Carbene complexes continued

Despite the unpromising UV-visible spectra and flash photolysis studies, the carbene complexes presented in this chapter have a rich photochemistry at wavelengths exceeding 300 nm. A wide range of synthetically useful transformations has been developed, and continued studies are likely to reveal more. 

Synthetic Applications of Heteroatom-Substituted Carbene Complexes 61 Table 2.19. continued. 

Synthetic Applications of Non-Heteroatom-Suhstituted Carbene Complexes 153 Table 3.18. continued. 

Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes 159 Table 3.19. continued. 

Synthetic Applications of Acceptor-Substituted Carbene Complexes 185 Table 4.5. continued. 

QCqmlfPFg] [C4C1im][Tf2N] Pd-phosphine carbene complex Pr3N 130-150 °C. Arylation of butyl acrylate with iodobenzene in a microflow-system with continuous recycling of the ionic liquid-catalyst phase product extracted with hexane. [77]... 

Investigations into the coordination chemistry of ylides and iminophosphor-anes continue apace, an interesting development being the use of iminophos-phorane fragments as the pendant arms on new pincer ligands which help to stabilise unusual samarium and zirconium carbene complexes. 

Chemists continue to explore ADMET using well-defined carbene complex catalysts, but the use of this mode of metathesis polymerization on a significant industrial scale awaits further research and development.63... 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

Further modification in snpport of the ruthenium carbene complex with ionic liquid media was described by Wakamatsu et al. [54]. The RCM of diethyl diallyl-malonate using a new catalyst was carried out under an argon atmosphere in the presence of 5 mol% catalyst in CHjClj/[bmim]BF (9 1) at room temperature. The starting material was consumed after 0.5 h to provide a product with 94% yield (Scheme 17.8). Continuous fast conversion was observed, and reliable reusability was realized by the fifth cycle. 

To a solution of 0.242 g carbene complex (0.80 mmol) in 50 mL dioxane was added 0.157 g alkyne (0.60 mmol) the solution was heated to 85°C. Once the coupling reaction was complete (disappearance of alkyne by TLC analysis), dimethyl acetylenedicarboxylate (DMAD, 0.20 mL, 1.63 mmol) was added immediately at the same temperature, and the reaction was continued for 2 h. The solvent was removed on a rotary evaporator, and the residue was purified by flash column chromatography on silica gel using EtOAc/hexanes (1 10) as an eluent, to give 0.235 g of final [8-h2] cycloaddition product, in a yield of 76% and 10 mg of [4+2] cycloaddition product (3% yield). 

In continuation of these studies we have used anionic Fischer carbene complexes [5] and the anionic thiocarb3me complex HB(3,5-dimethylpyrazolyl)3(OC)2MO=C-S" [6] as nucleophiles. The latter reaction represents a third method for a rational synthesis of hydrocarbon bridged complexes according to the scheme ... 

An example of a microflow continuous palladium-catalyzed Mizoroki-Heck coupling between iodobenzene with butyl acrylate, in combination with continuous microextraction/ catalyst recyclation was reported by Liu and coworkers (Liu et al., 2004). Their reaction was catalyzed by a [Pd(PPh3)Cl2(BMIM)] carbene complex, which was immobilized in the low-viscosity ionic liquid l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][(CF3S02)2N]). Using automated microflow apparatus (Figure 7), iodobenzene, butyl acrylate, and tripropylamine were introduced from one inlet of the micromixer (channel width 0.1 mm, inner volume 2 mL), and the ionic liquid containing the Pd catalyst was introduced from the other inlet. Two solutions were mixed in the microreactor and were pnimped into the temperature controlled residence time unit. 

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