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A singlet carbene contains an empty p orbital and a roughly sp 2 hybrid orbital that has two electrons. Singlet carbenes add stereospecifically to alkenes, and alkene stereochemistry is retained in the cyclopropane product. [1] The mechanism for addition of a carbene to an alkene is a concerted [2+1] cycloaddition (see figure).
Cyclopropanation is also stereospecific as the addition of carbene and carbenoids to alkenes is a form of a cheletropic reaction, with the addition taking place in a syn manner. For example, dibromocarbene and cis-2-butene yield cis-2,3-dimethyl-1,1-dibromocyclopropane, whereas the trans isomer exclusively yields the trans cyclopropane. [16]
Carbene addition to alkenes. Singlet and triplet carbenes exhibit divergent reactivity. [11] [page needed] [12] Triplet carbenes are diradicals, and participate in stepwise radical additions. Triplet carbene addition necessarily involves (at least one) intermediate with two unpaired electrons.
Wyatt et al. have studied the regioselectivity of the thermal Buchner reaction using Rh 2 (O 2 CCF 3) 4 and demonstrated that the electrophilic character of the rhodium carbene prefers reaction at the more nucleophilic π-bonds of the aromatic ring. [15] regioselectivity. The accepted carbene catalytic cycle [16] was proposed by Yates [17] in ...
Several synthetic routes to cyclopropyl and cyclopropenyl compounds involve the cycloaddition of a metal carbene to an alkene or alkyne. [ 4 ] [ 5 ] [ 6 ] Metal-stabilized allyl and pentadienyl complexes are used in [4+3] and [5+2] cycloadditions for preparing seven-membered rings.
Concerted addition of the metal carbene to the olefin (without direct coordination of the olefin to the metal) generates the observed cyclopropane product. [5] The configuration of the olefin is retained throughout the process; [6] however, metal carbenes with heterotopic faces may generate a mixture of diastereomers, as shown at the right of ...
In this reaction type either the two carbenic intermediates react or a carbenic intermediate reacts with a carbene precursor. [1] An early pioneer was Christoph Grundmann reporting on a carbene dimerisation in 1938. [2] In the domain of persistent carbenes the Wanzlick equilibrium describes an equilibrium between a carbene and its alkene.
The chemistry of the 1,2,3-triazol-5-ylidene system is a much more recently developed field. This system is based on the 1,2,3-triazole ring and had been indicated to have "non negligible lifetimes" in solution as early as 1975. [18] In 2008, 1,2,3-triazolium iodide salts were observed to react with transition metals to form metal-ligand ...