Organogold intermediates

In intramolecular reactions, both exo- and endo-mode cyclizations may be observed. In a study of chromene formation, endo-cyclization giving the chromene 6.194 was generally the preferred pathway the observation of the 5-exo product 6.193 was attributed to fragmentation of the organogold intermediate 6.191 prior to protonation, to give an allenic intermediate 6.192, followed by cyclization (Scheme 6.90). ... 

A reasonable reaction mechanism involves (a) coordination of the alkynyl moiety to the Au-catalyst (- 101), (b) domino nucleophilic attack ofROH and anti-endo-dig-cycHzation via oxirane oxygen to afford the organogold intermediates 101-103, and (c) protonation of 103 to regenerate the Au-catalyst and to produce dihydrofuran 104 followed by HOAc-elimination to the products 100. 

Carbenes are defined as molecular species with formally divalent and two-coordinate carbon atoms bearing various substituents X and Y and a lone pair of electrons. While the simple representatives are of low stability (such as CH2) and may only appear as short-lived reaction intermediates or in adducts with electron donors, some cyclic systems can be readily isolated. This is particularly true for many of the A-heterocyclic carbenes (NHCs), which are now widely applied as ligands to metals ( Wanzlick-Arduengo carbenes ). Such carbenes based on imidazol and benzimidazol have become the working horses in this branch of organogold chemistry (Scheme 54). 

Stable Au intermediates have been identified in analogous reactions of organogold compounds. See ref. S3, p. 386, and refs, therein. 

Pyrroles almost exclusively add to acetylenes as Ai-centered nucleophiles however, when the acetylene is activated by a strong electron-withdrawing substituent, pyrrole can add to the triple bond as a C-centered nucleophile. " In their gold-catalyzed alkynylation of pyrrole with silyl-protected alkynyl benziodoxolone hypervalent iodine reagent 6, members of the Waser laboratory only saw C-alkynylated products (Scheme 10.2). Using this method, 1/f-pyrrole (1) could be C2-funtionalized to afford 7 in 62% yield, whereas Ai-TIPS pyrrole (8) was selectively transformed into C3-product 9 in 79% yield. In both cases, auration of the heterocycle led to an organogold(III) intermediate that reductively eliminated to afford the alkynylated products. 

The same effect has been observed in the case of an encapsulated organogold catalyst of cycloi-somerization of enyne 441 by Scheme 5.23, and it has been attributed [22] to the hydrophobic environment within the coordination capsule 575, preventing carbenium-ionic intermediates of the catalytic process from their side reactions with water. The same caging catalyst has been used in [23] to perform the combined enzymatic and transition metal catalysis of tandem reactions by Scheme 5.24. The authors of this work also developed a tandem olefin isomerization-reduction reaction by Scheme 5.25 with the encapsulated organoruthenium cation as a catalyst on its... 

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