Unexpectedly facile racemization of 8-diphenylphosphinoyl-8′-methoxy-1,1′-binaphthyl
Unexpectedly facile racemization of 8-diphenylphosphinoyl-8′-methoxy-1,1′-binaphthyl
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8-二苯基膦酰基-8-甲氧基-1,1-联萘的意外轻松外消旋化
DOI：
10.1039/cc9960001609
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发表时间：
1996
影响因子：
4.9
通讯作者：
T. Tada
中科院分区：
文献类型：
--
作者：
K. Fuji*;M. Sakurai;Naoji Tohkai;A. Kuroda;T. Kawabata;Y. Fukazawa;T. Kinoshita;T. Tada
光学活性联萘在不对称合成中起着重要的作用。2，2‘-二羟基-1，L’-联萘是一种常用的化学计量和催化手性修饰剂。2相反，8，8‘-二羟基-1，1’-联萘基L3作为手性修饰剂很少受到关注。最近，我们报道了(R)-2与烯醇酸酯及相关氨基甲酸酯的不对称质子化反应。4为了进一步扩大这种新的手性来源1的用途，我们试图从1合成光学活性的膦配体8。在这里，我们报道了在合成8的过程中，我们遇到的膦氧化物5-7的外消旋。最近，Hayashi等人。介绍了手性单齿膦配体，如2-二苯基膦-2‘-甲氧基-1，L’-联萘11，用于钯催化的硅氢化、5氢硼6和烯丙基酯的还原。7将11的2-和2‘-取代基分别切换到8-和8’-位得到另一个膦8。由于8中8-磷原子到2‘-氢的距离应该与11中2-磷原子到8’-氢的距离相同，所以每个对映体应该独立存在。根据先前报道的方法，光学活性8预计可以容易地从光学活性1制备而不需要外消旋。8二苯基膦酰基的手性环境与11有很大的不同。位于8的近位的另一个萘环完全阻碍了从这一侧的途径，因此8的选择性可能比11.9高。Harris和他的同事10的广泛研究表明，1，L-联萘在8，8‘-位上带有sp3-取代基的光学稳定性比那些具有sp2-取代基的要更稳定。
Optically active binaphthyls play an important role in asymmetric syntheses. 2, 2 ‘-Dihydroxy-1, l’-binaphthyl is frequently used as a chiral modifier both stoichiometrically 1 and catalytically. 2 In contrast, 8, 8’-dihydroxy-1, 1’-binaphthyl l3 has received little attention as a chiral modifier. Recently, we reported the enantioselective protonation of enolates with (R)-2 and the related carbamates. 4 To further extend the utility of this new chiral source 1, we attempted to synthesize the optically active phosphine ligand 8 from 1. Here we report the unexpectedly easy racemization of the phosphine oxides 5-7 we encountered in the synthesis of 8. Recently, Hayashi et al. introduced chiral monodentate phosphine ligands, eg 2-diphenylphosphino-2’-methoxy- 1, l’-binaphthyl 11, for palladium-catalysed hydrosilylation, 5 hydroboration6 and the reduction of allylic esters. 7 Switching the 2-and 2’-substituents of 11 to the 8-and 8’-positions, respectively, gives another phosphine 8. Since the distance from the 8-phosphorus atom to the 2’-hydrogen in 8 should be the same as that from the 2-phosphorus atom to the 8’-hydrogen in 11, each enantiomer should exist independently. Optically active 8 was expected to be easily prepared from optically active 1 without racemization according to a previously reported procedure. 8 The chiral environment of the diphenylphosphinoyl group in 8 is quite different from that in 11. Another naphthyl ring located at the peri-position in 8 completely obstructs the approach from this side so that a higher selectivity may be expected for 8 than for 11.9 Extensive studies by Harris and coworkers 10 revealed that 1, l-binaphthyls with sp3-substituents at the 8, 8’-positions are optically more stable than those with sp2-substituents.