Destiny of Transient Phosphenium Ions Generated from the Addition of Electrophiles to Phosphaalkenes: Intramolecular C−H Activation, Donor−Acceptor Formation, and Linear Oligomerization

The reaction of the phosphaalkenes MesPCPh2 (Mes = 2,4,6-Me3C6H2) and Mes*PCH2
(Mes* = 2,4,6-tBu3C6H2) with Lewis (AlCl3, GaCl3, InCl3) and protic (HOTf) acids has been
examined to evaluate the feasibility of cationic polymerization for PC bonds. Addition of
GaCl3 to Mes*PCH2 generates the adduct Mes*(Cl3Ga)PCH2, which can be detected
spectroscopically at 193 K. At higher temperatures, GaCl3 migrates from phosphorus to
carbon to afford the fleeting phosphenium zwitterion Mes*PCH2GaCl3. This undetected
transient species immediately oxidatively adds to a C−H bond of an o-tBu group in the P-Mes*
substituent, resulting in a GaCl3-coordinated ylide that has been characterized crystallographically. The analogous reaction of GaCl3 with MesPCPh2 gives stable Mes(Cl3Ga)PCPh2, for which a crystal structure determination has been conducted. Significantly, treating
a highly concentrated solution of Mes*PCH2 with substoichiometric quantities of GaCl3
leads to linear dimerization following a cationic chain growth mechanism; however, the
oligomerization is terminated by intramolecular C−H activation. The novel coordinated linear
dimer (C−H activated Mes*)PCH2PH(Mes*)CH2GaCl3 has been characterized crystallographically. Interestingly, mechanistic studies reveal that the diphosphiranium ring
derived from the reaction of Mes*PCH2GaCl3 with Mes*PCH2 is an intermediate in this transformation. The reaction of phosphaalkenes with
phosphenium species appears to be a general method to prepare diphosphiranium ions. In
one case, NMR spectroscopic data suggests that treating MesPCPh2 with HOTf gives both
the diphosphiranium species and the adduct Ph2C(Mes)P→P(Mes)(CHPh2)]OTf. Remarkably, treating concentrated Mes*PCH2 solutions with HOTf
results in oligomers of up to six repeat units, as determined by ESI mass spectrometry.
These results suggest that it may be possible to initiate the polymerization of PC bonds
using cationic initiators and that the propagating species will be a cationic phosphenium
moiety.

本研究考察了磷杂烯烃（phosphaalkenes）MesP=CPh₂[Mes即2,4,6-三甲基苯基（2,4,6-Me₃C₆H₂）]与Mes*P=CH₂[Mes*即2,4,6-三叔丁基苯基（2,4,6-tBu₃C₆H₂）]分别与路易斯酸（AlCl₃、GaCl₃、InCl₃）及质子酸HOTf（三氟甲磺酸）的反应，以评估磷碳（P=C）键阳离子聚合的可行性。将GaCl₃加入Mes*P=CH₂中可生成加合物Mes*(Cl₃Ga)P=CH₂，该产物可在193 K下通过光谱法检测到。当温度升高时，GaCl₃会从磷原子迁移至碳原子，得到瞬态且未被检测到的膦正离子两性离子Mes*PCH₂GaCl₃。该未被检测到的瞬态物种会快速与P-Mes*取代基中邻位叔丁基的C-H键发生氧化加成，生成经晶体学表征的、由GaCl₃配位的叶立德。GaCl₃与MesP=CPh₂的类似反应则得到稳定的加合物Mes(Cl₃Ga)P=CPh₂，该产物已通过晶体结构测定完成表征。值得注意的是，对高浓度的Mes*P=CH₂溶液加入亚化学计量比的GaCl₃时，反应会按照阳离子链式增长机理发生线性二聚；但该低聚反应会因分子内C-H活化而终止。所得新型配位型线性二聚体(C-H活化的Mes*)PCH₂PH(Mes*)CH₂GaCl₃已通过晶体学表征。机理研究表明，由Mes*PCH₂GaCl₃与Mes*P=CH₂反应生成的二磷杂环丙烷鎓（diphosphiranium）环中间体是该转化过程的关键中间体。磷杂烯烃与膦正离子（phosphenium）物种的反应似乎是制备二磷杂环丙烷鎓离子的通用方法。在一例反应中，核磁共振光谱数据显示，将MesP=CPh₂与HOTf反应时，同时生成了二磷杂环丙烷鎓物种与加合物[Ph₂C=(Mes)P→P(Mes)(CHPh₂)]OTf。值得一提的是，对高浓度的Mes*P=CH₂溶液加入HOTf时，经电喷雾电离（ESI）质谱检测，产物为至多包含6个重复单元的低聚物。上述结果表明，使用阳离子引发剂可引发P=C键的聚合反应，且增长活性种为阳离子膦正离子片段。