Catalysis, 2,3 -sigmatropic

There arc no methods for the preparation of azepines by ring transformation of other seven-membered systems however, interconversion between the four tautomeric azepines, either by [1,5]-H sigmatropic shifts or under base catalysis, is common and almost always results in formation of a 3//-azepine. Such transformations are dealt with in Section 3.1.1.5.7. 

Various crown ethers have been constucted onto the 3- and 4- positions of coumarin by way of 3-diazo-2ff-lbenzopyran-2,4(3ff)-dione. Cafbenoid generation under Rh-catalysis is followed by formation of an oxonium ion by reaction with the ether solvent and a sigmatropic rearrangement completes the sequence <99T6577>. 

Stereoselective oxonium ylide reaction, in particular the asymmetric catalysis, has been a problem of considerable challenge in this field. Since the first report by McKervey and co-workers in the asymmetric induction in metal carbene-mediated ylide formation/[2,3]-sigmatropic rearrangement in 1992, there have been efforts being directed... 

Further study by Katsuki, McMillen, Hashimoto, and Wang improved the enantioselectivity up to a moderately high level.Wang and co-workers further extended the asymmetric catalysis to the [2,3]-sigmatropic rearrangement of propargyl sulfonium ylide to give allenic products with up to 81% ee (Equation (18))." ... 

Irradiation of 3,3-diphenylthiooxindole gives 9-phenylthioxanthene (72T597). It is likely that this transformation involves successive cheletropic extrusion of CO, electrocyclization and a photolytic 1,3-sigmatropic shift of hydrogen (Scheme 152). The last step can also be brought about by base catalysis. 

The asymmetric oxidation of sulphides to chiral sulphoxides with t-butyl hydroperoxide is catalysed very effectively by a titanium complex, produced in situ from a titanium alkoxide and a chiral binaphthol, with enantioselectivities up to 96%342. The Sharpless oxidation of aryl cinnamyl selenides 217 gave a chiral 1-phenyl-2-propen-l-ol (218) via an asymmetric [2,3] sigmatropic shift (Scheme 4)343. For other titanium-catalysed epoxidations, see Section V.D.l on vanadium catalysis. 

Pericyclic processes comprise a broad and important class of concerted reactions of both theoretical and practical interest. These transformations, which are especially useful in the construction of carbon-carbon bonds,93 include electrocyclic reactions, sigmatropic rearrangements, and cycloadditions. Because they are not typically subject to general acid-general base chemistry but can be highly sensitive to strain and proximity effects, they are attractive targets for antibody catalysis. 

Although acyloxy, phosphatyloxy, and tosyl allenones were obtained from the corresponding propargyl alcohol derivatives via silver catalysis through an overall process that resembled [3,3]-sigmatropic rearrangement (see Sections 3.41 and 3.43), the mechanism was not fully proved and is still in question. 

Enantioselective catalysis has also been used for the synthesis of optically active sulfimines [51]. By application of 5 mol % of the bisoxazoline-copper(I) catalyst 80, the sulfide 77 is oxidized catalytically to 78 which undergoes a [2,3]-sigmatropic rearrangement to give allyl amine 79 in 80% yield and with 58% ee (Eq. (20)). Other alkenes were found to give lower ee. 

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