Department of Organic Chemistry, Faculty of Chemistry , University of Murcia , Campus de Espinardo , 30071 Murcia , Spain.
Inorg Chem. 2018 Jul 16;57(14):8058-8064. doi: 10.1021/acs.inorgchem.7b02816. Epub 2018 Jul 5.
High-level single-reference calculations reveal that trimethylphosphine-mediated reductive dimerization of properly substituted (e.g., CF) ketones proceeds via initial formation of an oxaphosphirane intermediate, with the oxygen atom occupying an equatorial position at phosphorus. In the "oxirane route", this oxaphosphirane intermediate loses a trimethylphosphine oxide unit, thus behaving as a carbene transfer agent toward a second carbonyl molecule and giving rise to a carbonyl ylide that cyclizates to the corresponding oxirane. This in turn transfers the carbene unit to a second phosphine molecule, with loss of acetone, affording a phosphorane. The latter undergoes typical Wittig reaction to the final homocoupling product through the oxaphosphetane intermediate. The alternative direct conversion of oxaphosphirane into phosphorane constitutes the lowest energy path as it skips the highest barrier oxirane → phosphorane conversion. The oxirane route is favored by the use of polar solvents and electron deficient carbonyl components. The lowest barrier most exergonic process from oxaphosphirane is the pericyclic cycloaddition of the acetone C═O bond along the endocyclic P-C bond, furnishing the stable 1,3,2-dioxaphospholane product.
高水平的单参考计算表明，经过适当取代的（如 CF）酮的三甲基膦介导的还原二聚反应通过初始形成氧杂磷杂环戊烷中间体进行，其中氧原子占据磷原子的赤道位置。在“环氧乙烷路线”中，这种氧杂磷杂环戊烷中间体失去一个三甲基氧化膦单元，因此表现为向第二个羰基分子转移碳烯的试剂，并产生羰基叶立德，其环化得到相应的环氧乙烷。这反过来又将碳烯单元转移到第二个膦分子上，同时失去丙酮，得到磷烷。后者通过氧杂磷杂环戊烷中间体经历典型的魏悌希反应转化为最终的同偶联产物。氧杂磷杂环戊烷向磷烷的直接转化构成了最低能量路径，因为它跳过了最高能垒的环氧乙烷→磷烷转化。环氧乙烷途径受极性溶剂和缺电子羰基组分的影响。从氧杂磷杂环戊烷出发，最低能垒最放能的过程是丙酮 C=O 键沿着内环 P-C 键的周环加成，生成稳定的 1,3,2-二氧杂磷杂环戊烷产物。
