Electronic Control of Frustrated Lewis Pair Behavior: Chemistry of a Geminal Alkylidene-Bridged Per-pentafluorophenylated P/B Pair
2011-08-08T00:00:00Z (GMT) by
“Markovnikov” hydroboration of bis(pentafluorophenyl)(propynyl)phosphine (<b>12</b>) with Piers' borane [HB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] gives the frustrated Lewis pair (FLP) (C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>P–(μ-CCHCH<sub>3</sub>)–B(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> (<b>13</b>) as the major product in good yield. FLP <b>13</b> adds cleanly to 1-alkynes to form the five-membered heterocyclic products <b>15</b> (two examples). It adds to the CN double bond of arylisocyanates to yield the five-membered heterocycles <b>16</b> (two examples), and it regioselectively adds to aldehydes to yield the B–O/P–C bonded heterocycles <b>17</b> (from benzaldehyde) and <b>18</b> (from <i>trans</i>-cinnamic aldehyde), respectively. Dynamic <sup>19</sup>F NMR spectroscopy indicates a rapidly occurring reversible P–C bond dissociation process taking place in <b>17</b> and <b>18</b> [<b>17</b>: Δ<i>G</i><sup>⧧</sup><sub>diss</sub>(328 K) = 14.7 ± 0.3 kcal/mol; <b>18</b>: Δ<i>G</i><sup>⧧</sup><sub>diss</sub>(368 K) = 11.9 ± 0.2 kcal/mol]. The products <b>15a</b>, <b>15b</b>, <b>17</b>, and <b>18</b> were characterized by X-ray diffraction. This chemistry indicates that frustrated Lewis pair behavior can be induced electronically by the choice of suitable substituents at the Lewis base.
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