Furthermore, complexes 33 and 85 were able to perform the RCM reactions under the same conditions, with yields ranging from moderate to excellent Fig. RCM scope in air with catalysts 33, 85 and 98a.
Olefin metathesis in air.
Reaction conditions: Catalyst, substrate 0. GC conversion and isolated yield in parentheses. Schiff bases in metathesis are usually O,N-bidentate ligands and represent an interesting alternative family of ligands as [ 18 , 87 — 94 ]: 1 they can be produced in one high yielding step by condensation of an aldehyde and an amine, thus allowing the fine and facile tuning of ligand and catalyst steric and electronic properties; and 2 the two different donor atoms, O hard and N soft , offer different features and therefore can stabilize, respectively, high and low oxidation states.
This class of complexes showed high activity and very high stability to air and water, compared to Grubbs 1 st and 2 nd generation catalysts [ 7 ].
In the absence of SIMes, increasing the olefin substitution led to low yields in all catalytic systems. An electron-withdrawing substituent on the phenyl ring and a bulky group on the imine generally lead to higher activity for both mono- and bimetallic systems. SIMes-bearing complexes are more active than monometallic systems in all cases, and more active than bimetallic systems only when the iminic substituent is less bulky Table 6 , entries 2 and 3.
In , Raines et al. RCM of representative dienes catalysed by a—c under air. In , surely inspired by the aforementioned work, the Verpoort group reported a family of indenylidene Schiff base—ruthenium complexes a—f , Fig.
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They combined the higher thermal stability of indenylidene complexes and the tunability and stability of Schiff base ligands. These complexes were able to perform CM and RCM reactions in air with lower catayst loadings compared to a—f , a—f , a—f and a—c. RCM reactions proceeded smoothly using N,N -diallyltosylamide 46 giving, with all catalysts, quantitative yields.
When a more challenging substrate N -allyl- N - methallyl tosylamide, 79 was used, a 24 h reaction time was needed in all cases, with the exception of d Table 8. This remarkable activity higher than Hoveyda—Grubbs 2 nd generation catalyst, 33 was due to the presence of the electron-withdrawing substituents on the Schiff base.
RCM of N -allyl- N - methallyl tosylamide 79 with complexes a—f in air. Although metathesis-type reactions represent one of the most valuable strategies in modern organic synthesis, making this highly valuable tool more accessible and practical for routine use still remains a challenge. Ruthenium-based catalysts have been at the centre of recent advancements making possible their use in air, moreover these catalysts are becoming more and more stable, efficient and economically friendly with time.
As seen in this review, conducting metathesis-type reactions in air, in the presence of water and under high temperature has become more concrete, with several groups leading the charge [ 62 , 86 ]. National Center for Biotechnology Information , U. Beilstein J Org Chem. Published online Oct Karol Grela, Guest Editor. Author information Article notes Copyright and License information Disclaimer.
Corresponding author. Steven P Nolan: moc. Received Jul 21; Accepted Oct This article has been cited by other articles in PMC. Summary Since the discovery and now widespread use of olefin metathesis, the evolution of metathesis catalysts towards air stability has become an area of significant interest. Introduction Transition metal-catalyzed alkene metathesis [ 1 — 10 ], which involves a fragment exchange between alkenes, is nowadays one of the most used strategies for the formation of carbon—carbon bonds.
Review Well-defined ruthenium catalysts Although well-defined early transition metal-based catalysts formed the basis of early metathesis reactions and can be thought of as the forefathers of modern metathesis catalysts [ 27 — 30 ], these all showed poor tolerance towards air and water, because of their high oxophilicity [ 3 , 8 — 9 16 , 27 ].
Open in a separate window. Scheme 1. Scheme 2. Scheme 3. Figure 1. Hoveyda—Grubbs catalyst The next notable evolution in terms of higher catalyst stability came from the Hoveyda group in [ 51 ]. Scheme 4. Scheme 5. Figure 2. Scheme 6. Figure 3. Scheme 7. Figure 4. Figure 5. Figure Figure 6. Figure 7. Figure 8. Figure 9. Table 2 Metathesis reaction in water under air. Table 3 Metathesis reactions catalysed by Scorpio-type complexes in air.
Indenylidene complexes The indenylidene-bearing family of complexes has exhibited a rapid growth in use in recent years and is quickly becoming a mainstream catalyst in metathesis-type reactions Fig. Table 4 RCM with commercially available catalysts in technical grade solvents. Table 5 RCM and ene—yne reactions catalysed by 93a—f and 94 in air. Phosphite-based catalysts In , the Cazin group reported a study on the synthesis and activity of a new family of complexes 98a—d , Scheme 9 [ 84 ]; phophite-based complexes were thus synthesized to evaluate possible positive effects of these ligands in alkene metathesis reactions.
Scheme 9. Schiff bases Schiff bases in metathesis are usually O,N-bidentate ligands and represent an interesting alternative family of ligands as [ 18 , 87 — 94 ]: 1 they can be produced in one high yielding step by condensation of an aldehyde and an amine, thus allowing the fine and facile tuning of ligand and catalyst steric and electronic properties; and 2 the two different donor atoms, O hard and N soft , offer different features and therefore can stabilize, respectively, high and low oxidation states.
Scheme Schiff base—ruthenium complexes synthesized by Verpoort. Table 7 RCM of representative dienes catalysed by a—c under air. Substrate b Product b Solvent substrate conc. Catalyst 0. Conclusion Although metathesis-type reactions represent one of the most valuable strategies in modern organic synthesis, making this highly valuable tool more accessible and practical for routine use still remains a challenge. References 1. Angew Chem, Int Ed. Eur J Inorg Chem. Acc Chem Res.
Chem — Eur J. Adv Synth Catal. Chem Rev. Astruc D. New J Chem. Grela K, editor. Olefin Metatehsis: Theory and Practice. Hoboken, NJ, U. Prog Polym Sci.
Ring Closing Metathesis
Grubbs R H, editor. Handbook of Metathesis. New York, NY, U.
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Mol J C. J Mol Catal A: Chem. Adv Polym Sci. Buchmeiser M R.