Hydroxy functionalised complexes

The interest in functionalised carbene complexes of gold(I) derives not only from the longer known biological applications [21-23], but also from a recent interest in their performance in catalysis [14,15,24,25]. Ray et al. tested a series of nonfunctionalised, hydroxy functionalised and hydroxy/keto difunctionalised NHC ligands in the ROP of L-lactide [14]. There is no significant difference in the activities of the two functionalised NHC... 

Figure 4.4 Syntheses of a hydroxy functionalised bis-imidazolium salt and its silver(l) and copper I) complexes.

Once the hydroxy functionalised imidazolium salt is formed, it can be deprotonised and reacted with various metal complexes to form (transition) metal carbene complexes. The hydroxy group ensures that the ligand can be coordinated even to metals that are normally reluctant to form stable carbene complexes. A good example is the deprotonation of a hydroxyethyl functionalised imidazolium salt with potassium hydride [36]. The potassium cation coordinates to the oxygen atom of the alkoxide sidechain and forms cubes as structural elements (see Figure 4.6). The carbene end then coordinates to the respective... 

The titanium(IV) complex just seen is not the only titanium carbene complex generated with this hydroxy functionalised NHC ligand. Arnold and coworkres reacted the potassium salt also with [Ti(thf)3Cl3], a titanium(III) precursor for which they established anew, cost efficient synthesis [39]. The product is a nonsymmetrical titanium(in) complex with octahedral geometry (see Figure 4.8). 

Schwarz et al. [326] synthesised a functionalised bis-imidazolium salt with hydroxy end groups on the wingtips [327,328] and used it in the formation of chelating cw-bis-carbene complexes of palladium(ll) applied as catalysts in the Heck reaction. The functional groups were needed to immobilise the catalyst by attachment to a polymeric support [329] (see Figure 3.104). 

Figure 4.3 Synthesis of a hydroxy cyclohexyl functionalised imidazolium salt and its copperf II) complexes.

Milione et al. reacted the same phenoxyimino functionalised imidazolium salt with trialkyl alanes in an attempt to form the corresponding carbene complex of aluminium(III) [71] (see Figure 4.19). Reaction of the phenoxyimino functionalised imidazolium salt with AIMOj proceeds smoothly with the hydroxy group reacting with the alane under elimination of methane. The imidazolium end does not react under methane elimination, but remains pendant. Aluminium carbene adducts are still rare [72-75] and are usually synthesised by a reaction between the alane and the free carbene. 

A detailed mechanistic study of the reaction between stabilised ylides (42) and 1,2-dioxines has been reported by Taylor and coworkers. This reaction provides a facile route to functionalised cyclopropanes (Scheme 12). The same group has also discovered that addition of chiral 3-ketoiminato (43) or cobalt salen (44) complexes, leads to a catalytic asymmetric ring-opening of me so 1,2-dioxines, affording enantio-enriched cis y-hydroxy enones that react with the stabilised ylides to give enantio-enriched cyclopropanes. ... 

Gagosz reported the 5-endo hydroxy- and alkoi -cyclisation of 1,5-enynes to afford functionalised cyclopentenes. Complex XXIV (1 mol%) was used as catalyst, the reaction proceeded at room temperature and the final products were isolated in moderate to excellent yields. Primary and secondaiy alcohols, phenols and water were successfully used as nucleophiles and incorporated in the products (Scheme 16.67). ... 

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