Carbene catalysts

The 1-arenesulfonylprolinate catalysts have been studied computationally.209 A computed TS and conceptual model that is consistent with experimentally observed enantioselectivity is shown in Figure 10.11. The arenesulfonyl groups block one of the directions of approach to the carbene catalyst and also orient the alkene substituent away from the metal center. 

The first examples of microwave-assisted cross-couplings with organozinc compounds were recently reported [47]. In addition, the first high-speed synthesis of aryl boronates (Suzuki coupling reactants) has been performed under the action of single-mode irradiation with an in-situ-generated palladium carbene catalyst [48],... 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

Although the bulk of this review is concerned with well-defined metal carbene catalysts, it is important to note the contributions made to cross-metathesis chemistry by ill-defined or multicomponent catalysts. A brief discussion of the cross-metathesis reactions of functionalised alkenes using catalysts of this type will therefore be included here [1]. 

With the development of an analogous ruthenium benzylidene catalyst 17 by Grubbs and co-workers in 1995, a ruthenium carbene catalyst suitable for the cross-metathesis reaction was in place [34]. Benzylidene 17 exhibited the same impressive tolerance of air and moisture, and the same stability towards functional groups as its predecessor 4, but benefited from easier preparation [35,36] and much improved initiation rates. 

The drawback of the CVD method is eliminated in ROMP, which is based on a catalytic (e.g., molybdenum carbene catalyst) reaction, occurring in rather mild conditions (Scheme 2.3). A living ROMP reaction ofp-cyclophanc 3 or bicyclooctadiene 5 results in soluble precursors of PPV, polymers 4 [31] and 6 [32], respectively, with rather low polydispersity. In spite of all cis (for 4) and cis and trans (for 6) configuration, these polymers can be converted into aW-trans PPV by moderate heating under acid-base catalysis. However, the film-forming properties of ROMP precursors are usually rather poor, resulting in poor uniformity of the PPV films. 

The mechanism involves a [2 + 2] cycloaddition reaction between an alkene and a transition metal carbene (Scheme 10.13). In the absence of a transition metal carbene catalyst, the reaction between two alkenes is symmetry forbidden and only takes place photochemically. However, the d-orbitals on the metal catalyst (typically Grubbs s catalyst as shown in Scheme 10.13), break the symmetry and the reaction is facile. 

Figure 16.16. The first isolated ruthenium carbene catalyst...

Recendy, we found that A -allyl-o-vii rlaniline 44 gave 1,2-dihydroquinoline 45 by normal RCM and developed silyl enol ether-ene metathesis for the novel synthesis of 4-siloxy-1,2-dihydroquinoline and demonstrated a convenient entry to quinolines and 1,2,3,4-tetrahydroquinoline [13], We also have found a novel selective isomerization of terminal olefin to give the corresponding enamide 46 using rathenium carbene catalyst [Ru] and silyl enol ether [14], which represented a new synthetic route to a series of substituted indoles 47 [12], We also succeeded an unambiguous characterization of mthenium hydride complex [RuH] with ACheterocyclic carbene... 

Diastereoselective hydrogenations of this type have been reported by Burgess and coworkers [54—59] using chiral-protected and -unprotected allylic and homo allylic alcohols as substrates with their carbene catalyst lr(9). Catalyst control was found to be dominant, but depending on the position and nature of the oxygen substituents, moderate to strong match/ mismatch effects were observed. 

Triene 257 was transformed into the racemic cyclopentenone 258 by a ring closing metathesis using Grubbs iST-heterocyclic carbene catalyst (102) [55]. 

The Tins group used Grubbs AT-heterocyclic carbene catalyst (102) for the ring closing metathesis of the triene 262 (Eq. 4) [55, 133] ... 

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