Intramolecular activation of aromatic C–H bonds at tantalum(V) metal centers: evaluating cyclometallation ‘resistant’ and ‘immune’ aryloxide ligation
作者:Jonathan S. Vilardo、Mark A. Lockwood、Linda G. Hanson、Janet R. Clark、Bernardeta C. Parkin、Phillip E. Fanwick、Ian P. Rothwell
DOI:10.1039/a702325a
日期:——
The trichloride compounds
[Ta(OC
6
HPh
2
-2,6-R
2
-3,5)Cl
3
]
(1: R = H a, Ph b, Me c, Pr
i
d or
Bu
t
e) have been obtained by treating
[Ta
2
Cl
10
] with the corresponding
3,5-disubstituted-2,6-diphenylphenols IaâIe. The solid-state
structures of 1c and 1d show a square-pyramidal structure with an axial
aryloxide ligand. The reaction of 1 with
LiCH
2
SiMe
3
(3 equivalents) led to the isolation of
the tris(alkyls)
[Ta(OC
6
HPh
2
-2,6-R
2
-3,5)
2
(CH
2
SiMe
3
)
3
] (4aâ4d) except in
the case of the 3,5-di-tert-butyl derivative 1e which generated
the alkylidene compound
[Ta(OC
6
H
3
Ph
2
-2,6-Bu
t
-3,5)
2
(CHSiMe
3
)(CH
2
SiMe
3
)] 6e. The alkylidenes 6aâ6d can be produced by photolysis of the
corresponding tris(alkyls) 4aâ4d. The alkylidenes 6aâ6d
undergo intramolecular cyclometallation of the aryloxide ligand
(addition of an aromatic CâH bond to the tantalum alkylidene) at
a rate which is extremely dependent on the meta substituents on
the phenoxide nucleus. Kinetic studies show that conversion of
6aâ6d into monometallated 7aâ7d is first order with the
phenyl, methyl and isopropyl substituents slowing the ring closure down
by factors of 20, 90 and 360 respectively. The tert-butyl
substituent completely shuts down cyclometallation of the adjacent
phenyl ring. It is argued that bulky substituents inhibit rotation of
the ortho-phenyl ring into a conformation necessary for
CâH bond activation. Structural analysis of the torsion angles
between ortho-phenyl and phenoxy rings has been carried out.
The use of
1
H NMR chemical shifts has been demonstrated to be
a valuable tool to probe the average conformations adopted in
solution.
通过将[Ta 2 Cl 10 ]与相应的 3,5-二取代-2,6-二苯基酚 IaâIe 处理,得到了三氯化物[Ta(OC 6 HPh 2 -2,6-R 2 -3,5)Cl 3 ](1:R = H a、Ph b、Me c、Pr i d 或 Bu t e)。1c 和 1d 的固态结构显示出一种带有轴向芳氧基配体的方阵结构。1 与 LiCH 2 SiMe 3(3 个等价物)反应,分离出三(烷基)[Ta(OC 6 HPh 2 -2,6-R 2 -3,5) 2 (CH 2 SiMe 3 ) 3 ](4aâ4d),但 3,5-二叔丁基衍生物 1e 除外,它生成了亚烷基化合物 [Ta(OC 6 H 3 Ph 2 -2,6-Bu t -3,5) 2 (CHSiMe 3 )(CH 2 SiMe 3 )] 6e。6e.通过光解相应的三烷基 4aâ4d 可以制得亚烷基 6aâ6d。亚烷基 6aâ6d 会发生芳氧基配体的分子内环金属化反应(在钽亚烷基上添加一个芳香族 CâH 键),反应速度与氧化酚核上的元取代基密切相关。动力学研究表明,6aâ6d 转化为单金属化 7aâ7d 的过程是一阶的,苯基、甲基和异丙基取代基会使闭环速度分别减慢 20、90 和 360 倍。叔丁基取代基完全阻止了邻近苯基环的环金属化。有观点认为,笨重的取代基抑制了正苯基环旋转到 CâH 键活化所需的构象。研究人员对正苯基环和苯氧环之间的扭转角进行了结构分析,结果表明,使用 1 H NMR 化学位移是探测溶液中平均构象的重要工具。