Carbenes, coupling rearrangements

Sect. 2.1.1) and [3C+2S] cyclopentene derivatives. The product distribution can be controlled by choosing the appropriate reaction conditions [72]. Moreover, the cyclopentene derivatives are the exclusive products from the coupling of fi-pyrrolyl-substituted carbene complexes [72b,c] (Scheme 25). The crucial intermediate chromacyclobutane is formed in an initial step by a [2+2] cycloaddition. This chromacyclobutane rearranges to give the rf-complex when non-coordinating solvents are used. Finally, a reductive elimination leads to the formal [3C+2S] cyclopentene derivatives. 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

One can correct the observed distribution of 1,2-C and 1,2-H products so as to reflect only the carbenic pathway.28 The corrected 1,2-C/1,2-H ratio (4.8), coupled with a pyridine ylide absolute rate constant for overall carbene rearrangement (kc + % = 6.8 x 107 s 1), gives the partitioned rate constants kc = 5.6 x 107 s 1 and ifeH = 1.2 x 107 s-1.28 The dominance of 1,2-C over 1,2-H in this system will be discussed below. 

The reaction of two equivalents of W(C=CC=CH)(CO)3Cp with Ru3(CO)io (NCMe)2 gives the RU3W cluster 149 (Scheme 30), which is also obtained from 135 and W(C=CC=CH)(CO)3Cp. The extended organic ligand is formed by coupling of two molecules of the diynyl complex with two of CO, to form a cyclopen-tadienone attached by a carbenic interaction to the cluster W atom, and featuring formylethynyl and C=CW(CO)3Cp substituents. " One of the elementary steps in the reaction mechanism may involve formal rearrangement of the diyne to a dicarbyne. 

Carbene 132 is implicated in the photolysis of 1 since the observed289 photodimerization to 9,10-dihydrophenanthrene and -anthracene is best explained by head-to-head and head-to-tail coupling of this species. Moreover, the fact that allene 134 is isolated289,290 as the major product from irradiation of diesters 31 (equation 35) is fully consistent with a photo-Wolff rearrangement of the carbene. The minor product here involves cyclization... 

Reactions of alkynyliodonium salts 119 with nucleophiles proceed via an addition-elimination mechanism involving alkylidenecarbenes 120 as key intermediates. Depending on the structure of the alkynyliodonium salt, specific reaction conditions, and the nucleophile employed, this process can lead to a substituted alkyne 121 due to the carbene rearrangement, or to a cyclic product 122 via intramolecular 1,5-carbene insertion (Scheme 50). Both of these reaction pathways have been widely utilized as a synthetic tool for the formation of new C-C bonds. In addition, the transition metal mediated cross-coupling reactions of alkynyliodonium salts are increasingly used in organic synthesis. 

Abstract Major advances in transition-metal catalyzed reactions have taken place since the discovery of N-heterocydic carbenes (NHCs). This review provides a summery of recent M-NHC catalyzed reactions including cycloadditions, rearrangements, coupling reactions, polymerizations, and the additions of H-X. 

Coupling of a Fischer carbene complex with an alkene can generate a vinylcarbene intermediate 12 via an insertion-rearrangement reaction, which can then further react with a double bond. For intramolecular reactions of tethered enynes 10, the products formed are bicyclic cyclopropanes 14 intermolecular reactions lead to cycloalkenylcyclopropanes. 

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