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Rh-POP Pincer Xantphos Complexes for C-S and C-H Activation. Implications for Carbothiolation Catalysis
Abstract
The neutral Rh(I)–Xantphos complex [Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)Cl]<sub><i>n</i></sub>, <b>4</b>, and cationic Rh(III) [Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)(H)<sub>2</sub>][BAr<sup>F</sup><sub>4</sub>], <b>2a</b>, and [Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos-3,5-C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)(H)<sub>2</sub>][BAr<sup>F</sup><sub>4</sub>], <b>2b</b>, are described [Ar<sup>F</sup> = 3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>; Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; Xantphos-3,5-C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub> = 9,9-dimethylxanthene-4,5-bis(bis(3,5-bis(trifluoromethyl)phenyl)phosphine]. A solid-state structure of <b>2b</b> isolated from C<sub>6</sub>H<sub>5</sub>Cl solution shows a κ<sup>1</sup>-chlorobenzene adduct, [Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos-3,5-C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)(H)<sub>2</sub>(κ<sup>1</sup>-ClC<sub>6</sub>H<sub>5</sub>)][BAr<sup>F</sup><sub>4</sub>], <b>3</b>. Addition of H<sub>2</sub> to <b>4</b> affords, crystallographically characterized, [Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)(H)<sub>2</sub>Cl], <b>5</b>. Addition of diphenyl acetylene to <b>2a</b> results in the formation of the C–H activated metallacyclopentadiene [Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)(ClCH<sub>2</sub>Cl)(σ,σ-(C<sub>6</sub>H<sub>4</sub>)C(H)CPh)][BAr<sup>F</sup><sub>4</sub>], <b>7</b>, a rare example of a crystallographically characterized Rh–dichloromethane complex, alongside the Rh(I) complex <i>mer</i>-[Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)(η<sup>2</sup>-PhCCPh)][BAr<sup>F</sup><sub>4</sub>], <b>6</b>. Halide abstraction from [Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)Cl]<sub><i>n</i></sub> in the presence of diphenylacetylene affords <b>6</b> as the only product, which in the solid state shows that the alkyne binds perpendicular to the κ<sup>3</sup>-POP Xantphos ligand plane. This complex acts as a latent source of the [Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)]<sup>+</sup> fragment and facilitates <i>ortho</i>-directed C–S activation in a number of 2-arylsulfides to give <i>mer</i>-[Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)(σ,κ<sup>1</sup>-Ar)(SMe)][BAr<sup>F</sup><sub>4</sub>] (Ar = C<sub>6</sub>H<sub>4</sub>COMe, <b>8</b>; C<sub>6</sub>H<sub>4</sub>(CO)OMe, <b>9</b>; C<sub>6</sub>H<sub>4</sub>NO<sub>2</sub>, <b>10</b>; C<sub>6</sub>H<sub>4</sub>CNCH<sub>2</sub>CH<sub>2</sub>O, <b>11</b>; C<sub>6</sub>H<sub>4</sub>C<sub>5</sub>H<sub>4</sub>N, <b>12</b>). Similar C–S bond cleavage is observed with allyl sulfide, to give <i>fac</i>-[Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)(SPh)][BAr<sup>F</sup><sub>4</sub>], <b>13</b>. These products of C–S activation have been crystallographically characterized. For <b>8</b> in situ monitoring of the reaction by NMR spectroscopy reveals the initial formation of <i>fac</i>-κ<sup>3</sup>-<b>8</b>, which then proceeds to isomerize to the <i>mer</i>-isomer. With the <i>para</i>-ketone aryl sulfide, 4-SMeC <sub>6</sub>H<sub>4</sub>COMe, C–H activation <i>ortho</i> to the ketone occurs to give <i>mer</i>-[Rh(κ<sup>3</sup>-<sub>P,O,P</sub>-Xantphos)(σ,κ<sup>1</sup>-4-(COMe)C<sub>6</sub>H<sub>3</sub>SMe)(H)][BAr<sup>F</sup><sub>4</sub>], <b>14</b>. The temporal evolution of carbothiolation catalysis using <i>mer</i>-κ<sup>3</sup>-<b>8</b>, and phenyl acetylene and 2-(methylthio)acetophenone substrates shows initial fast catalysis and then a considerably slower evolution of the product. We suggest that the initially formed <i>fac</i>-isomer of the C–S activation product is considerably more active than the <i>mer</i>-isomer (i.e., <i>mer</i>-<b>8</b>), the latter of which is formed rapidly by isomerization, and this accounts for the observed difference in rates. A likely mechanism is proposed based upon these dataSimilar works
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Last time updated on 17/02/2017