Ruthenium addition initiated

Two mechanistic pathways, which differed in the way of ruthenium-mediated initial cleavage of formyl C-H or amido N-H bond, were proposed for the catalytic cycle. As shown in Scheme 7.3, an irreversibly cleavage of formyl C-H bond by the active ruthenium complex was followed by reversible insertion of the olefin into the Ru-H bond, which afforded either six-membered or seven-membered ruthenacycle. After reductive elimination, indolin-2-ones or 3,4-dihydroquinolin-2-one was formed. According to isotopic studies, pathway leading to six-membered lactams is postulated to be less favored. Another cyclization process initiated by Ru-catalyzed oxidative addition of formyl N-H bond (Scheme 7.4) was similar to Carreira s proposal for their hydrocarbamoyla-tion reaction of allylic formamides under similar ruthenium catalysis conditions [7]. The 6-endo cyclization process is proposed to be favored under the catalytic system B. 

Developments in the area of initiators for ROMP have resulted in the creation of a large armory of transition-metal compounds suited to these purposes. In addition, while the mechanistic details of Schrock-type initiators are well established, the mechanistic understanding of ROMP-particularly with ruthenium-based initiators-has experienced impressive progress such that today, highly sophisticated polymeric architectures can be produced that quite recently were barely achievable with other polymerization methods. Yet, further progress in both polymer and materials science may well be expected. 

A Belgian patent (178) claims improved ethanol selectivity of over 62%, starting with methanol and synthesis gas and using a cobalt catalyst with a hahde promoter and a tertiary phosphine. At 195°C, and initial carbon monoxide pressure of 7.1 MPa (70 atm) and hydrogen pressure of 7.1 MPa, methanol conversions of 30% were indicated, but the selectivity for acetic acid and methyl acetate, usehil by-products from this reaction, was only 7%. Ruthenium and osmium catalysts (179,180) have also been employed for this reaction. The addition of a bicycHc trialkyl phosphine is claimed to increase methanol conversion from 24% to 89% (181). 

Since Chatt and Davidson13 observed the first clear example of simple oxidative addition of a C—H bond of naphthalene to a ruthenium metal center, Ru(dmpe)2 (dmpe = Me2PCH2CH2PMe2), hydrocarbon activation has been the subject of many transition metal studies.11 c Sometimes, the efforts in this field have ended in findings different from the initial objectives, which have been the starting point for the development of novel organometallic chemistry. 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

A carbazole-functionalized norbornene derivative, 5-CN-carbazoyl methy-lene)-2-norbornene, CbzNB, was polymerized via ROMP using the ruthenium catalyst Cl2Ru(CHPh)[P(C6Hii)3]2 [100]. The polymerization was conducted in CH2C12 at room temperature, to afford products with polydispersity indices close to 1.3. Subsequent addition of 5-[(trimethylsiloxy)methylene]-2-norbornene showed a clear shift of the SEC trace of the initial polymer, indicating that a diblock copolymer was efficiently prepared in high yield. 

Synthesis of block copolymers of norbornene derivatives, with different side groups, has been reported via ROMP [101]. Initially, exo-N-bulyl-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboximide was polymerized in acetone at room temperature with a ruthenium initiator (Scheme 40). The conversion of the reaction was quantitative. Subsequent addition of norbornene derivative carrying a ruthenium complex led to the formation of block copolymers in 85% yield. Due to the presence of ruthenium SEC experiments could not be performed. Therefore, it was not possible to determine the molecular weight... 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

In 1957 (2) platinic, ruthenium, and iridium chlorides were shown to be catalysts leading to very rapid additions, sometimes below room temperature, of many kinds of SiH compounds. These findings initiated much activity, chiefly in industrial research laboratories, in several countries, because they indicated that the manufacture of new organosilicon monomers and many new silicone polymers and copolymers would become commercially practicable for the first time. 

The addition to a double bond is observed in aromatic substrates where the reaction is assisted by chelation. The initial success of such reactions was achieved with the double alkylation of phenol with ethene (Equation (2)).1 This reaction occurs at the or/ -positions selectively by using an orthometallated ruthenium phosphate complex 1. 

A mechanistic pathway is proposed based upon the observed regioselectivities and other results that were obtained during the exploration of the scope and limitations of the Alder-ene reaction.38 Initially, coordination of the alkene and alkyne to the ruthenium catalyst takes place (Scheme 5). Next, oxidative addition affords the metallocycles 42 and 43. It is postulated that /3-hydride elimination is slow and that the oxidative addition step is reversible. Thus, the product ratio is determined by the rate at which 42 and 43 undergo /3-hydride elimination. 

For the synthesis of carbohydrate-substituted block copolymers, it might be expected that the addition of acid to the polymerization reactions would result in a rate increase. Indeed, the ROMP of saccharide-modified monomers, when conducted in the presence of para-toluene sulfonic acid under emulsion conditions, successfully yielded block copolymers [52]. A key to the success of these reactions was the isolation of the initiated species, which resulted in its separation from the dissociated phosphine. The initiated ruthenium complex was isolated by starting the polymerization in acidic organic solution, from which the reactive species precipitated. The solvent was removed, and the reactive species was washed with additional degassed solvent. The polymerization was completed under emulsion conditions (in water and DTAB), and additional blocks were generated by the sequential addition of the different monomers. This method of polymerization was successful for both the mannose/galactose polymer and for the mannose polymer with the intervening diol sequence (Fig. 16A,B). 

Otherwise, the reactions of indenyl-ruthenium(II) allenylidenes [RuCty -CgHy) =C=C=C(R )Ph (PPh3)2][PF6] (R = H, Ph) with ynamines R C CNEtj (R = Me, SiMea) have been reported to yield the alkenyl(amino)allenylidene complexes 41 via insertion of the ynamine into the Cp=Cy allenylidene bond (Scheme 10) [52, 53], This insertion process involves an initial nucleophilic addition of the ynamine at Cy atom of the cumulene, which leads to the cationic alkynyl intermediate complexes 39. Further ring closing, involving the Cp atom, generates the [2+2]... 

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