Donor-acceptor-substituted cyclopropane derivatives and their application in organic synthesis.

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Metal-Assisted Cycloaddition Reactions in Organotransition Metal Chemistry

The formal combination of a carbene and an organometallic fragment yields carbene complexes. They are commonly classified as Fischerand Schrock-type complexes. While Schrock-type compounds (first described in the early 1970s) play an important role in olefin metathesis (acknowledged by the Nobel Prize 20051), Fischer carbene complexes, which were first reported in 19642a and 1965,2b have been developed into powerful reagents for organic synthesis.3 This review concentrates on group 6 metal carbene complexes with a special focus on chromium complexes since, because of their balance of reactivity and stability combined with easy accessibility (see below), they are the most prominent members of the Fischer carbenes and have found the broadest application. While most of the recent chemistry of Fischer carbene complexes has been periodically summarized during the last 5-10 years,4 this review, apart from the basic characteristics of Fischer carbene complexes in the introductory part, will focus on the developments since the mid1990s. The literature is covered up to April 2009.

Fischer Carbene Complexes in Organic Synthesis: Metal-Assisted and Metal-Templated Reactions

Thermal rearrangements of vinylcyclopropanes to cyclopentenes.

The metal carbene complexes, discovered by E. O. Fischer at the start of the 1960s and carrying his name, have since proved themselves to be irreplaceable building blocks for organic synthesis. In particular, since the discovery of the Dotz reaction, a formal cycloaddition of Fischer alpha,beta-unsaturated carbene complexes to alkynes with CO insertion, this area of chemistry has become increasingly interesting to organic chemists. In spite of the considerable diversity of reactions performed with these complexes, proper selection of substrates and careful adjustment of the reaction conditions have allowed, in most cases the perfectly selective preparation of individual compounds of this enormous range of products. The spectrum of new successes begins with the conventional Diels-Alder reaction of alkynylcarbene complexes and the formal regioselective [3+2] cycloaddition of alkenylcarbene complexes to alkynes. It extends much further, however, from cascade reactions with the formation of oligofunctional and oligocyclic products of impressive molecular complexity to complex, formal [3+6] cocyclizations in which six bonds are formed in a single operational step. Beyond doubt, the methodological arsenal of preparative organic chemistry cannot be imagined any more without the valuable transformations of the Fischer carbene complexes; it only remains to be seen whether one or other of the numerous new types of cocyclization products of these complexes can establish itself as a lead structure in the search for biologically active compounds.

Fischer Carbene Complexes as Chemical Multitalents: The Incredible Range of Products from Carbenepentacarbonylmetal alpha,beta-Unsaturated Complexes.

Regioselective and stereoselective synthesis of vinylcyclopropane derivatives from 1,3-dienes and a Fischer carbene complex

The thermal reaction of Fischer carbene complex 1 with electron–deficient 1,3–dienes such as methyl (E,E)-2,4–hexadienoate (2) provided functionalized vinylcyclopropanes like 3 in good yields. Similar results were obtained by employing related unsaturated esters 4, 17, and 21 or nitrile 6. The periselectivity as well as regioselectivity of these carbene–transfer reactions are generally very high, and the diastereomer with the methoxy group cis-positioned with respect to the olefinic moiety is largely favoured (>85:15). A mechanistic rationale of these observations is given. Double adducts were only formed as minor sideproducts in these reactions, but by employing amide 13 as electron–deficient diene they were very easily formed with 1. The [2 + 1] cycloaddition could also be extended to methylcarbene complex 24 and diene 2, but the corresponding vinylcyclopropane 25a was so far only obtained in low yield.

Highly Functionalized Vinylcyclopropane Derivatives by Regioselective and Stereoselective Reactions of Fischer Carbene Complexes with 1,4–Disubstituted Electron–Deficient 1,3–Dienes†

New Surprises with Fischer Carbene Complexes: Formal [3+2] Cycloadditions with and without Preeceding Carben–Ligand Matathesis

Thermal reactions of Fischer carbene complex 1 with trisubstituted electron-deficient 1,3-dienes 7, 8, 23, 25, 32, and 35 provided highly substituted vinylcyclopropanes in good yields. The carbene transfer proceeds highly regioselectively favouring cyclopropanation of the double bond not bearing the ester function. In addition, the diastereoselectivity is generally fairly high in preference of cyclopropanes with the methoxy group cis-positioned with respect to the olefin moiety. The reaction of methylcarbene complex 2 with diene ester 8 displays inversed regioselectivity. These observations are discussed together with solvent effects, and a mechanistic rationale is presented.

On the Regioselectivity and Stereoselectivity of the Carbene Transfer from Fischer Carbene Complexes to Trisubstituted Electron‐Deficient 1,3‐Dienes

Interaction ofα,β-unsaturated Fischer carbene complexes with various siloxydienes yields a wide range of cycloheptadiene and cyclopentene derivatives [Eq (1)]. The exact outcome is determined by the electronic nature of the substituents on both the carbene and the diene. A novel mechanism is proposed for some reactions, in which diene dimerization with fragmentation of a methylene unit was observed. TBS=SitBuMe2.

Reactions of Fischer Carbene Complexes with Siloxydienes: Formation of Cycloheptadiene and Cyclopentene Derivatives—Formal [2+1] Cycloaddition Followed by Cope Rearrangement Versus Formal [3+2] Cycloaddition with or without Preceding Carbene‐Ligand Metathesis

Matthias Buchert

Papers

Regioselective and stereoselective synthesis of vinylcyclopropane derivatives from 1,3-dienes and a Fischer carbene complex

Highly Functionalized Vinylcyclopropane Derivatives by Regioselective and Stereoselective Reactions of Fischer Carbene Complexes with 1,4–Disubstituted Electron–Deficient 1,3–Dienes†

New Surprises with Fischer Carbene Complexes: Formal [3+2] Cycloadditions with and without Preeceding Carben–Ligand Matathesis

On the Regioselectivity and Stereoselectivity of the Carbene Transfer from Fischer Carbene Complexes to Trisubstituted Electron‐Deficient 1,3‐Dienes

Reactions of Fischer Carbene Complexes with Siloxydienes: Formation of Cycloheptadiene and Cyclopentene Derivatives—Formal [2+1] Cycloaddition Followed by Cope Rearrangement Versus Formal [3+2] Cycloaddition with or without Preceding Carbene‐Ligand Metathesis