Platinum cycloisomerization

Fiirstner and coworkers developed a new Pt- and Au-catalyzed cycloisomerization of hydroxylated enynes 6/4-141 to give the bicylo[3.1.0]hexanone skeleton 6/4-143, which is found in a large number of terpenes [317]. It can be assumed that, in the case of the Pt-catalysis, a platinum carbene 6/4-142 is formed, which triggers an irreversible 1,2-hydrogen shift. The complexity of the product/substrate relationship can be increased by using a mixture of an alkynal and an allyl silane in the presence of PtCl2 to give 6/4-143 directly, in 55 % yield (Scheme 6/4.36). 

Allenes, while arguably underused in synthesis as a whole, have become popular functionalities in cycloisomerization chemistry and provide access to a wide variety of products. Ruthenium, cobalt, platinum, palladium, rhodium, and iridium catalysts are efficient in the transition metal-catalyzed Alder-ene reactions of allenes. 

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)... 

PtCh is also a catalyst of choice to perform the electrophilic activation of enynes towards the formation of cyclopropane derivatives [103,104], The sequence ruthenium-catalyzed C - C bond formation/platinum-catalyzed cycloisomerization has been successfully carried out in one pot at 60 °C to form... 

Platinum(ll)-catalyzed carbocyclization of a.ty-enynes has been intensively researched lately. For instance, 2-disubstituted l-en-6-ynes react with methanol in the presence of PtCl2 as catalyst to form carbocycles with exocyclic aUcenes. The analogous reactions of 3-allyl propargyl ether or 3-allyl propargyl tosylamine in water using the same catalysts will generate cyclopropyl aldehydes besides the product of cycloisomerization (Scheme 82). The coordination of the... 

Pt(II) to the alkyne of the substrate likely triggers all these events. The cycloisomerization might undergo a metallacyclic intermediate that proceeds to eliminate /3-H. The formation of cyclopropanes is presumably succeeded via alkenyl platinum carbene followed by platina(IV)cyclobutane intermediates. The extension using formal metathesis of the enynes includes two transformations, the formation of 1,3-diene moieties and the stereoselective tetrasubstituted aUcene derivatives via O C allyl shift, both leading to diverse structural motifs and serving as the key step in the total synthesis of bioactive targets (Scheme 83). 

Rhodium is a rare white-silvery metal classified as a member of the platinum group metals. As a result, rhodium is commonly used as a catalyst in chemical reactions. It is also used in several chemical feedstock processes, including hydroformylation. Furthermore, rhodium has been used to catalyze more complex processes, such as higher order cycloaddition reactions. In addition, rhodium has been used in simpler reaction types such as hydrogenation and cycloisomerization. The fact that rhodium is effective for a wide range of chemical processes makes it an attractive metal for catalysis. 

An enantioselective version of the gold-catalyzed cyclopropanation was reported by Michelet and co-workers. In the presence of [Au2 (/ )-3,5-/-Bu-4-MeO-MeOBIPHEP Cl2] and AgOTf, 3-oxabicyclo[4.1.0]hept-4-ene derivatives were obtained with excellent enantioselectivities (Scheme 4-26). Both electron-rich and electron-poor aryl groups are tolerated under these conditions. Lower reactivities and stereoselectivities were observed in the corresponding platinum- or iliodium-catalyzed cycloisomerizations. ... 

The cycloisomerization of 1,4-enynes in the presence of platinum(II) catalyst gives 1,2,3-trisubstituted IH-indenes via a [l,2]-alkenyl rearrangement (Scheme 157). ... 

The platinum-catalysed cycloisomerization of ene-yne 6.272 to give diene 6.273 was employed in a synthesis of streptorubin 6.277 (Scheme 6.110). After cyclization, the more electron poor of the two double bonds was reduced using a palladium-catalysed reduction with a tin hydride reagent. The ketone 6.274 was reduced and the resulting alcohol was excised by the Barton-McCombie method. The resulting... 

Scheme 11 Platinum and ruthenium-catalyzed cycloisomerization of l-alkyl-2-ethynylbenzenes...

Functional 1,5-enynes containing a propargyl alcohol motive in their structure can be cycloisomerized in the presence of platinum and gold catalysts following the same endo-dig pathway and H-shift, to directly form [3.1.0]-hexanones (Eq. 7) [41,42]. 

Cycloisomerization of 1,6-enynes in the presence of a dicationic platinum(IV) catalyst has been reported to give five-membered ring systems (Scheme 146) ° ... 

Enantiospecific platinum-catalysed cycloisomerizations of 1,6-enynes give oxabi-cyclo[4.1.0]heptenes and a model for the stereochemical transfer has been proposed (Scheme 148). (g)... 

The cycloisomerization of l, -enynes promoted by gold complex catalysts, are known to go through only the third pathway. This high selectivity is due to two main reasons (1) the fragment [AuL]" has only one vacant site, thus it can not coordinate simultaneously the alkyne and the alkene moieties, (2) oxidative addition processes are not facile for gold complexes [24, 157, 158]. In general, gold(I) complexes surpass the reactivity shown by platinum(ll) and other electrophilic metals for the reaction of enynes. ... 

In addition, 1,7-enynes possessing an acyclic alkene and a haloalkyne moiety II-2 produce cyclobutene compounds via platinum(II)-[6] or ruthenium(II)-cata-lyzed [7] cycloisomerization (Scheme 3.2). 

Scheme 3.5 Synthesis of cyclobutanones via platinum(II)-catalyzed cycloisomerization of 1,7-ene-ynamides II-6...

Cycloisomerizations in Cascade Sequences Cycloisomerizations as the final step of a sequence led mainly to cyclobutenes or cyclobutanes containing an exocyclic double bond. A pioneering study by the Eiirstner s group on a platinum-catalyzed transformation of 1,6-enynes 118 showed the versatile character of the reaction leading to bicycles 119 (Scheme 5.43, eq. 1) or strained tficycles... 

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

Mamane, V., Gress, T., Krause, H., Furstner, A. (2004). Platinum- and gold-catalyzed cycloisomerization reactions of hydroxylated enynes. Journal of the American Chemical Society, 126, 8654-8655. 

Fehr, C., Galindo, J. (2006). Synthesis of (—)-cubebol by face-selective platinum-, gold-, or copper-catalyzed cycloisomerization evidence for chirality transfer. Angewandte Chemie - International Edition, 45, 2901-2904. 

Taking advantage of the rich chemistry of transition-metal-catalyzed cycloisomerization of 1,6-enynes, the electron-rich, conformationally blocked cyclohepta-1,3, 5-triene has been envisioned as a 6-% nucleophilic component [59]. Thus, cycloisomerization of l-(pent-4-ynyl)cyclohepta-l,3,5-trienes in the presence of catalytic amounts of platinum(II) chloride led to a formal intramolecular [64-2] cycloaddition in good to excellent yields [60]. These reactions are conducted at room temperature in toluene as the solvent. A heteroatom in the tether between the unsaturated subunits is tolerated, although in these cases other catalytic pathways were also observed. A mechanism involving cationic intermediates resulting from the nucleophilic attack of the triene on the metal-alkyne moiety has been proposed (Scheme 8.38). The occurrence of ionic intermediates was supported with... 

R. Spina, E. Colacino, J. Martinez, F. Lamaty, Poly(ethylene glycol) as a reaction matrix in platinum- or gold-catalyzed cycloisomerization a mechanistic investigation, Chem. Eur. J. 19 (2013) 3817-3821. 

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