Cationic Palladium Complex-Catalyzed Hetero Diels-Alder Reaction

以羰基化合物为亲二烯基团的杂化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，要在温和的条件下顺利地进行反应，必须使用含强给电子基的活性二烯和/或含吸电子基团的活性羰基化合物。在本研究项目中，我们发现未活化的单二烯与醛的杂化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的反应在2ﾟ%的阳离子钯配合物[Pd(Dppp)(Phcn)_2](BF_4)_2存在下，在50℃下反应20 h，反应顺利进行。在1997年的研究中，我们利用手性钯配合物研究了不对称杂化Diels-Alder反应。苯乙二醛与2，3-二甲基-1，3-丁二烯、1，3-环己二烯和1，2-二甲基环戊烷在[Pd(S-BINAP)(PHCN)_2](BF_4)_2存在下反应，分别得到95%~99%ee的环化产物。可以认为，苯乙二醛的两个羰基与钯的螯合配位导致了高的对映体选择性。

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.