Yannick Borguet

Microwave-Assisted Ruthenium-Catalyzed Reactions

Since the first reports on the use of microwave irradiation to accelerate organic chemical transformations, a plethora of papers has been published in this field. In most examples, microwave heating has been shown to dramatically reduce reaction times, increase product yields, and enhance product purity by reducing unwanted side reactions compared with conventional heating methods. The present contribution aims at illustrating the advantages of this technology in homogeneous catalysis by ruthenium complexes and, when data are available, at comparing microwave-heated and conventionally heated experiments. Selected examples refer to olefin metathesis, isomerization reactions, 1,3-dipolar cycloadditions, atom transfer radical reactions, transfer hydrogenation reactions, and H/D exchange reactions.

Low-catalyst concentration atom transfer radical polymerization of a phosphonium salt-type monomer

Polymeric phosphonium salts are an important class of polyelectrolytes that are currently gaining increasing interest due to their solution properties and reactivity. The successful controlled/living radical polymerization of a phosphonium salt-type monomer, 4-vinylbenzyltriphenylphosphonium tetrafluoroborate (4VBTPPBF4), under initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) conditions employing a low concentration of catalyst is reported. After optimization of the reaction conditions, high monomer conversions to the polymeric phosphonium salt, poly4VBTPPBF4, were reached with very good polymerization control. The living character of the polymerizations was further evidenced via chain extension experiments that yielded either higher molecular weight homopolymers or block copolymers. It is also shown that poly4VBTPPBF4 can be efficiently converted to linear poly(4-vinylstyrene) using the Wittig olefination reaction.

Tandem catalysis of ring-closing metathesis/atom transfer radical reactions with homobimetallic ruthenium–arene complexes

Summary The tandem catalysis of ring-closing metathesis/atom transfer radical reactions was investigated with the homobimetallic ruthenium–indenylidene complex [(p-cymene)Ru(μ-Cl)3RuCl(3-phenyl-1-indenylidene)(PCy3)] (1) to generate active species in situ. The two catalytic processes were first carried out independently in a case study before the whole sequence was optimized and applied to the synthesis of several polyhalogenated bicyclic γ-lactams and lactones from α,ω-diene substrates bearing trihaloacetamide or trichloroacetate functionalities. The individual steps were carefully monitored by 1H and 31P NMR spectroscopies in order to understand the intimate details of the catalytic cycles. Polyhalogenated substrates and the ethylene released upon metathesis induced the clean transformation of catalyst precursor 1 into the Ru(II)–Ru(III) mixed-valence compound [(p-cymene)Ru(μ-Cl)3RuCl2(PCy3)], which was found to be an efficient promoter for atom transfer radical reactions under the adopted experimental conditions.

Assessing the ligand properties of 1,3-dimesitylbenzimidazol-2-ylidene in ruthenium-catalyzed olefin metathesis

The deprotonation of 1,3-dimesitylbenzimidazolium tetrafluoroborate with a strong base afforded 1,3-dimesitylbenzimidazol-2-ylidene (BMes), which was further reacted in situ with rhodium or ruthenium complexes to afford three new organometallic products. The compounds [RhCl(COD)(BMes)] (COD is 1,5-cyclooctadiene) and cis-[RhCl(CO)2(BMes)] were used to probe the steric and electronic parameters of BMes. Comparison of the percentage of buried volume (%V(Bur)) and of the Tolman electronic parameter (TEP) of BMes with those determined previously for 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-dimesitylimidazolin-2-ylidene (SIMes) revealed that the three N-heterocyclic carbenes (NHCs) had very similar profiles. Nonetheless, changes in the hydrocarbon backbone subtly affected the stereoelectronic properties of these ligands. Accordingly, the corresponding [RuCl2(PCy3)(NHC)(=CHPh)] complexes displayed different catalytic behaviors in the ring-closing metathesis (RCM) of α,ω-dienes. In the benchmark cyclization of diethyl 2,2-diallylmalonate, the new [RuCl2(PCy3)(BMes)(=CHPh)] compound (1d) performed slightly better than the Grubbs second-generation catalyst (1a), which was in turn significantly more active than the related [RuCl2(PCy3)(IMes)(=CHPh)] initiator (1b). For the formation of a model trisubstituted cycloolefin, complex 1d ranked in-between catalyst precursors 1a and 1b, whereas in the RCM of tetrasubstituted cycloalkenes it lost its catalytic efficiency much more rapidly.

Controlled radical polymerization of a styrenic sulfonium monomer and post-polymerization modifications

The controlled radical polymerization (CRP) of a sulfonium salt-type monomer, 4-vinylbenzyltetrahydrothiophenium tetrafluoroborate (4VBThtBF4), was investigated under initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) and reversible addition–fragmentation chain-transfer (RAFT) polymerization conditions. A progressive loss of control over the polymerization was observed under ICAR ATRP conditions when an alkyl bromide initiator was used, due to the reaction of the bromine-capped dormant chain ends with tetrahydrothiophene which is formed in the reaction mixture by slow decomposition of the monomer. The use of a chloride initiator (less sensitive towards nucleophiles) leads only to a limited improvement because of the comparatively low initiation efficiency. 4VBThtBF4 can be polymerized in a controlled fashion under RAFT conditions using a tertiary trithiocarbonate chain transfer agent (CTA). The polymerizations proceeded quickly to high monomer conversion (>85% within 5 hours) and afforded materials with narrow molecular weight distribution dispersities (around 1.15). The obtained polymeric sulfonium salt (poly4VBThtBF4) was used successfully as a building block in the synthesis of block copolymers with polystyrene segments. The reaction of poly4VBThtBF4 with sodium benzylthiolate and sodium azide was investigated in detail, and optimized to afford the corresponding polymeric thioethers (poly4VBSBn) and azides (poly4VBN3). Finally, the pendant benzylic methylene groups in poly4VBThtBF4 were deprotonated and the formed polymers with multiple attached ylide functionalities (not isolated) were reacted with benzaldehyde to yield polymers with pendant epoxide groups.

Recent Applications of Alkene Metathesis in Fine Chemical Synthesis

During the last decade or so, the emergence of the metathesis reaction in organic synthesis has revolutionised the strategies used for the construction of complex molecular structures. Olefin metathesis is indeed particularly suited for the construction of small open-chain molecules and macrocycles using crossmetathesis and ring-closing metathesis, respectively. These reactions serve, inter alia, as key steps in the synthesis of various agrochemicals and pharmaceuticals such as macrocyclic peptides, cyclic sulfonamides, novel macrolides, or insect pheromones. The present chapter is aiming at illustrating the great synthetic potential of metathesis reactions. Shortcomings, such as the control of olefin geometry and the unpredictable effect of substituents on the reacting olefins, will also be addressed. Examples to be presented include epothilones, amphidinolides, spirofungin A, and archazolid. Synthetic approaches involving silicon-tethered ring-closing metathesis, relay ring-closing metathesis, sequential reactions, domino as well as tandem metathesis reactions will also be illustrated.

Microwave-Assisted Olefin Metathesis

Since the first reports on the use of microwave irradiation to accelerate organic chemical transformations, a plethora of papers have been published in this field. In most examples, microwave heating has been shown to dramatically reduce reaction times, increase product yields, and enhance product purity by reducing unwanted side reactions compared to conventional heating methods. The present contribution aims at illustrating the advantages of this technology in olefin metathesis and, when data are available, at comparing microwave-heated and conventionally heated experiments

Mono- and Bimetallic Ruthenium-Arene Catalysts for Olefin Metathesis: A Survey

In this chapter, we summarize the main achievements of our group toward the development of easily accessible, highly efficient ruthenium—arene catalyst precursors for olefin metathesis. Major advances in this field are presented chronologically, with an emphasis on catalyst design and mechanistic details. The first part of this survey focuses on monometallic complexes with the general formula RuCl2(p-cymene)(L), where L is a phosphine or N-heterocyclic carbene ancillary ligand. In the second part, we disclose recent developments in the synthesis and catalytic applications of homobimetallic ruthenium—arene complexes of generic formula (p-cymene)Ru(μ-Cl)3RuCl(η2-C2H4)(L) and their derivatives resulting from the substitution of the labile ethylene moiety with vinylidene, allenylidene, or indenylidene units