Metal-Catalyzed Sigmatropic Rearrangements

Many metals catalyze sigmatropic rearrangements including several that likely function by it-activation such as mercury, palladium, platinum, and, most recently. 

Understanding OrganometaUic Reaction Mechanisms and Catalysis Computational and Experimental Tools, First Edition. Edited by Valentine P. Ananikov. 

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Cope Rearrangements The metal-promoted sigmatropic rearrangement of 1,5-hexadienes also known as the Cope rearrangement (X=Y=X=C Eq. (5.1)) has been widely studied [3]. Overman and Knoll achieved the first Pd(II)-catalyzed... 

Keyword Carbocycles a Cascade Reactions a Cycloadditions a Combinatorial Chemistry a Domino Reactions a Enantioselective Transformations a Ene Reactions a Eieterocydes a Natural products a Preservation of Resources and Environment a Sigmatropic Rearrangements a Tandem Reactions a Transition Metal-Catalyzed Transformations... 

The close association between metal ions and p-benzoquinones catalyzes their Diels-Alder reactions with anthracenes. The efficiency of the metal cations correlates with their Lewis acidity171. A mechanism proceeding via radical-anions for a [3,3] sigmatropic rearrangement was established172. 

The [3,3]-sigmatropic rearrangement of a 1,5-hexanediene is known as the Cope rearrangement and usually proceeds through a chair transition state. Generally, a large substituent at C-3 (or C-A) prefers to adopt an equatorial-like confirmation.303 304 As the reaction is concerted, chirality at C-3 (or C-4) is transferred to the new chiral center at C-l (or C-6). The reaction can be catalyzed by transition metals.305 The use of a palladium catalyst allows for the reaction to be conducted at room temperature instead of extremely high temperatures (Scheme 26. lO).306-307... 

These results clearly demonstrated that, depending on substrates, apparent [3,3]-sigmatropic shifts actually resulting from 1,2 shifts could occur. Further studies are thus clearly needed to better understand the mechanism of apparent sigmatropic rearrangements catalyzed by silver and other metals. 

The Cope- and Claisen rearrangements are also catalyzed by square-planar transition metal complexes. Palladium complexes have been responsible for spectacular increases in the rates of Cope rearrangements by factors of up to 10 . Certain other sigmatropic rearrangements are also catalyzed by transition metal complexes. 

The term sigmatropic rearrangement is used here only in a formal sense, i.e., for a process described by the formulae above, without implication of reaction mechanism. Therefore, thermal, metal-catalyzed and even radical-induced rearrangements, which fulfill this requirement, are included in this section. However, with respect to nomenclature, sigmatropic rearrangements are described in the way introduced by Woodward and Hoffmann, i.e., by stating the number of atoms from the cleavage site to the centers where the new n-bond will be formed. 

In 1995 the first examples of enantioselective, metal-catalyzed [2,3]-sigmatropic sulfurylide rearrangements appeared (Scheme 96) [225]. When -cinnamylphenyl sulfide 285 was reacted with ethyl diazoacetate in the presence of a catalytic amount of CuOTf and bisoxazohne 394 [226], a di-astereomeric mixture close to 1 1 of homoallyhc sulfides 393 was produced with 20% ee for the major diastereomer (configuration was not assigned). 

Similarly to the mechanism of the Claisen rearrangement, the Overman rearrangement is a suprafacial, concerted, nonsynchronous / 3,37-sigmatropic rearrangement. The reaction is irreversible, which is the result of the significant driving force associated with the formation of the amide functionality. The mechanism of the metal catalyzed reaction is believed to proceed via an iminomercuration-deoxymercuration sequence and it is only formally a [3,3]-sigmatropic shift. 

Nagashima, H., Wakamatsu, H., Ozaki, N., Ishii, T., Watanabe, M., Tajima, T., Itoh, K. Transition metal catalyzed radical cyclization new preparative route to y-lactams from allylic alcohols via the [3.3]-sigmatropic rearrangement of allylic trichloroacetimidates and the subsequent ruthenium-catalyzed cyclization of N-allyltrichloroacetamides. J. Org. Chem. 1992, 57, 1682-1689. 

Prior chapters have covered the use of transition metals in asymmetric hydrogenations ( 6.2 and 7.1), hydroborations ( 7.3), hydrosilylations and hydro-cyanations ( 6.3, 6.4, 7.4 and 7.5), cyclopropanations ( 7.19), aldol reactions ( 6.11), allylations of carbanions ( 5.3.2), and some sigmatropic rearrangements ( 10.3). This chapter covers other reactions catalyzed by transition metal complexes including coupling of organometallic reagents with vinyl, aryl or allyl derivatives, Heck reactions allylamine isomerizations, some allylation reactions, car-bene insertions into C-H bonds and Pauson-Khand reactions. 

Certain other sigmatropic rearrangements are also catalyzed by transition metal complexes. 

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