Synthesis, Structure,
Dynamics, and Selective Methylation of Platinum and Palladium Diphosphametallacyclobutane
Complexes
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Abstract
Treatment of M(dppe)Cl<sub>2</sub> (M = Pd, Pt) or Pt((<i>R,R</i>)-Me-DuPhos)Cl<sub>2</sub> with IsHPCH<sub>2</sub>PHIs
(<b>1</b>; Is = isityl = 2,4,6-(<i>i</i>-Pr)<sub>3</sub>C<sub>6</sub>H<sub>2</sub>) and 2 equiv of NaOSiMe<sub>3</sub> gave
the mononuclear diphosphametallacyclobutane complexes M(dppe)(IsPCH<sub>2</sub>PIs) (M = Pd (<b>2</b>), Pt (<b>3</b>)), or Pt((<i>R,R</i>)-Me-DuPhos)(IsPCH<sub>2</sub>PIs) (<b>4</b>).
Dynamic processes involving phosphorus inversion and rotation about
the P–C(Is) bonds in <b>2</b>–<b>4</b> were
characterized by variable-temperature NMR spectroscopy, which suggested
that each existed as a single <i>C</i><sub>2</sub>-symmetric
diastereomer in solution, consistent with their solid-state structures
determined by X-ray crystallography. The MiniPhos derivative IsMePCH<sub>2</sub>PMeIs (<b>5</b>) was prepared as a 5.5/1 <i>rac</i>/<i>meso</i> mixture by sequential arylation and methylation
of Cl<sub>2</sub>PCH<sub>2</sub>PCl<sub>2</sub>. Alternatively, the
catalyst precursor Pt((<i>R,R</i>)-Me-DuPhos)(Ph)(Cl) mediated
alkylation of secondary phosphines in the presence of NaOSiMe<sub>3</sub> to yield selectively <i>meso</i>-<b>5</b> either from PHMe(Is) and CH<sub>2</sub>I<sub>2</sub> or from <b>1</b> and MeI. Recrystallization and chromatography yielded diastereomerically
enriched <i>rac</i>-<b>5</b> and <i>meso</i>-<b>5</b>. Treatment of M(dppe)(OTf)<sub>2</sub> (M = Pd, Pt)
or Pt((<i>R,R</i>)-Me-DuPhos)(OTf)<sub>2</sub> with <i>meso</i>-<b>5</b> gave the dications <i>meso</i>-[M(diphos)(IsMePCH<sub>2</sub>PMeIs)][OTf]<sub>2</sub> (M(diphos)
= Pd(dppe) (<b>6</b>), Pt(dppe) (<b>8</b>), Pt((<i>R,R</i>)-Me-DuPhos) (<b>10</b>)). Similar reactions of <i>rac</i>-<b>5</b> yielded the dications <i>rac</i>-[M(diphos)(IsMePCH<sub>2</sub>PMeIs)][OTf]<sub>2</sub> (M(diphos)
= Pd(dppe) (<b>7</b>), Pt(dppe) (<b>9</b>)) and a 1/1
mixture of the <i>C</i><sub>2</sub>-symmetric diastereomers
[Pt((<i>R,R</i>)-Me-DuPhos)(IsMePCH<sub>2</sub>PMeIs)][OTf]<sub>2</sub> (<b>11a</b>,<b>b</b>). Treatment of <b>2</b> and <b>3</b> with 2 equiv of methyl triflate gave the dications <b>6</b>–<b>9</b> as ca. 1/1 <i>meso</i>/<i>rac</i> mixtures, and methylation of <b>4</b> selectively
gave one of the <i>C</i><sub>2</sub>-symmetric diastereomers, <b>11a</b>. These alkylations proceeded via the observable monomethylated
intermediates [M(diphos)(IsMePCH<sub>2</sub>PIs)][OTf] (<b>12</b>–<b>14</b>)
