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Lecture 14 - Nitrenes and Carbenes

Electron dona5ng groups such as alkoxy subs5tuents

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Unformatted text preview: enoids have been developed. Note that a CyclopropanaKon with Rh and Cu carbenoids: Diazo- general cataly5c enan5oselec5ve Simmon- Smith rxn is compounds are common precursors to metal carbenoids s5ll not been developed. Below some examples: (Chem. Rev. 1986, 86, 919) F. Sladojevich Reactivity: C-H insertion C- H inserKon reacKons: (see, Davies Chem. Rev. 2003, 103, 2861 and Doyle Chem. Rev. 2010, 110, 704) Highly valuable conversion that do not depend on " c l a s s i c " f u n c 5 o n a l g r o u p s . S e l e c 5 v e C - H func5onaliza5on is the ideal transforma5on in organic synthesis. In the ideal case it allows to shorten reac?on steps reduce side products. Usually diazo- compounds are the most common carbenoids precursors. Used in the presence of a transi5on metal (most common Rh) and a ligand. •  Reac5vity of diazo- compounds change according to subs5tu5on: Chem 106 The mechanism of these C- H inser5ons is being disputed but it is believed to proceed in a concerted, asychronous fashion, involving a 3- centerd transi5on state. F. Sladojevich Reactivity: C-H insertion Chem 106 I ntramolecular C- H inser?on: forma5on of five membered rings is highly favoured against different ring sizes! 4 and 6 membered rings can be obtained in the presence of appropriate direc5ng groups: C- H ac5va5on preferen5ally occurs at sites that stabilize buildup of posi5ve charge at the carbon undergoing C- H cleavage. Electron- dona5ng groups such as alkoxy subs5tuents direct C- H ac5va5on to the adjacent C- H bond, whereas electron- withdrawing groups such as ester and acetoxy groups are strongly deac5va5ng. Electronic preferences, however, are not totally dominant. Electronic effects may be outweighed by steric and even conforma5onal factors inherent in the substrate...
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