DDQ-Supported Alkoxylation of 2‑Aza-21-carbaporphyrin and Noncatalyzed Transetherification of Its 3,21-Dialkoxy Derivatives

An oxidative addition of primary alkoxyls into two sites
of N-confused
porphyrin (NCP) has been accomplished by means of alcohols in the
presence of a stoichiometric amount of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ). The resulting aromatic monocationic species 1-R3 (R = Me, Et) were characterized by the NMR and, in the case
of triethoxy-NCP, by monocrystal X-ray diffraction analysis. One alkoxy
group is located in position 3, on the macrocycle’s perimeter,
while two −OR moieties are attached to the internal carbon
(position 21) of the confused pyrrole. The 3-EtO-21-Cl-NCP 2, which is formed as a byproduct, was also structurally characterized
by means of X-ray diffraction. Reduction of 3-RO-21-(RO)2-NCP with hydrazine hydrate gave selectively a neutral and intrinsically
chiral 3,21-bis­(alkoxy)­NCP 3-R2. Dealkylation
of the externally bonded 3-OR fragment under basic conditions leading
to a zwitterionic aromatic 3-oxo-species 4-R2, which still possesses the internal ketal functionality, was established
by the NMR and X-ray diffraction methods. An unprecedented transetherification
for the internal alkoxyl of 3-R2 can be achieved
under very mild conditions and without catalyst. One of the alkoxyl-exchange
products, i.e., 3-ethoxy-21-methoxy-NCP, was characterized by the
X-ray diffraction method. The substitution proceeds via an associative
(SN2) mechanism resulting in an inversion of the chirality
of 3-RR′, which was shown by means of the NMR
and chirooptical methods.

以醇类为反应物，在化学计量量的2,3-二氯-5,6-二氰基-1,4-苯醌（DDQ）作用下，伯烷氧基成功发生氧化加成反应，加成至氮混杂卟啉（N-confused porphyrin, NCP）的两个位点上。所得芳香性单阳离子物种1-R3（R=Me, Et）经核磁共振波谱（NMR）表征；对于三乙氧基取代的NCP，还通过单晶X射线衍射分析完成了结构表征。其中一个烷氧基位于大环外围的3号位，而两个-OR基团则连接在混杂吡咯的内部碳原子（21号位）上。作为副产物生成的3-EtO-21-Cl-NCP 2同样通过X射线衍射方法完成了结构表征。采用水合肼对3-RO-21-(RO)2-NCP进行还原，可选择性得到中性且具有固有手性的3,21-双(烷氧基)NCP 3-R2。研究发现，在碱性条件下，外环键合的3-OR片段会发生脱烷基反应，生成仍保留内部缩酮官能团的两性离子芳香性3-氧代物种4-R2，该过程通过核磁共振波谱与X射线衍射方法得以证实。3-R2内部烷氧基可在无催化剂、极为温和的条件下发生前所未有的转醚化反应。其中一种烷氧基交换产物——3-乙氧基-21-甲氧基NCP，经X射线衍射方法完成了表征。该取代反应通过双分子亲核取代（SN2）的缔合机理进行，伴随3-RR'手性中心的构型翻转，该结论通过核磁共振波谱与旋光光学方法得到了验证。