Studies on The Ruthenium Catalyzed Carbon-Carbon Bond Formation Reaction
Part 1. Ruthenium-Catalyzed Heck Olefination : Studies on the Nature of Catalytic Species
Ruthenium-catalyzed Heck olefination and Suzuki cross coupling reactions have been reported. With ruthenium complex $[RuCl_2(p-cymene)]_2$ as a homogeneous catalyst precursor, induction periods were observed and ruthenium colloids of zero oxidation state were generated. Isolated ruthenium colloids carried out the olefination, implying that active catalytic species are ruthenium nanoclusters. To support this hypothesis, ruthenium nanoparticles stabilized with dodecylamine were independently prepared via a hydride reduction procedure, and their catalytic activity was subsequently examined. Olefination of iodobenzene with ethyl acrylate was efficiently catalyzed by the ruthenium nanoparticles under the same conditions, which could be also reused for the next runs. No residual ruthenium was detected from the filtrate at the end of the reaction. In poisoning experiments, the conversion of the olefination was completely inhibited in the presence of mercury, thus supporting our assumption on the nature of catalytic species. On the basis of the postulation, a heterogeneous catalyst system of ruthenium supported on alumina was consequently developed for the Heck olefination and Suzuki cross coupling reactions for the first time. It turned out that substrate scope and selectivity were significantly improved with the external ligand-free catalyst even under milder reaction conditions when compared to results with the homogeneous precatalyst. It was also observed that the immobilized ruthenium catalyst was recovered and reused up to several runs with consistent efficiency. Especially in the Suzuki couplings, the reactions could be efficiently carried out with as low as 1 mol % of the supported catalyst over a wide range of substrates and could be scaled up to a few grams without any practical problems, giving coupled products with high purity by a simple workup procedure.
Part 2. Ruthenium-catalyzed hydroesterification of alkynes based on a chelation-approach
Based on a chelation-assistance strategy, efficient hydroesterification of alkynes and dienes has been achieved using ruthenium catalyst without the need of high pressuric external CO atmosphere, and a diverse range of α,β -unsaturated esters and mono-olefinic esters could be obtained with high stereo- and regioselectivity. It was observed that a complete stereoselectivity of syn-addition to triple bonds with moderate regioselectivity. In addition, in the case of diolefinic substrates, only mono-esters were selectively produced with high efficiency without isomerization of double bonds.
Part 3. Studies of Catalytic Behavior of Dimeric Ruthenium Complexes on the Basis of Electrochemical Approaches
We have recently developed a series of efficient and selective organic transformations with the use of a dimeric ruthenium catalyst, $[RuCl_2(p-cymene)]_2$. It was rather surprising that an identical catalyst precursor carries out a diverse range of reactions such as oxidation of alcohols and silanes, activation of Si-H and alkynyl C-H bonds, and dehydration of aldoximes. It was previously observed by spectroscopic means that dissociation of the dimeric species into monomeric adducts takes place under certain circumstances. In our previous $^1H$ NMR study quantitative formation of a new monomeric ruthenium complex coordinated with the added base (1:1 ratio) was also observed by addition of pyrrolidine to a solution of $[RuCl_2(p-cymene)]_2$. Here, we observed that Electrochemical parameters measured from a series of ruthenium complexes are nicely correlated with both structural changes and catalytic activities of those species under the reaction conditions. This study provides possibilities that electrochemistry may offer a facile tool for better understanding of some transition metal-catalyzed reactions.
Studies on The Ruthenium Catalyzed Carbon-Carbon Bond Formation Reaction
Part 1. Ruthenium-Catalyzed Heck Olefination : Studies on the Nature of Catalytic Species
균일 촉매인 $[RuCl_2(p-cymene)]_2$ 루테늄 촉매를 이용하여 Heck olefination과 Suzuki cross coupling반응을 수행하였다. 관찰된 Induction period에서 산화상태가 0인 루테늄 콜로이드가 형성되어짐을 TEM을 찍어 확인하였다. 균일촉매로부터 생성된 루테늄 콜로이드가 olefination을 수행할 것이라는 가정을 확인하기 위해 hydride reduction 방법을 가지고 dodecyamine으로 안정화시킨 루테늄 나노파티클을 만들어 촉매활성을 확인해보았다. 같은 조건에서 루테늄 나노파티클을 이용하여 Iodobenzene과 ethyl acrylate를 반응하여 정량적으로 olefination을 수행함을 확인하였고 촉매의 재활용이 가능함을 확인하였다. 앞서 내린 가정을 뒷받침하고자 poisoning 실험을 수행한 결과 수은에 의해 반응이 억제되어짐을 확인하였다. 앞서 얻어진 가정을 통해 불규일 촉매 시스템으로써 알루미나에 흡착된 루테늄촉매를 이용하여 Heck 반응과 Suzuki 반응을 처음으로 수행하였다. 균일촉매와 비교하여 온화한 조건에서 리간드의 첨가 없이 기질의 범위와 선택성이 향상되어졌다. 촉매의 활성도는 유지된 채 촉매의 재활용이 4-5회까지도 계속 유지되어짐을 관찰하였다. 특히 Suzuki 반응의 경우에는 그램단위로 촉매 1 mol%에서 workup과정만을 통해서 높은 순도의 생성물을 얻을 수 있었다.
Part 2. Ruthenium-catalyzed hydroesterification of alkynes based on a chelation-approach
Chelation-Assistance 방법을 이용하여 높은 기압의 CO 가스없이 루테늄 촉매 조건에서 alkyne과 diene의 hydroesterification을 수행하였다. alkyne으로부터 a,b-unsaturated ester를, diene으로부터 mono-olefinic ester를 입체선택적이며 위치선택적으로 얻을 수 있었다. 특히, 삼중결합으로 완벽하게 syn-addition하여 생성물이 형성되어짐을 확인하였다. Diolefin의 경우에는 두 개의 이중결합 중에서 한 개의 ester만이 선택적으로 얻어지며 이중결합의 isomerization이 일어남이 없이 효과적으로 반응을 수행함을 확인하였다.
Part 3. Studies of Catalytic Behavior of Dimeric Ruthenium Complexes on the Basis of Electrochemical Approaches
우리는 최근 dimer 형태의 $[RuCl_2(p-cymene)]_2$ 촉매를 사용하여 효율적이며 선택적인 유기반응을 개발하였다. 특히, 동일한 촉매를 가지고 alcohol이나 silane의 산화반응, Si-H 결합이나 alkyne의 C-H 결합의 activation 그리고 aldoxime의 dehydration등을 수행하였다. 앞서 $^1H$ NMR을 통해서 $[RuCl_2(p-cymene)]_2$ 용액에 pyrrolidine을 넣음으로써 새로운 monomer형태의 루테늄 화합물이 형성되어짐을 확인하였다. 여기서, 우리는 전기화학적 파라미터를 측정하여 구조변화와 촉매의 활성도간의 관계를 관찰함으로써 여러 전이금속 촉매 반응을 더 잘 이해할 수 있는 도구로써 전기화학의 가능성을 제시하고자 한다.
