Reactions of Neutral Scandium/Phosphorus Lewis Pairs with Small Molecules

Treating a scandium mixed alkyl/diaryloxide complex with phosphino-substituted alcohols resulted in the formation of scandium mixed alkoxyl/diaryloxide complexes Sc­[OC­(CH₃)₂CH₂PR₂]­[O-2,6-<i>t</i>Bu₂-C₆H₃]₂[THF] (<b>6</b>, R = Ph; <b>8</b>, R = <i>t</i>Bu). The reactivity of Sc-based Lewis pair complexes <b>6</b> and <b>8</b> toward a wide range of substrates was investigated. A ligand substitution reaction occurred when complex <b>6</b> was treated with benzophenone. The reaction of complex <b>6</b> with sulfur afforded a phosphane oxidative product. Complexes <b>6</b> and <b>8</b> also exhibited typical Sc/P frustrated Lewis pair (FLP) reactivity. They added to the carbonyl group of phenyl isocyanate to form seven-membered metallacycles. Complex <b>6</b> underwent selective Sc/P 1,4-addition reactions with α,β-unsaturated carbonyl compounds to give the corresponding nine-membered metallacyclic products. The reaction of <b>6</b> with 0.5 mol of dimethyl acetylenedicarboxylate led to the formation of bicyclo[7.7.0]­cetane-derived metallacycle <b>16</b> in a distinctive double 1,4-addition pattern. With benzil complex <b>6</b> underwent a 1,4-addition reaction to afford a <i>cis</i>-enediolate which was coordinated to the Sc/P FLP through Sc–O and P–O bonds. Treatment of complex <b>6</b> with cyclopropyl phenyl ketone afforded the 10-membered metallaheterocycle <b>18</b>, in which a scandium Lewis acid added to the oxygen atom and the phosphine Lewis base attacked the cyclopropyl group with ring opening. A ring-opening reaction also took place when epoxycyclohexane was employed as a substrate. Complex <b>6</b> successfully cleaved the C–Br bond of benzyl bromide to give zwitterionic complex <b>20</b> with newly formed Sc–Br and P–C bonds. Scandium phosphazine complex <b>21</b> was generated by a Sc/P 1,1-addition to the terminal nitrogen atom of (trimethylsilyl)­diazomethane. Aided by the phosphorus Lewis base, scandium Lewis acid was able to abstract chloride from (Ph₃P)­AuCl to produce heterobimetallic Sc/Au complexes <b>22</b> and <b>23</b>. Many complexes in this study were characterized by single-crystal X-ray diffraction.