Silver catalysis cycloisomerization

The chemical reactions possible with silver catalysis are multiple and cover cycloadditions, cycloisomerizations, allylations, aldol reactions, and even C-H bond activation. Also, asymmetric versions are known, even though they still need to be improved.3-10... 

Silver salts or reagents have received much attention in preparative organic chemistry because they are useful catalysts for various transformations involving C-G and C-heteroatom bond formation.309 Especially, the silver(i)/ BINAP (2,2 -bis(diphenylphosphino)-l,T-binaphthalene) system is a very effective catalyst for a variety of enantio-selective reactions, including aldol, nitroso aldol, allylation, Mannich, and ene reactions. Moreover, silver salts are known to efficiently catalyze cycloisomerization and cycloaddition reactions of various unsaturated substrates. Recently, new directions in silver catalysis were opened by the development of unique silver complexes that catalyze aza-Diels-Alder reactions, as well as carbene insertions into C-H bonds. 

Moreover, following the cycloisomerization reaction, a tandem dimerization reaction is also possible on the same substrates under Pd11, Ag1, and Aura catalysis, leading to different substituted furans (4 or 6) depending on the nature of the catalyst used (Scheme 5.5). Indeed, from compound 3 (Scheme 5.5), palladium(II) catalysis led to a 59% yield of 4, whereas silver(I) and gold(III) catalysis led to furans 5 and 6.41... 

Few examples of ene-yne cycloisomerization reactions are seen in the literature. The first results for ene-yne cycloisomerizations were with systems bearing an heteroatom (amine or oxygen) next to the alkene counterpart (forming an enamine or an enol ether). Indeed, Dake s group reported the cyclization of enesulfonamides on alkynes (69-70, Scheme 5.30) under catalysis by platinum and silver salts.85 Catalysis using AgOTf (1 1 mol%) was particularly efficient with systems such as 69 (Scheme 5.30)... 

The activation of allenes is a rather new, but particularly promising area of gold catalysis.381,400 The first example for such a transformation is the cycloisomerization of allenic ketones 480 to furans 482 which probably occurs via intermediate 481 (Scheme 147). Hashmi et /.401,401a showed that this reaction proceeds much faster when gold(m) chloride in acetonitrile is employed as the precatalyst instead of the traditionally used silver salts (cf. Section 9.12.3.2). The products are usually contaminated by substituted furans originating from a Michael addition of aurated 482 to the substrates 480, thereby indicating that the gold catalyst is also capable to activate C-H bonds of furans. 

The cycloisomerization of a-allenyl ketones to the corresponding substituted furans was the first example of a gold-catalyzed addition of an oxygen nucleophile to an allene (Scheme 4-86). Traditionally, silver or palladium catalysts were employed for cyclizations of this type advantages of gold catalysis incluiie shorter reaction times, milder conditions, an or lower catalyst loadings. Variable amounts of... 

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