Cyclizations platinum-catalyzed

When the platinum-catalyzed hydrothiolation was performed for acetylenic alcohols, intramolecular cyclization took place to afford a-methylene lactone 25 in up to 67% yield (Eq. 7.19) [30]. 

For the total synthesis of heliophenanthrone, the Dyker group has recently used a platinum-catalyzed cyclization of an o-alkynylbenzaldehyde leading to an isoben-zopyrylium cation which underwent a subsequent intramolecular Diels-Alder reaction to give a diastereomeric mixture of the tricyclic structure [319]. 

Intramolecular nucleophilic additions by nitrogen functional groups onto pendant alkynes and allenes represent an important class of type la approaches to functionalized pyrroles. A platinum-catalyzed (PtCl4) cyclization of homopropargyl azides provided an entry to 2,5-disubstituted pyrroles and 4,5,6,7-tetrahydroindoles (fused pyrroles) <06OL5349>. 

The final cyclization manifold has been realized with a different ruthenium catalyst (Scheme 22). The cationic [Cp Ru(MeCN)3]PF6 induces exclusive endo-dig cyclization of both homopropargylic and bis-homopropargylic alcohols.29 73 The clean reaction to form a seven-membered ring is noteworthy for several reasons intramolecular exo-dig cyclization with bis-homopropargylic alcohols is not well established, the platinum-catalyzed case has been reported to be problematic,80 and the selectivity for seven-membered ring formation over the exo-dig cyclization to form a six-membered ring is likely not thermodynamic. The endo-dig cyclization manifold was thus significant evidence that a re-examination of alkyne hydrosilylation mechanisms is necessary (see Section 10.17.2). 

In addition to the stepwise mechanism, Dautzenberg and Platteeuw proposed another platinum-catalyzed cyclization mechanism (23). This might correspond simply to a disguised stepwise aromatization where the further reaction of unsaturated intermediates is very rapid compared with their desorption. Thus, hydrogen pressure would govern the probability of desorption versus further reaction. Since the cyclization of triene is irreversible, a very low steady-state surface triene concentration must be sufficient to ensure a measurable reaction rate. 

On the basis of a number of precedents, a plausible mechanism for the platinum-catalyzed cyclization/... 

Platinum-catalyzed cyclization of alkylaromatics is faster than the cyclization of paraffins because the presence of the aromatic ring enhances the rate. The rate of dehydrocyclization further increases with the number of aromatic rings in the feed molecule. A comparative study of the de-... 

Platinum(ii)-catalyzed cyclization of 2-alkenyl indoles forms tetrahydrocarbazole 535 and related derivatives (Scheme 108), but all efforts to realize the platinum-catalyzed cyclization-carboalkoxylation of 2-alkenyl indoles have been unsuccessful <2004JA3700>. In an optimized procedure l-methyl-2-(4-pentenyl)indole 534 gave tetrahydrocarbazole 536 in 83% yield as a single regioisomer (Scheme 108) <2004JA10250>. Unprotected, electron-rich, and electron-poor 2-(4-alkenyl)indoles underwent efficient palladium-catalyzed cyclization-carboalkoxylation. 

As can be seen in Equation (99), a platinum-catalyzed intramolecular cyclization was also utilized in the formation of benzo[, ]furan-based tricycles. A platinum carbene was proposed as an intermediate in this reaction <2003JA5757>. 

Platinum-catalyzed cyclization of a 2,3-dihydrofuran to the tethered alkyne provided the fused tricyclic compound 238, as shown in Scheme 71. Acid-promoted benzannulation of 238 then produced the dihydrobenzofuran, presumably via a retro-hetero-Diels-Alder opening of the dihydropyran ring <20040L3191>. 

Recently, a platinum-catalyzed cyclization/elimination was accomplished on heating dienyl acetate (260a) with 3.5 mol % of Pt(PPh3)4 in acetic acid at 80 °C, which furnished the expected cyclopentane (261a) in 84% yield (Scheme 54 Table 17, entry... 

Scheme 53 Platinum-catalyzed a-alkenylation of cyclic ethers based on cascade [l,5]-HT/cyclization...

Intramolecular Hydrosilation of Allyl Amines. The amino group of an allyl amine is silylated with chlorodimethylsilane, and then the resulting silazane is subjected to standard platinum-catalyzed cyclization and oxidation steps to form a syn-2-amino alcohol (eq 1). Exclusive formation of the four-membered cyclic product and the high stereoselectivity should be noted. ... 

Nevado, C., Ferrer, C., Echavarren, A. (2004). New annulations via platinum-catalyzed enyne cyclization and cyclopropane cleavage. Organic Letters, 6, 3191-3194. 

Diphenylacetylene undergoes unusual dimerization forming 1,2,3-triphenyl-azulene upon treatment with aluminum trichloride and sulfenyl chloride (Scheme 7.39) [54]. Although the reaction mechanism is unclear, one benzene ring is expanded into a seven-membered ring. An analogous ring expansion process is observed in a platinum-catalyzed cyclization reaction of arylalkynes [55]. 

Barluenga J, Fernandez A, Di6guez A, Rodriguez F, Fananas FJ (2009) Gold- or platinum-catalyzed cascade processes of alkynol derivatives involving hydroalkoxylation reactions followed by Prins-type cyclizations. Chem-Eur J 15 11660-11667. doi 10.1002/ chem.200900856... 

M. Yoshida, M. Al-Amin, K. Shishido, Synthesis of substituted furans and pyrroles by platinum-catalyzed cyclizations of propargylic oxi-ranes and aziridines in aqueous media. Synthesis (2009) 2454r-2466. 

A platinum-catalyzed intramolecular hydroarylation of ethyl 2-benzyl-2,3-alkadienoates afforded an efficient synthesis of 1,4-dihydronaphthalenes having various alkyl groups on the 4-positions and/or ethoxycarbonyl group on the 2-position, through 6-endo cyclization (Scheme 18.20) [19]. The same Pt-catalyzed hydroarylation of ethyl 2-phenethyl-2,3-butadienoate gave a seven-membered product in 64% yield. 

In 2003, the Uemura group reported a diversity-oriented approach to highly substituted furans based on ruthenium- and platinum-catalyzed intramolecular annulation of propargylic alcohols 59 and ketones 60 (Scheme 19.28) [22]. This reaction is related to the aforementioned pyrrole synthesis (Scheme 19.13). Based on the time profile of the reaction, in which rapid formation/consumption of the alk-4-yn-l-one 63 was seen but only gradual formation/consumption of the diketone 122 was observed, they proposed a mechanism including alkyne hydration and subsequent cyclization of the resulting diketones. However, this reaction can be considered as a furan synthesis by formal reaction between the C=0 and alkyne groups. 

The Iwasawa group developed a platinum-catalyzed generation of unsaturated carbene complexes 325 by cyclization of methoxyalkyne-substituted aniline derivatives 321 and elimination of methoxide as depicted in 324 (Scheme 19.86) [159]. The carbene complexes 325 are useful for [3 + 2] cycloaddition with enol ethers 322, leading to efficient formation of cyclopentene-fused indoles 323. In this case, the indolylmetal species 326 would act as the nucleophile to promote the second cyclization with the oxonium moiety. 

Previously, Hashmi et al. found the gold-catalyzed cyclization of furans bearing a terminal alkyne moiety 75 into the phenol derivatives 76 (Scheme 21.32) [42]. Martin-Matute et al. reported a similar platinum-catalyzed reaction in which platinum-carbene complexes were proposed as reactive intermediates [43]. However, some results of experimental and DFT studies by Hashmi, RabaS, and co-workers indicated... 

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