Photochemical generation of the N-isopropylbenzonitrilium ion and unusually slow trapping by azide ion
作者:Patrick H. Ruane、Robert A. McClelland、A. Frank Hegarty、Steen Steenken
DOI:10.1039/a807346e
日期:——
The imidate ester 4-cyanophenyl N-isopropylbenzimidate undergoes efficient photoheterolysis in water producing the 4-cyanophenoxide ion and the N-isopropylbenzonitrilium ion. The latter is detected as a transient intermediate in flash photolysis experiments and has a relatively long lifetime (1/kw) in water of 2.5 ms. The rate constants for hydroxide and azide are respectively 5.2 × 106 and 3.9 × 106 dm3 mol–1 s–1. This pattern is not seen with sp2 hybridized cations, where kaz is normally significantly greater than kOH. Further indications that azide ion is unexpectedly poor as a trap for this cation is seen in considering the kaz∶kw value in the context of ratios directly measured for sp2 hybridized cations. For the latter a kaz∶kw of less than 105 can be taken to mean that the reaction with azide is at (or certainly very close to) the diffusion limit (i.e. 5–10 × 109 dm3 mol–1 s–1). The nitrilium ion 3 has kaz∶kw equal to only 104 mol–1 dm3 and yet its azide reaction is three orders of magnitude slower than diffusion.
亚氨酸酯 4-氰基苯基 N-异丙基苯亚氨酸在水中发生高效光解,产生 4-氰基苯氧化物离子和 N-异丙基苯甲硝鎓离子。后者在闪光光解实验中作为瞬时中间体被检测到,在水中的寿命(1/kw)相对较长,为 2.5 毫秒。氢氧化物和叠氮化物的速率常数分别为 5.2 × 106 和 3.9 × 106 dm3 mol-1 s-1。这种模式在 sp2 杂化阳离子中并不常见,在 sp2 杂化阳离子中,kaz 通常明显大于 kOH。如果将 kaz∶kw 值与直接测量到的 sp2 杂化阳离子的比率联系起来看,就会发现叠氮离子对这种阳离子的捕获能力出乎意料地差。对于后者,kaz∶kw 值小于 105 意味着与叠氮化物的反应处于(或肯定非常接近)扩散极限(即 5-10 × 109 dm3 mol-1 s-1)。亚硝酸离子 3 的 kaz∶kw 仅等于 104 mol-1 dm3,但其叠氮反应却比扩散慢三个数量级。