As a chemist, I am interested in constructing molecules that mimic aspects of biology for pharmaceutical development. Bringing undergraduate students along in this creative endeavor is one of the most rewarding aspects of teaching at Colgate. Asterisks indicate undergraduate student co-authors who conducted their research in collaboration with a faculty member at Colgate. Description of Current Research Project Synthesis of Methylene Isosteres of Glycosyl Amino Acids and Glycosyl Phosphates Glycosylation of proteins and lipids is a post-translational modification important for numerous physiological processes 1.
As well as this general difference in behaviour towards alkenes, first- and second-generation catalyst systems will react differently with some substitution patterns found in substrates.
Type I alkenes react and dimerise quickly, but dimers are consumable; a number of alkenes have been charged to CM reactions as the dimer.
Type II alkenes react and dimerise more slowly, and the corresponding dimers are consumed slowly.
Categorising each CM partner for a given reaction allows the outcome to be predicted. The category into which a given motif falls is a function of the pre-catalyst, with different behaviour observed for molybdenum complexes, and for first- and second-generation ruthenium complexes.
Notably, the reactivity of second-generation complexes varies considerably, depending on the nature of the dissociating ligand and on the identity of the NHC ligand.
The degree of thermodynamic control in a metathesis reaction will also depend on the pre-catalyst system; the issue of thermodynamic control has been reviewed recently by Fogg et al.
This was demonstrated experimentally by Percy and co-workers, who showed that the RCM of 1,8-nonadiene using different loadings of G2 and the ROMP of cycloheptene in the presence of ethene led to the same equilibrium mixture.
While RCM to form smaller ca. Scheme 10 A prototypical cross-metathesis reaction. Cavallo explored the early stages of metathesis with a series of complexes, from initiation to MCB formation, in one of the first DFT studies of metathesis that considered untruncated structures Scheme In addition, the steric pressure exerted by the S IMes ligands destabilised the phosphine-bound and olefin-bound intermediates, leading to more energetically favourable metathesis but not increased initiation rate.
Scheme 11 Structures explored by Cavallo. For second-generation complexes, the energy difference between reactive and unreactive conformers was smaller than for first-generation complexes, explaining the observed reactivity differences. Attempts to optimise the structure of the reactive conformer led to MCB formation.
A later study evaluated the bonding in active ruthenium carbene complexes to rationalise the effect of the NHC on their stabilisation. This is, of course, an interaction that is not available in first generation complexes.
Metathesis catalysts can react with alkynes and allenes, as well as alkenes, allowing elaborate cascade reactions. Sohn and Ihee used time-dependent fluorescence quenching studies to explore the affinity of metathesis catalysts for alkenes, allenes and alkynes.
During enyne metathesis, G1 favoured reaction with the alkene terminus first. Understanding this order of selectivity is important in the design of cascade metathesis reactions.
The use of NHCs as ligands for metathesis pre-catalysts has allowed access to a vast range of interesting catalyst structural motifs, each with different reactivity. More favourable MCB formation in particular leads to an enhancement of catalytic performance.
The highly flexible NHC scaffold has allowed for a wide variety of complexes to be prepared; for example, N-alkyl-N-aryl-bearing NHCs have found application as selective ethenolysis catalysts, 93 and in the preparation of small cyclic oligomers instead of long-chain polymers.
CM reactions seek the selective coupling of two different alkene partners, and therefore the challenge lies in this selectivity. In contrast, the RCM reactions of dienes can be complicated by competing CM processes to produce dimers or polymers.
An understanding of the kinetics and thermodynamics of these processes can help mitigate the impact of these deleterious reactions on RCM. While the concept of effective molarity EM 95,96 Scheme 12 has been applied widely in the study of acid- and base-catalysed nucleophilic ring-closing chemistry, there are relatively few examples of this concept in metathesis chemistry.
The authors also proposed that oligomeric material is an intermediate in the preparation of medium 7, 8 and 9-membered rings.ChemInform Abstract: Metathesis Reactions of Carbohydrates: Recent Highlights in Alkyne Metathesis.
Transformations are catalyzed by ≤ mol % of a Ru catechothiolate complex and afford trisubstituted allylic alcohols and ethers in up to 81% yield and >98% stereoisomeric purity. The method has considerable scope, as olefins containing an alcohol, an aldehyde, an epoxide, a carboxylic acid, or an alkenyl group may be used.
A new approach towards reprocessing cross-linked rubbery materials by catalytic disassembly of polymer chains, which eliminates the need for energy intensive mechanical processes, is demonstrated.
First and second generation (G1 and G2) Grubbs’ ruthenium catalysts break down polybutadiene (PBd) networks at their double bonds via cross-metathesis (CM) reactions to produce readily soluble. In particular, a procedure for employing 3E-1,3-dienes in Z-selective homodimerization and cross-metathesis with terminal alkenes is detailed.
The reaction takes advantage of the pronounced chemoselectivity of a recently reported ruthenium-based catalyst containing a cyclometalated NHC ligand for terminal alkenes in the presence of internal E -alkenes.
ChemInform Abstract: Carbohydrate Carbocyclization by a Novel Zinc-Mediated Domino Reaction and Ring-Closing Olefin Metathesis.
Abstract: A new family of 1,3-dimesityl-4,5-dihydroimidazolylidene-substituted ruthenium-based complexes 9a−c has been prepared starting from RuCl 2 (CHPh)(PCy 3) 2 leslutinsduphoenix.com air- and water-tolerant complexes were shown to exhibit an increased ring-closing metathesis. Analogous metathesis of (triple-bonded) alkynes has been applied as well. Fürstner () reviews recent developments in the continuing optimization of this extraordinarily versatile reaction class. A new approach towards reprocessing cross-linked rubbery materials by catalytic disassembly of polymer chains, which eliminates the need for energy intensive mechanical processes, is demonstrated. First and second generation (G1 and G2) Grubbs’ ruthenium catalysts break down polybutadiene (PBd) networks at their double bonds via cross-metathesis (CM) reactions to produce readily soluble.
Metathesis reactions are chemical reactions in which two hydrocarbons (alkanes, alkenes or alkynes) are converted to two new hydrocarbons by the exchange of carbon–carbon single, double or.