OCH<sub>2</sub>-3-(DippNCH)C<sub>8</sub>H<sub>4</sub>NLi]<sub>2</sub> (<b>3</b>), respectively.
The bonding modes of the indolyl ligand were kept in <b>1</b> by coordination with donor solvent, affording [η<sup>1</sup>:(μ<sub>2</sub>-η<sup>1</sup>:η<sup>1</sup>)-1-Bn-3-(DippNCH)C<sub>8</sub>H<sub>4</sub>NLi(THF)]<sub>2</sub> (<b>4</b>).
The trinuclear complex <b>2</b> was converted to dinuclear form
with a change of bonding modes of the indolyl ligand by treatment
of <b>2</b> with donor solvent THF, producing [η<sup>1</sup>:(μ<sub>2</sub>-η<sup>1</sup>:η<sup>1</sup>)-1-Bn-3-(<sup><i>t</i></sup>BuNCH)C<sub>8</sub>H<sub>4</sub>NLi(THF)]<sub>2</sub> (<b>5</b>). X-ray diffraction established
that compounds <b>1</b>, <b>3</b>, <b>4</b>, and <b>5</b> crystallized as dinuclear structures with the carbanionic
sp<sup>2</sup> carbon atoms of the indolyl ligands coordinated to
lithium ions in a μ<sub>2</sub>-η<sup>1</sup>:η<sup>1</sup> manner, while compound <b>2</b> crystallized as a trinuclear
structure and the carbanionic atoms of the indolyl moieties coordinated
to lithium ions in μ<sub>2</sub>-η<sup>1</sup>:η<sup>1</sup> and μ<sub>3</sub>-η<sup>1</sup>:η<sup>1</sup>:η<sup>1</sup> manners. When the lithiation reaction of <b>HL</b><sup><b>1</b></sup> with 1 equiv of <sup><i>n</i></sup>BuLi was carried out in THF, the monomeric lithium complex
{η<sup>1</sup>:η<sup>1</sup>-1-Bn-3-(DippNCH)-2-[1′-Bn-3′-(DippNCH)C<sub>8</sub>H<sub>5</sub>N]C<sub>8</sub>H<sub>4</sub>NLi(THF)}
(<b>6</b>) having coupled indolyl moieties was obtained. The
compound <b>6</b> can also be prepared by the reaction of <b>1</b> with 0.5 equiv of <b>HL</b><sup><b>1</b></sup> with a higher isolated yield. Accordingly, the lithium complexes
[η<sup>1</sup>:η<sup>4</sup>-1-Bn-3-<sup><i>t</i></sup>BuNCH-2-(1′-Bn-3′-<sup><i>t</i></sup>BuNCHC<sub>8</sub>H<sub>5</sub>N)C<sub>8</sub>H<sub>4</sub>NLi(L)] (L = THF, <b>7a</b>; L = Et<sub>2</sub>O, <b>7b</b>) with the coupled indolyl moieties in η<sup>4</sup> mode were
isolated by treatment of <b>HL</b><sup><b>2</b></sup> with <b>2</b> in THF or Et<sub>2</sub>O. All complexes were characterized
by spectroscopic methods, and their structures were determined by
X-ray diffraction study
