Lecture 14 - Nitrenes and Carbenes

Lecture 14 Nitrenes and Carbenes

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Unformatted text preview: carbenes and CO. F. Sladojevich Reactivity: Addition to double bonds Chem 106 q༇  ReacKvity of carbenes Synthesis from hydrazones (Bamford- Stevens ReacKon): Shapiro Org. Rxns. 1976, 23, 405. A way to generate AddiKon to double bonds: This is one the most common carbenes from tosylhydrazones. Is a good alterna5ve to the reac5ons of carbenes and carbenoids. direct decomposis5on of diazocompounds, that are onen unstable and poten5ally explosive. Stability and synthesis of diazo- compounds: alkyl- diazocompounds are generally unstable and poten5ally explosive when not in solu5on. Aryl- subs5tuted diazo- compounds are more stable than alkyl diazo- compounds and carbonyl- diazo- compounds are even more stable. Ethyl diazo - acetate is yellow liquid commercially available. The outcome of the reac5on change if the carbene reacts as singlet or triplet. •  Singlet: Stereospecific reac5on. E- alkenes: trans- cyclopropanes / Z- alkenes: cis cyclopropanes. •  Triplet: mixture of cis/trans cyclopropanes With singlet carbenes the process is concerted and therefore stereospecific. (see, Fleming pag. 95- 96) the reac5on is classified as chelotropic reac?on, a cycloaddi5on where, on one component, both new bonds are made to the same atom. If the carbene approach the olefin in a straight line, the HOMO/LUMO interac5ons are an5bonding. The carbene approaches the double bond sideways. F. Sladojevich Reactivity: Addition to double bonds Chem 106 Simmons- Smith ReacKon: Cyclopropana5on of alkenes via Zn carbenoids. When the carbene reacts in the triplet state the mechanism is not concerted but radical. The diradical that forms aner ini5al addi5on of the carbene can not form the cyclopropane. A spin inversion must occur and this process is slower than rota5on around the σ- bond. F. Sladojevich Reactivity: Addition to double bonds Chem 106 Several cataly5c enan5oselec5ve cyclopropana5on using Rh or Cu carb...
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This document was uploaded on 02/27/2014 for the course CHEM 106 at Harvard.

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