Heterobimetallic Reductive Cross-Coupling of Benzonitrile with Carbon Dioxide, Pyridine, and Benzophenone

Described herein are heterobimetallic radical cross-coupling reactions between the benzonitrile adduct of the molybdenum(III)
complex Mo(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2) and titanium(III)
complexes with carbon dioxide, pyridine, and benzophenone. The
titanium(III) system employed was either Ti(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2) or Ti(N[t-Bu]Ph)3. Crystal structure studies are described
for the Mo/PhCN/CO2/Ti coupled system and for an analogue of
the Mo/PhCN/Ph2CO/Ti coupled system in which PhCN is replaced
with 2,6-Me2C6H3CN. In the case of the couplings involving pyridine
and benzophenone, C−C bond formation takes place with
dearomatization, with the new C−C bond being formed between
the nitrile carbon of PhCN and the para carbon of pyridine or one
of the benzophenone phenyl groups. Of the radical metal complex/substrate adducts invoked in this work, that between titanium(III)
and CO2 is the only one not directly observable. In all cases, the
selective cross-coupling reactions are interpreted as arising by
heterodimerization of titanium(III) substrate complexes (substrate
= CO2, py, or Ph2CO) with the persistent molybdenum−PhCN
radical adduct. All of the heterobimetallic coupling products are
diamagnetic, and the metal ions Ti and Mo in them both are
assigned to the formal 4+ oxidation state.

本文报道了异双金属自由基交叉偶联（heterobimetallic radical cross-coupling）反应：钼(III)配合物（molybdenum(III) complex）Mo(N[t-Bu]Ar)3（Ar = 3,5-C6H3Me2，即3,5-二甲基苯基）的苯甲腈（benzonitrile，PhCN）加合物，与以二氧化碳（carbon dioxide，CO2）、吡啶（pyridine，py）及二苯甲酮（benzophenone，Ph2CO）为底物的钛(III)配合物（titanium(III) complex）之间发生的偶联反应。本次研究所采用的钛(III)体系为Ti(N[t-Bu]Ar)3（Ar = 3,5-C6H3Me2）或Ti(N[t-Bu]Ph)3。本文还对Mo/PhCN/CO2/Ti偶联体系，以及将PhCN替换为2,6-二甲基苯甲腈（2,6-Me2C6H3CN）的Mo/PhCN/Ph2CO/Ti偶联体系类似物开展了晶体结构表征。对于涉及吡啶与二苯甲酮的偶联反应，反应过程伴随脱芳构化（dearomatization），新生成的C-C键形成于苯甲腈的氰基碳与吡啶的对位碳，或是二苯甲酮其中一个苯环的对位碳之间。在本工作中涉及的自由基金属配合物/底物加合物中，仅钛(III)与二氧化碳的加合物无法直接被观测到。在所有反应体系中，选择性交叉偶联反应的机理被认为是：钛(III)底物配合物（底物为CO2、py或Ph2CO）与稳定的钼-苯甲腈自由基加合物发生异二聚化（heterodimerization）。所有异双金属偶联产物均为反磁性（diamagnetic）物质，其中所含的钛(Ti)与钼(Mo)金属离子的形式氧化态（formal oxidation state）均被定为+4价。