Stoichiometric and Catalytic Cross Dimerization between Conjugated Dienes and Conjugated Carbonyls by a Ruthenium(0) Complex: Straightforward Access to Unsaturated Carbonyl Compounds by an Oxidative Coupling Mechanism
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A series of stoichiometric and catalytic cross dimerizations between conjugated dienes and conjugated carbonyls are studied. The reaction of Ru(η4-cisoid-1,3-butadiene)(η4-1,5-COD)(NCMe) (2a) with methyl acrylate gives a Ru(0) complex, Ru[methyl η4-cisoid-(2E,4E)-hepta-2,4-dienoate](η4-1,5-COD)(NCMe) (3aa) in 97% yield. Similar treatments of 2a with a series of tert-butyl acrylate, methyl crotonate, 3-buten-2-one, and N,N-dimethylacrylamide produce similar analogues of 3ac. When (E)-1,3-pentadiene complex 2d is employed in the reaction with methyl acrylate, the branched coupling product Ru[methyl η4-cisoid-(2E,4E)-4-methylhepta-2,4-dienoate](η4-1,5-COD)(NCMe) (3da-b) is dominantly obtained in 65% yield along with the linear product in 19% yield. In the case of the (E)-2,5-dimethylhexa-1,3-diene complex 2e, the corresponding branch product is exclusively obtained in 86% yield. The catalytic cross dimerizations between conjugated dienes and conjugated carbonyls are established by 2. The origin of the present chemoselectivity is the η4-coordination of a conjugated diene and η2-coordination of an electron-deficient alkene to formal 6e coordination sites at Ru(0), and the regioselectivity being prone to giving branched products is interpreted as an oxidative coupling mechanism, involving nucleophilic attack of the coordinated diene to the coordinated electron-deficient alkene.
本研究系统探究了共轭二烯烃与共轭羰基化合物之间的多组化学计量及催化交叉二聚反应。将配合物Ru(η⁴-顺式-1,3-丁二烯)(η⁴-1,5-环辛二烯（COD）)(乙腈（NCMe）)（2a）与丙烯酸甲酯反应，以97%的产率得到零价钌配合物Ru[η⁴-顺式-(2E,4E)-庚-2,4-二烯酸甲酯](η⁴-1,5-环辛二烯（COD）)(乙腈（NCMe）)（3aa）。采用丙烯酸叔丁酯、巴豆酸甲酯、3-丁烯-2-酮以及N,N-二甲基丙烯酰胺对2a进行相似处理，可得到3ac的系列类似物。当使用(E)-1,3-戊二烯配合物2d与丙烯酸甲酯反应时，主产物为支化偶联产物Ru[η⁴-顺式-(2E,4E)-4-甲基庚-2,4-二烯酸甲酯](η⁴-1,5-环辛二烯（COD）)(乙腈（NCMe）)（3da-b），产率达65%，同时伴随19%产率的直链偶联产物生成。对于(E)-2,5-二甲基-1,3-己二烯配合物2e，则仅以86%的产率得到对应的支化产物。配合物2a可介导共轭二烯烃与共轭羰基化合物的催化交叉二聚反应。本研究中观测到的化学选择性源于：共轭二烯烃的η⁴配位与缺电子烯烃的η²配位共同占据零价钌中心形式上的6电子配位位点；而倾向于生成支化产物的区域选择性可通过氧化偶联机理阐释，即配位态二烯烃对配位态缺电子烯烃发生亲核进攻。
