Carbene-catalyzed ring-opening

Remarkable development over the last 10-15 years in the synthesis of well-defined functional-group-tolerant ruthenium carbenes (Grubbs-related catalysts) also caused real development of the metathesis-based reactions in organosilicon polymers. For recent reviews on metathesis of organosilicon compounds see Refs. [6,7]. Unsaturated organosilicon polymers can be synthesized via ruthenium carbene catalyzed ring-opening metathesis polymerization (ROMP) of silylsubstituted cycloalkenes (Eq. 113). 

Equilibrium studies. Acidities of thiazolium cations, such as 213, cannot be measured in basic aqueous solution because the hydroxyl adduct ( pseudobase ) 216 of the thiazolium cation is formed rapidly and is subject to base-catalyzed ring opening (Scheme 36).151 In DMSO, formation of the carbene dimer 217 from 213 and 214 is a complicating factor.8 If indicator anion (In") was added to a solution of 213, a very rapid drop in absorbance was followed by a somewhat... 

Two types of addition reactions involving carbenoid species have been used to synthesize cyclopropanes. The first approach, the reaction of a vinyl carbene species, obtained by a metal-catalyzed ring-opening reaction of cyclopropenes, with alkenes is discussed in Section 1.1.6.3.2., as it is, from a topological point of view, a codimerization of eyclopropene and an alkene. In this case the ring carbons of the cyclopropane are not identical to those of the eyclopropene substrate. 

Ratio (ZiE) 75 25. Both products are apparently derived via a singlet carbene acid-catalyzed ring opening gave only the major isomer. Possibly via a di-7i-methane rearrangement via a spiropentane diradical. 

The enantioseiective hydrogenation of a-amino ketones has been applied extensively to the synthesis of chiral drugs such as the / -agonist SR 58611 (Sanofi Cie). m-Chlorstyreneoxide was obtained via carbene-induced ring closure of the amino alcohol. Epoxide-opening by a chiral amine obtained via a ruthenium-catalyzed hydrogenation of an enamide has led to the desired compound where... 

Metal oxides were also chirally modified and few of them showed a significant or at least useful e.s. Thus, while Al203/alkaloid [80] showed no enantiodifferentiation, Zn, Cu, and Cd tartrate salts were quite selective for a carbene addition (45% e.e.) [81] and for the nucleophilic ring opening of epoxides (up to 85% e.e.) [82], Recently, it was claimed that /(-zeolite, partially enriched in the chiral polymorph A, catalyzed the ring opening of an epoxide with low but significant e.s. (5% e.e.) [83], All these catalysts are notyet practically important but rather demonstrate that amorphous metal oxides can be modified successfully. 

Fig. 3.47. Mechanism of ring-opening metathesis polymerization catalyzed by carbene complexes.

However, the reaction was shown to be catalyzed by a methylidene tungsten-carbene complex rather than the Fischer tungsten carbene complex. They proposed that the reaction would proceed by [2 + 2] cycloaddition of the tungsten carbene complex with the alkyne in Equation (3), ring opening, and another [2 + 2] cycloaddition with the alkene moiety to finally give the cyclized product. 

Strained rings may be opened by a ruthenium carbene-catalyzed reaction with a second alkene following the mechanism of the Cross Metathesis. The driving force is the relief of ring strain. As the products contain terminal vinyl groups, further reactions of the Cross Metathesis variety may occur. Therefore, the reaction conditions must be optimized to favour the desired product. 

The observation that the metal carbene complex, (CO)5W = C(Ph)2 [22], catalyzed the polymerization of cyclic olefins to ring opened polymers containing the diphenylmethylene unit of the catalyst provided additional evidence that carbenes were involved in the catalytic cycle. The formation of the initiating metal carbenes in the classic systems that consist of transition metal halides and alkylating agents was proposed to involve metal alkylation followed by oc-hydrogen loss, Eq. (6). Methane and propene were detected in the early stages of these reactions [23]. 

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