Lecture 14 - Nitrenes and Carbenes

Some nhcs are stable solids at room temperature and

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Unformatted text preview: e structures: It is evident from the resonance structure above that carbenes subs5tuted with π- donors ligands will display a nucleophilic character and are onen referred as nucleophilic carbenes. On the other hand, carbenes susb5tued with π- acceptors ligands will display an electrophilic character and are called electrophilic carbenes. Stability: Most carbenes are highly reac5ve species, prac5cally all having life5mes considerably under 1 s. Most carbenes have been isolated only by entrapment in matrices at low temperatures (77 K or less). In many cases the evidence of carbene forma5on is deducted by the product obtained in the reac5on. F. Sladojevich Generation of Carbenes There are some excepKons to this, the most notable are N- Heterocyclic Carbenes (NHC’s). Some NHC’s are stable solids at room temperature and can be stored undefinetly under nitrogen atmosphere. NHC’s are extremely important and have many applica5ons as ligands in organometallic chemistry and as organocatalysts. Will be treated in detail in a separate lecture. q༇  Synthesis of carbenes Synthesis from Diazo compounds: Diazocarbonyl compounds can be decomposed to carbenes by heat or light. The forma5on of stable nitrogen compensates for the forma5on of the unstable carbene. Chem 106 A more common way of decomposing diazocompounds In organic chemistry is the use of transi5on metals, such as Rh and Cu. In these cases metal- carbene complexes with similar reac5vity to the free- carbenes are obtained (carbenoids). Synthesis via α- eliminaKon: Common method for the prepara5on oh halo- carbenes . For example dichlorocarbene can be obtained trea5ng CHCl3 with hydroxide. Synthesis from ketenes: on hea5ng or irradia5on ketenes can decompose to...
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