Hydroxyl-carbene complexes

Youngs and co-workers extended the chemistry toward synthesizing water-soluble silver carbene complexes 68a-68c (Figure 28) by using hydroxyl-containing pincer-type iV-heterocyclic carbenes as ligands.1 4 It is interesting... 

A bromoallene was demonstrated to act as an allyl dication equivalent. When treated with Pd(0) in an alcoholic solvent, an ei-hydroxybromoallene provides a mediumsized heterocycle (Scheme 16.101) [106]. The oxidative addition of a bromoallene to Pd(0) generates an allenylpalladium species, which is successively transformed into a Jt-allylpalladium complex through the attack of the hydroxyl group on the sp carbon followed by the protonation of the resulting Pd-carbene complex. Finally, the products are provided as a mixture of regioisomers by the nucleophilic attack of the external methanol. 

When an isonitrile ligand containing a suitable hydroxyl group was reacted with AUCI4, it underwent isomerization (147) via a carbene complex (Scheme 1). With CN(CH2)3OH, in the presence of a noncoordinating anion, the bis(carbene)gold(I) complex [Au tO(CH2)3l(jH 2]+BPhJ was formed. 

The conventional vinylidene complex could be isolated when the hydroxyl group was protected as the tetrahydropyranyl ether derivative reaction of this with acid immediately gave the cyclic carbene complex, even under mild conditions. The reaction is related to the formation of similar nickel(II)- and platinum(IV)-carbene complexes from the [Pg.71]

Most of the work has been done with RUCI3, OSCI3 or IrCl3 as catalysts at 50-80 °C in water, aqueous emulsion, an aromatic solvent, or mixtures of an alcohol and water. Tungsten or molybdenum carbene complexes in toluene are effective at 20 °C with monomers that do not contain hydroxyl groups. Thus 8W (R = Me) gives polymers of very high... 

Generally, arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-l-naphthols, with the more hindered substituent ortho to the hydroxyl group . Upon treatment with alkynes, aryl(dialkylamino)carbene chromium complexes do not yield aminonaphthols, but they form indene derivatives . Vinyl(dialkylamino)carbene complexes, however, react with alkynes to yield aminophenols as the main products The solvent is one of the many factors that affects this type of reaction, for which the most important is the polarity and/or coordinating ability of the solvent. The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. 

Bridged calix[4]arenes in which two phenolic hydroxyl groups are derivatized as 4-pentenyl ethers undergo RCM in the presence of a carbene complex 2." Another impressive result concerns the construction of a cylindrical tricyclic peptide. This compound contains a 38-membered ring and is formed in 65% yield on exposure to 1. The two-step reaction involves RCM in the final stage. 

The metabolism of aryldioxymethylene compounds to catechol, carbon monoxide and formic acid is consistent with hydroxylation of the carbene complex (Scheme 3) [5],... 

Protons attached to the a-carbon atom in metal-carbene complexes undergo rapid base-catalyzed hydrogen-deuterium exchange with hydroxylic solvents (Kreiter, 1968). For example, the half-life for exchange of the a protons in VII in acetone-D20 with no added base is 23 min at 40° (Casey, 1970). This rapid exchange implies the existence of an intermediate carbanion. These... 

Fiirstner and coworkers developed a new Pt- and Au-catalyzed cycloisomerization of hydroxylated enynes 6/4-141 to give the bicylo[3.1.0]hexanone skeleton 6/4-143, which is found in a large number of terpenes [317]. It can be assumed that, in the case of the Pt-catalysis, a platinum carbene 6/4-142 is formed, which triggers an irreversible 1,2-hydrogen shift. The complexity of the product/substrate relationship can be increased by using a mixture of an alkynal and an allyl silane in the presence of PtCl2 to give 6/4-143 directly, in 55 % yield (Scheme 6/4.36). 

Although the ruthenium allenylidene complexes 2 have not yet been as comprehensively studied as their carbene counterparts, they also seem to exhibit a closely related application profile [6]. So far, they have proven to tolerate ethers, esters, amides, sulfonamides, ketones, acetals, glycosides and free secondary hydroxyl groups in the substrates (Table 1). 

Although the C=C bond of allyl alcohols is frequently less susceptible to reaction with dihalocarbenes, insertion of the carbene into the C=C bond invariably occurs (Table 7.4) to the exclusion of reaction at the hydroxyl group (see Section 7.5) [98]. A complex mixture of products is obtained from the reaction of dichlorocarbene with allyl alcohol, but the cyclopropane can be obtained in high overall yield (>70%) via... 

The link between the dioxymethylene function and P450 inhibition, the requirement for catalytic activation of the inhibitor, and the fact that the dioxymethylene group is oxidized implicate this function in the inhibitory events. An inhibitory role has been postulated for free radical, carbocation, and carbanion intermediates, but formation of the carbene from the bridge-hydroxylated metabolite or from its radical precursor is most consistent with the results (Figure 7.12). Substituents other than an alkoxy group on the dioxymethylene group suppress complex formation The retention of... 

Whereas the reactions included in the second group require direct contact between a molecule of the C-H compound and the metal complex (albeit via the ligand). In the processes belonging to the third type, the complex activates initially not the hydrocarbon but the other reactant (e.g H2O2 or O2) The reactive species formed (a carbene or radical, e.g hydroxyl radical) attacks then the hydrocarbon molecule without any participation in the latter process of the metal complex which has generated this species. Oxidation of alkanes by Fenton s reagent [4a-c] is an example of a such type process ... 

---