Gold catalysis cycloisomerization

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... 

The development of new procedures for the creation of C-C bonds is of major importance in organic synthesis. In this respect, gold catalysis has emerged as a very efficient synthetic tool, allowing the generally easy and efficient formation of such bonds by addition of various carbone nucleophiles onto alkynes, allenes, and alkenes. These transformations, which can be performed in an intra- or intermolecular manner, are extremely varied. It should however be noted that tlie cycloisomerization of ene-ynes, diene-ynes, or ene-allenes remains the most frequently encountered. A very short selection of such transformations is presented in Scheme 16.16 [15g,k, 20]. 

Keywords Catalysis Cycloisomerization Gold Heterocycles Intermediates Ligands Proton transfer Transmetalation VinyUdene... 

The cycloisomerization of 1,6-enynes is one of the most widely studied and developed reaction within gold catalysis. In the absence of nucleophiles, a variety of products can be obtained (Scheme 1.14) [8, 159-164]. 

SCHEME 7.55 Cycloisomerization of polyene propargylic alcohols under gold catalysis. 

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... 

In a continuation of their work on the synthesis of azaphilone natural products and derivatives [135], Porco and co-workers disclosed in 2012 the synthesis of azaphilone-based chemical libraries [136]. Phenolic ortAo-alkynylbenzaldehydes 260 were cycloisomerized via a gold(lll) catalysis in the presence of TFA to afford... 

Zhang, L., Sun, J., Kozmin, S. A. (2006). Gold and platinum catalysis of enyne cycloisomerization. Advanced Synthesis Catalysis, 348, 2271-2296. 

Odabachian, Y, Gagosz, F. (2009). Cyclobutenes as isolable intermediates in the gold(I)-catalyzed cycloisomerization of 1,8-enynes. Advanced Synthesis Catalysis,... 

It is only in recent years, following what is commonly referred to as the Gold Rush in the scientific literature, that NHC-Au complexes have gained popularity and have seen their potential unfold. The first application in catalysis of a NHC-gold species appeared in 2003, but the real surge in publication only started in 2006. Since then, there is hardly a week without a report on the activity of NHC-Au catalysts in organic synthesis is published. The field is dominated by two main topics of crucial importance in modern catalysis, namely cycloisomerization of polyunsaturated substrates and hydrofunctionalization of 7i-bonds. NHC-Au species have been the subject of several reviews... 

Zhang, L., Sun, J. and Kozmin, S.A. (2006) Gold and platinum catalysis of enyne cycloisomerization. Advanced Synthesis Catalysis, 348, 2271-2296. Khramchikhin, V.A., Dogadina, A.V., Khramchikhin, A.V. and lonin, B.I. (2012) Heterocyclization reaction of chloroacetylenephosphonates with 2-acylamidomalonates into 5-(dialkoxyphosphorylmethylidene)oxazolines. Russian Journal of General Chemistry, 82(4), 776-778 Zhurnal Obshchei Khimii, 82(4), 694-696. 

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