Five-membered rings Rhodium carboxylates

The first type of application is already described in Chap. 6. The metal elements in the active metal center react easily with CO2 to give carboxylic acid derivatives. For example, the cyclometalation of 2-phenylpyridine as a substrate in the presence of a rhodium compound proceeds easily to give a five-membered ring rhodium intermediate. CO2 can be inserted into the rhodium-phenyl carbon bond, and a methyl ester is then formed from rhodium and a carboxylate through reaction with TMSCH2N2, as shown in Eq. (6.5) [89]. The reaction mechanism is proposed as shown in Scheme 6.2 [89]. 

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazo ketones and esters.215 In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into methine hydrogen is preferred to a methylene hydrogen. Intramolecular insertion can be competitive with intramolecular addition. Product ratios can to some extent be controlled by the specific rhodium catalyst that is used.216 In the example shown, insertion is the exclusive reaction with Rh2(02CC4F9)4, whereas only addition occurs with Rh2(caprolactamate)4, which indicates that the more electrophilic carbenoids favor insertion. 

Intramolecular carbene C-H insertion frequently leads to the formation of five-membered rings [967,990,1021,1113-1128], In particular l-diazo-2-alkanones tend to yield cyclopentanones exclusively when treated with rhodium(ll) carboxylates. The use of enantiomerically pure catalysts for diazodecomposition enables the preparation of non-racemic cyclopentane derivatives [1005,1052,1074,1092,1129]. Intramolecular 1,5-C-H insertion can efficiently compete with 1,2-C-H insertion... 

Since the observation that Rh(II) carboxylates are superior catalysts for the generation of transient electrophilic metal carbenoids from a-diazocar-bonyls compounds, intramolecular carbenoid insertion reactions have assumed strategic importance for C-C bond construction in organic synthesis [1]. Rhodium(ll) compounds catalyze the remote functionalization of carbon-hydrogen bonds to form carbon-carbon bonds with good yield and selectivity. These reactions have been particularly useful in the intramolecular sense to produce preferentially five-membered rings. 

In this respect, the recent finding that intramolecular C-H insertion in long-chain diazoderivatives is smoothly catalyzed by rhodium(II) carboxylates to selectively yield five-membered rings is of major synthetic importance [88],... 

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazoketones and esters. In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into a methine carbon-hydrogen bond is preferred to insertion at a... 

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