Cationic Palladium Complex-Catalyzed Hetero Diels-Alder Reaction

以羰基化合物为亲双烯体的杂Diels-Alder反应是构建二氢吡喃骨架的一种非常有用的方法。然而，必须使用含有强供电子基团的活化二烯和/或含有吸电子基团的活化羰基化合物，以在温和条件下顺利进行反应。在本研究项目中，我们发现非活化简单二烯与醛的杂Diels-Alder反应是由阳离子钯（II）配合物催化的。因此，2-甲基和2，3-二甲基取代的1，3-丁二烯与芳香醛和脂肪醛在2mol%的阳离子钯配合物[Pd（dppp）（PhCN）_2]（BF_4）_2存在下，于50 ℃反应20 h，反应顺利进行。在该反应中，我们假设钯络合物作为刘易斯酸催化剂。在1997年的研究中，我们利用手性钯配合物研究了不对称杂原子Diels-Alder反应。在[Pd（S-BINAP）（PhCN）_2]（BF_4）_2存在下，苯基乙二醛分别与2，3-二甲基-1，3-丁二烯、1，3-环己二烯和1，2-二亚甲基环戊烷反应，得到95 ~ 99%ee的环化产物。可以认为，苯基乙二醛的两个羰基与钯的螯合配位产生了高的对映选择性。

The hetero Diels-Alder reaction using carbonyl compounds as dienophiles has been a very useful method to construct the dihydropyran skeleton. However, it is necessary to use activated dienes containing strong electron-donating groups and/or activated carbonyl compounds containing electronwithdrawing groups to conduct the reaction smoothly under mild conditions. In this research project, we found that the hetero Diels-Alder reaction of nonactivated simple dienes with aldehydes is catalyzed by cationic palladium (II) complexes. Thus, the reaction of 2-methyl and 2,3-dimethyl substituted 1,3-butadienes with aromatic and aliphatic aldehydes proceeded smoothly at 50ﾟC for 20 h in the presence of 2 mol% of the cationic palladium (II) complexes, such as [Pd (dppp) (PhCN) _2] (BF_4) _2. In this reaction, we assume that the palladium complex acts as a Lewis acid catalyst. In the research of 1997, we investigated asymmetric hetero Diels-Alder reaction using chiral palladium complex. The reaction of phenylglyoxal with 2,3-dimethyl-1,3-butadiene, 1,3-cyclohexadiene, and 1,2-dimethylenecyclopentane in the presence of [Pd (S-BINAP) (PhCN) _2] (BF_4) _2 afforded the cyclization products in 95 to 99 %ee, respectively. It can be assumed that chelate coordination of the two carbonyl group of phenylglyoxal to palladium give rise to the high enantioselectivity.