Ambident reactivity of aryloxide ions towards the super-electrophile, 4,6-dinitrobenzofuroxan. Kinetics, thermodynamics and stereoelectronic factors on regioselectivity
作者:Erwin Buncel、Richard A. Manderville、Julian M. Dust
DOI:10.1039/a605907d
日期:——
Reactions of the aryloxide ions, phenoxide
(PhO-)
and 3,5-di-tert-butylphenoxide
(3,5-DTBPhO-), with the super-electrophilic
heteroaromatic substrate, 4,6-dinitrobenzofuroxan (DNBF, 1), have been
examined by 400 MHz 1H NMR spectroscopy in
acetronitrile–dimethoxyethane
([2H3]MeCN∶[2H10]DME
1∶1, v/v) as a function of varying temperature (-40 to
23 °C) and in dimethyl sulfoxide
([2H6]DMSO) at room temperature. We herein report
the first observation and full characterization of the
O-bonded σ-adduct
(DNBF·OPh-, 3a) formed by attack of
PhO-, acting as an O-nucleophile, at
the C-7 super-electrophilic site of 1. No C-5 adduct was seen in the
initial spectrum (-40 °C,
[2H3]MeCN∶[2H10]DME)
or in subsequent monitoring of the reaction. These results suggest that
PhO- displays K7T7 regioselectivity towards DNBF
wherein attack at the C-7 site is favoured by both kinetics and
thermodynamics, comparable to the behaviour shown by
PhO- towards 2,4,6-trinitroanisole where the C-1 adduct
is the product of both kinetic and thermodynamic control
(i.e. K1T1 regioselectivity). Upon warming the
reaction mixture to ambient, the C-7 O-adduct,
DNBF·OPh-, 3a, gives way to the more stable C-7
C-bonded σ-adducts
(DNBF·ortho-PhOH- adduct, 4,
and DNBF·para-PhOH- adduct, 5,
in a ratio of ca. 1∶6). The C-7 hydroxide
adduct, DNBF·OH-, 2a, and phenol are detected at
this temperature. The C-adducts, 4 and 5, are the sole
PhO- adducts previously observed in the
DNBF–PhO- reaction system (in
[2H6]DMSO at room temperature). When
C-attachment is precluded by steric hindrance, as in
the reaction of 1 with 3,5-DTBPhO-, the C-7
DNBF·OPhDTB- adduct, 3b, is observed by
1H NMR spectroscopy even in [2H6]DMSO
under ambient conditions. The results of the kinetics and thermodynamics
of aryloxide adduct formation with DNBF, including the ambident
reactivity found, are discussed with regard to stereoelectronic
stabilization in the adducts and with comparison to relevant
4-nitrobenzofuroxan (NBF), 2-(nitroaryl)-4,6-dinitrobenzotriazole
1-oxides (2-Ar-4,6-DNBT) systems and to the normal electrophile,
1,3,5-trinitrobenzene (TNB).
芳氧基离子、
酚盐的反应
(PhO-)
和3,5-二
叔丁基苯氧基
(3,5-
DTBPhO-),具有超亲电性
杂芳族底物 4,6-二硝基
苯并呋喃 (DNBF, 1)
通过 400
MHz 1H NMR 光谱检查
乙腈-
二甲氧基乙烷
([2H3]MeCN∶[2H10]
DME
1∶1,v/v)作为变化温度(-40 至
23°C)和
二甲亚砜中
([2H6]
DMSO) 在室温下。我们在此报告
的第一次观察和全面表征
O-键 σ-加合物
(DNBF·OPh-, 3a) 由攻击形成
PhO-,作为 O-亲核试剂,在
1 的 C-7 超亲电子位点。在 1 中没有看到 C-5 加合物
初始光谱(-40°C,
[2H3]MeCN∶[2H10]
DME)
或在随后的反应监测中。这些结果表明
PhO- 显示 K7T7 对 DNBF 的区域选择性
其中 C-7 位点的攻击受到动力学和
热力学,与以下所示的行为相当
PhO- 朝向
2,4,6-三硝基苯甲醚,其中 C-1 加合物
是动力学和热力学控制的产物
(即 K1T1 区域选择性)。加热后
反应混合物至环境温度,C-7 O-加合物,
DNBF·OPh-,3a,让位于更稳定的 C-7
C-键 σ-加合物
(DNBF·邻-PhOH-加合物,4,
和 DNBF·对-PhOH-加合物,5,
比例为约。 1∶6)。 C-7 氢氧化物
加合物、DNBF·OH-、2a 和
苯酚在
这个温度。 C-加合物 4 和 5 是唯一的
PhO-加合物先前在
DNBF-PhO-反应体系(在
[2H6]
DMSO(室温)。什么时候
C-连接被空间位阻排除,如
1 与 3,5-
DTBPhO-(C-7)的反应
DNBF·OPhDTB-加合物,3b,通过以下方式观察到
1H NMR 光谱,甚至在 [2H6]
DMSO 中
在环境条件下。动力学和热力学结果
与 DNBF 形成芳氧基加合物,包括环境
发现的反应性,讨论了立体电子学
加合物的稳定性并与相关的比较
4-硝基
苯并呋喃 (NBF), 2-(硝基芳基)-4,6-
二硝基苯并三唑
1-氧化物(2-Ar-4,6-D
NBT)系统和正常的亲电子试剂,
1,3,5-三硝基苯(TNB)。