Alkene-Chelated Ruthenium Alkylidenes: A Missing Link to New Catalysts

A variety
of heteroatom-chelated ruthenium alkylidenes have been
developed as metathesis-active catalysts. Alkene-chelated ruthenium
alkylidenes, however, have not been considered as a viable alternative
because alkene coordination is a necessary step in the catalytic cycle.
Relying on common design principles with varying steric and electronic
factors, a series of structurally diverse alkene-chelated ruthenium
alkylidene complexes were prepared by trapping the intermediates of
enyne ring-closing metathesis (RCM) of 1,n-enynes
and diynes with a stoichiometric amount of an initiator ruthenium
complex. One of the crucial structural elements that promotes the
formation of 1,5-alkene-chelates is the exo-Thorpe–Ingold
effect, exerted by a gem-dialkyl moiety. These alkene-chelated
complexes show a trans relationship between the N-heterocyclic carbene (NHC) ligand and the chelated alkene. On the
other hand, η3-vinyl alkylidene complexes were generated
from the RCM of ynamide-tethered 1,n-enynes. The
presence of an ynamide moiety with a right connectivity is essential
for the formation of these rare η3-vinyl alkylidene
complexes with a cis relationship between the N-heterocyclic
carbene (NHC) ligand and the chelated alkene. The stability and reactivity
of these alkene-chelated ruthenium alkylidenes could be finely tuned
to show characteristic behaviors in RCM, cross-metathesis (CM), and
ring-opening metathesis polymerization (ROMP) reactions.

多种杂原子螯合钌亚烷基配合物已被开发为具备复分解催化活性的催化剂。但烯烃螯合钌亚烷基配合物此前并未被视作可行的替代催化剂，这是因为烯烃配位是催化循环中的必要步骤。本研究依托兼具不同空间位阻与电子效应的通用设计原则，通过以化学计量比的引发剂钌配合物捕获1,n-炔烯与二炔的炔烃闭环复分解（enyne ring-closing metathesis, RCM）中间体，合成了一系列结构多样的烯烃螯合钌亚烷基配合物。促进1,5-烯烃螯合物生成的关键结构要素之一，是偕二烷基基团所产生的外向索普-英格尔（exo-Thorpe–Ingold）效应。这类烯烃螯合配合物的氮杂环卡宾（N-heterocyclic carbene, NHC）配体与螯合烯烃之间呈反式构型。另一方面，η³-乙烯基亚烷基配合物可通过连有炔酰胺的1,n-炔烯的RCM反应制得。带有正确连接方式的炔酰胺基团，是形成这类罕见η³-乙烯基亚烷基配合物的必要条件，此类配合物的氮杂环卡宾（NHC）配体与螯合烯烃之间呈顺式构型。这些烯烃螯合钌亚烷基配合物的稳定性与反应活性可被精细调控，从而在RCM、交叉复分解（cross-metathesis, CM）以及开环复分解聚合（ring-opening metathesis polymerization, ROMP）反应中展现出特征性的催化行为。