Alkoxycarbene ligands, formation

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

When the ruthenium precursor contains an electron-rich phosphine such as tris(p-methoxyphenyl)phosphine, and if a large excess of this ligand is used in the catalytic reaction, the formation of valerolactones by oxidation of a postulated transient alkoxycarbene ruthenium moiety is favored (Scheme 10) [20]. 

The formation of the products 15 and 16 obviously involved complex, multistep reactions, and proposing a meaningfijl mechanistic sequence would require additional work. Nevertheless, the outlined results demonstrate several points, including the deprotonation of N-alkylimidazole at C5 and subsequent formation of abnormal NHCs, the formation of alkoxycarbenes via intramolecular attack of a C2-deprotonated N-alkylimidazole ligand to a CO coHgand, the CO activation by the attack of two C2-deprotonated N-alkyhmidazole ligands, and the dramatic effect of the employed strong base on the nature of the products. 

However, in the presence of (cyclopentadienyl)ruthenium complexes bearing an electron-rich ligand such as tris(p-methoxyphenyl)phosphine in the presence of a large excess of the same ligand (catalytic system M), the selective formation of lactones was obtained. The elimination of the organic ligand as a six-membered lactone was made possible by oxidation of an intermediate cyclic alkoxycarbene-metal with A -hydroxysuccinimide, a mild oxidant which did not destroy the catalyst (Scheme 26) [110]. 

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