Unsymmetric salen ligands bearing a Lewis base: intramolecularly cooperative catalysis for cyanosilylation of aldehydes
作者:Ye-Qian Wen、Wei-Min Ren、Xiao-Bing Lu
DOI:10.1039/c1ob05695f
日期:——
A series of unsymmetric salen ligands derived from 1,2-diaminocyclohexane bearing an appended Lewis base on the three-position of one aromatic ring were synthesized by the reaction of various functional salicyaldehydes with the condensation product of 1,2-diaminocyclohexane mono(hydrogen chloride) and 3,5-di-tert-butylsalicylaldehyde. These ligands in conjunction with Ti(OiPr)4 exhibited excellent activity in catalyzing the cyanosilylation of aldehydes with trimethylsilyl cyanide (TMSCN) at mild conditions. The highest activity was observed in the catalyst system with regard to the salen ligand bearing a diethylamino group, which proved to be active even at a high [aldehyde]/[catalyst] ratio up to 50000. In a low catalyst loading of 0.05 mol%, the quantitative conversion of benzaldehyde to the corresponding cyanosilylation product was found within 10 min. at ambient temperature. An intramolecularly cooperative catalysis was proposed wherein the central metal Ti(IV) is suggested to play a role of Lewis acid to activate aldehydes while the appended Lewis base to activate TMSCN.
一系列基于1,2-二氨基环己烷并带有附加路易斯碱于一个芳环3号位的不对称salen配体,通过不同功能的水杨醛与1,2-二氨基环己烷单(盐酸)和3,5-二叔丁基水杨醛的缩合产物反应合成。这些配体与Ti(OiPr)4联合展现出了在温和条件下催化醛与三甲基氰硅烷(TMSCN)的氰硅化反应的优秀活性。在含有二乙氨基的salen配体的催化体系中观察到了最高的活性,即使在高达50000的[醛]/[催化剂]比例下也显示出活性。在低催化剂负载量0.05 mol%下,室温下10分钟内可以发现苯甲醛到相应氰硅化产物的定量转化。提出了一种分子内协同催化,其中中心金属Ti(IV)被认为扮演了激活醛的路易斯酸的角色,而附加的路易斯碱则激活了TMSCN。