Substituents in ring A of chlorophylls can exert profound effects on spectral properties. A de novo route to synthetic chlorins employs a tetrahydrodipyrrin reactant containing pyrrole and pyrroline rings. Complexation of the tetrahydrodipyrrin with a dialkylboron motif caused electrophilic substitution (bromination, formylation) to proceed predominantly at the β7- rather than α-position of the pyrrole ring, whereas an analogous dihydrodipyrrin underwent substitution equally at the 7- and 8-positions. The fully unsaturated dipyrrinâdifluoroboron complex is known to undergo electrophilic substitution at the 8-position. The 7-position of the hydrodipyrrin ultimately gives rise to substituents at the chlorin 2-position (ring A), which heretofore has been little accessed. The position of substitution was confirmed by four single-crystal X-ray structures. Two isomeric formylchlorins were prepared by Pd-mediated carbonylation of the corresponding bromochlorins. Access to a 2-formylchlorin relied on bromination of the tetrahydrodipyrrinâdibutylboron complex, whereas a 3-formylchlorin was prepared by installation of the bromo group in the earliest precursor, pyrrole-2-carboxaldehyde. The two formylchlorins differ in absorption spectral properties: the Qy absorption maximum is 654 or 664 nm for the 2- or 3-formylchlorin, respectively. The synthetic formylchlorins provide initial models for understanding the strong red absorption of native 2- or 3-formylchlorophylls (f and d).
叶绿素A环中的取代基可以对光谱特性产生深远的影响。一种合成叶绿素的新方法采用了含有
吡咯和
吡咯啉环的四氢二
吡咯反应物。四氢二
吡咯与二烷基
硼基团的络合使得电亲核取代(
溴化、甲酰化)主要发生在
吡咯环的β7位,而非α位,而一个类似的二氢二
吡咯则在7位和8位的取代反应是相同的。已知完全不饱和的二
吡咯–二
氟硼络合物在8位进行电亲核取代。氢二
吡咯的7位最终产生在叶绿素2位(A环)的取代基,而这一位置此前未被广泛研究。取代位置通过四个单晶X射线结构得到了证实。通过
钯介导的羰基化反应制备了两种异构
甲酰氯素。获得2-
甲酰氯素依赖于对四氢二
吡咯–
二丁基硼络合物的
溴化,而3-
甲酰氯素则是通过在最早的前体
吡咯-2-甲醛中引入
溴基来制备的。这两种
甲酰氯素在吸收光谱特性上有所不同:对于2-或3-
甲酰氯素,Qy吸收最大值分别为654或664 nm。合成的
甲酰氯素为理解天然2-或3-甲酰叶绿素(f和d)的强红光吸收提供了初步模型。