Transition metal complexes Claisen rearrangement

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 catalytic influence of ammonium chloride on the rate of the reaction was discussed by Claisen in his first report19. Since then, numerous catalysts have been introduced to affect rate enhancements of Claisen rearrangements, e.g., Bronsted and Lewis acids, bases or transition metal complexes. The literature concerning catalytic effects in the Claisen rearrangement has been thoroughly covered until 1984 0,122. 

Tetraethynylmethane (39), a potential monomer for a three-dimensional superdiamonoid carbon network [1], was elusive for many years [51, 52], until its synthesis was accomplished in 1993 by Feldman and co-workers [53]. The key step in the synthesis was the acid-mediated Johnson orthoester variant of the Claisen rearrangement, which provided the central quaternary methane C-atom with suitable functional groups for the ultimate transformation into 39 [Scheme 13-9(b)]. Solid 39, like tetraethynylethene (20), decomposes rapidly at room temperature in either the presence or absence of oxygen. The earlier efforts to prepare tetraethynylmethane had yielded the peralkynylated derivatives 40-42 [Scheme 13-9(c, d)] [51, 52]. Tetraethy-nylallene represents another potential precursor for a three-dimensional carbon network [1], but remains elusive of the perethynylated [K]cumulenes, so far only the silyl-protected [3]cumulenes 43a and 43b [Scheme 13-9 (e)] have been prepared [54]. With 44 [Scheme 13-9 (f)], the first transition metal complex of a perethynylated ligand is now available [55]. 

In sharp contrast, the late transition metals are more coordinative to soft carbon-carbon multiple bonds rather than hard oxygen. These binding modes are further classified into mono- and bi-dentate coordinations, depending on the ligands on the metal catalysts, or the substituent pattern in the Claisen diene systems and solvents employed. Bi-dentate coordination of the Claisen substrate is advantageous over the weak mono-dentate coordination of the Claisen rearrangement product, y,d-unsaturated carbonyl compounds, to release the metal complex allowing the catalytic cycle. Furthermore, enantiodiscrimination by chiral late transition metal complexes is based on the discrimination of two enantiotopic diene faces in the enantiomeric six-membered transition states. 

A comprehensive review of the catalysis of the Cope and Claisen rearrangements includes catalysis by transition metal complexes. ... 

Although most catalysts for the various rearrangements are complexes of the late transition metals, TiCLi catalyzes the amino-Claisen rearrangement shown in equation (39). In this case, the thermal rearrangement gives identical prodnct yields. 

The Claisen rearrangement of 2-allylthiopyridine to l-allyl-2-pyri-dinethione is also catalyzed by transition metals. The role of the catalyst, PdCl2(PhCN)2, is particularly important in this case since the reaction is thermodynamically unfavorable, and presumably the product is stabilized by complex formation. When the product is freed from the metal by addition of excess pyridine, it readily reverts to starting material on heating (80JOC5221). 

Transition metal catalysed prenylation. There is a new one-step N-tert-prenylation of indole developed by Baran and co-workers [42] which still outcom-petes the chemoenzymatic approach (Scheme 5). Isobutene (21) as prenyl source is reacted with side-chain Fmoc-protected tryptophan methyl ester 20 in the presence of catalytic amounts of Pd(OAc)2 and superstoichiometric amounts of Ag(I) trifluoroacetate and Cu(II) acetate. The protocol also requires the presence of oxygen. After about 1 day at 35°C, the N-tert-prenylated indole is obtained in a yield of about 60%. The mechanism has not been elucidated, but may involve a 7i-allyl-Pd(II) complex which is coordinated by the indole nitrogen or by C3. In the latter case, a Pd-Claisen rearrangement of a 3-palladated indole would follow. Ag (I) functions as reoxidant of Pd(0). 

In contrast, the late transition metal palladium is more coordinative to soft carbon-carbon multiple bonds rather than hard oxygen. The bidentate coordination is further advantageous over the weak monodentate coordination of y,6-unsaturated carbonyl product to set the catalytic cycle. Recently we reported the (R)-DABNTf-Pd(II) complex as an effective catalyst for asymmetric Claisen rearrangement. (R)-DABNTf-Pd(II) catalyst gave the (RyR)-anti-50 in 83% ee (Scheme 35) [171]. 

Late transition-metal-catalyzed asymmetric Claisen rearrangement takes place in a different mode from that of Lewis-acid-catalyzed Claisen rearrangement Late transition metal catalysis is based on affinity for the Claisen diene system. Among late transition metals, palladium complexes are the most useful and effective for the Claisen rearrangement. 

Transition-metal-catalyzed processes often work nicely for the aromatic Claisen rearrangement. Mechanistic aspects might be different from those under usual thermal conditions. Platinum complexes catalyzed the reaction of allyl 2-naphthyl ether 41 to afford l-allyl-2-naphthol 42 regioselectively in excellent yield [40]. Molybdenum hexacarbonyl also catalyzed the one-pot conversion of allyl aryl ethers to coumaran derivatives such as 43 from 17 [41]. 

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