A Frustrated Lewis Pair Based on a Cationic Aluminum Complex and Triphenylphosphine

The highly Lewis acidic, cationic aluminum species [DIPP-nacnacAlMe]+[B­(C6F5)4]− (1, DIPP-nacnac = [HC­{C­(Me)­N­(2,6-iPr2C6H3)}2]−) has been shown to undergo reactions with a wide variety of small molecules, in both the presence and absence of an external weak phosphine base, PPh3. Cycloaddition reactions of unsaturated C–C bonds across the aluminum diketiminate framework are reported, and the first structural confirmation of this type of cycloaddition product is presented. Addition of PPh3 to 1 produces the cationic aluminum phosphine complex [DIPP-nacnacAl­(Me)­PPh3]+[B­(C6F5)4]−, which undergoes fluxional dissociation/coordination of the phosphine in solution. This weak Al–P interaction can be utilized in frustrated Lewis pair type reactions to activate alkenes, alkynes, CO2, propylene oxide, and the C–Cl bonds of CH2Cl2. The CO2 adduct [DIPP-nacnacAl­(Me)­OC­(PPh3)­O]+[B­(C6F5)4]− undergoes further stoichiometric reduction with Et3SiH to produce an aluminum formate species.

高路易斯酸性的阳离子铝物种[DIPP-nacnacAlMe]+[B(C6F5)4]−（化合物1，其中DIPP-nacnac = [HC{C(Me)N(2,6-iPr2C6H3)}2]−）已被证实可在存在或不存在外源性弱膦碱三苯基膦（PPh3）的条件下，与多种小分子发生反应。本文报道了不饱和C-C键在铝二酮亚胺骨架上的环加成反应，并首次给出了该类环加成产物的结构确证。向化合物1中加入三苯基膦（PPh3）可得到阳离子铝膦配合物[DIPP-nacnacAl(Me)PPh3]+[B(C6F5)4]−，该配合物在溶液中会发生膦配体的动态解离与配位过程。这种弱的Al-P相互作用可应用于受阻路易斯对（frustrated Lewis pair）类型的反应中，用于活化烯烃、炔烃、二氧化碳（CO2）、环氧丙烷以及二氯甲烷（CH2Cl2）的C-Cl键。该CO2加合物[DIPP-nacnacAl(Me)OC(PPh3)O]+[B(C6F5)4]−可与三乙基硅烷（Et3SiH）发生进一步的化学计量还原反应，生成铝甲酸酯物种。