PtCl₂‐Catalyzed [6+2] Cycloaddition of Alkynes Tethered to Cycloheptatriene

Abstract

Transition-metal-catalyzed cycloisomerization reactions of enynes have attracted considerable attention because of their high synthetic potential for the construction of useful carbo-and heterocyclic structural features.[1] Important advances in this timely area of research have been achieved using late-transition metals (for example, Ru, Pt, and Au) as p-acid catalysts.[2] In this context, novel modes of connection between alkenes and alkynes still remain an attractive field of investigation, although the chemoselectivity of the reactions often depends on the structural patterns of the substrates. The cycloisomerization of polyene-ynes takes advantage of additional unsaturated bond(s) to trap transient reactive intermediates, thus allowing the elaboration of complex polycycles. For example, the gold-catalyzed cycloisomerization of dienynes affords Diels-Alder adducts, [3] whereas cyclohexadienyl alkynes generate tetracycles. [4,5] In the particular case of aryl-tethered alkynes, the cycloisomerization with various electrophilic metal catalysts afforded Friedel-Crafts-type compounds, but these reactions generally required electronrich arenes.[6] As part of our recent studies on the cobaltcatalyzed intermolecular [6+2] cycloaddition of cycloheptatriene (CHT) with alkynes, [7] we were intrigued by the behavior of CHT tethered to alkynes in cycloisomerization reactions involving p-acid catalysts. In these reactions, nucleophilic attack of CHT on an electrophilic metal-coordinated alkyne B is expected to generate a pentadienyl cation C which may evolve through ring closure to produce [2+2], [4+2], and/or [6+2] adducts (Scheme 1).Herein, we report the platinum-catalyzed cycloisomerization of 1-(pent-4-ynyl)-1,3,5-cycloheptatrienes, in which connections are made between the carbon termini of the triene and the acetylenic carbon atoms, to afford tricyclotrienes F, similar to a formal intramolecular [6+2] cycloaddition.Exposure of triene-yne 1 a to catalytic amounts of PtCl 2 (5 mol %) in toluene at room temperature resulted in a complete and clean conversion into a single adduct 2 a [8] in 94 % yield (Table 1, entry 10). The cycloisomerization proceeded with excellent chemoselectivity, whereas heating at 80 8C in toluene in the absence of catalyst resulted in the recovery of unchanged 1 a (Table 1, entry 1). To the best of our knowledge, intramolecular [6+2] cycloaddition reactions between alkynes and CHT have been only been achieved by using tricarbonyl(h 6 -7-exo-alkynyl-1,3,5-cycloheptatriene)-chromium(0) complexes at high temperatures (140-170 8C).[9] In comparison, the present cycloisomerization is performed catalytically and under mild conditions (room temperature), and affords higher yields without preliminary coordination of the triene to the metal. A catalyst loading as little as 1 mol % can be used, but the reaction time increases

Transition‐Metal‐Catalyzed Cycloadditions for the Synthesis of Eight‐Membered Carbocycles

“…An intramolecular [(6+2)] cycloaddition between CHT and an alkyne to afford cycloheptatrienes was developed recently by Tenaglia41 and co‐workers using PtCl 2 as the catalyst (Scheme ). Complex heterocyclic compounds were obtained when this reaction was applied to substrates containing a heteroatom (O or N) in the tether.…”

Synthesis of Functionally Substituted Bicyclo[4.2.1]nona-2,4-dienes and Bicyclo[4.2.1]nona-2,4,7-trienes by Cobalt(I)-catalyzed [6π + 2π] Cycloaddition of 2-Tropylcyclohexanone

“… 4 − 7 The catalytic versions of these transformations include Ti-, Co-, Mo-, Rh-, and Pt-catalyzed cycloaddition reactions of CHT and its derivatives. 8 − 12 …”

Platinum‐Catalyzed Intramolecular Cyclizations of Alkynes: Efficient Synthesis of Pyrroloazepinone Derivatives

“…Transition-metal-catalyzed cyclizations of aromatic and heteroaromatic compounds with alkynes have attracted much attention in the last decade. In particular, intramolecular gold-and platinum-catalyzed hydroarylations, [1] cycloisomerizations, [2] and cycloadditions [3] offer new ways for the efficient construction of biologically interesting carbo-and heterocycles. Recent elegant examples also include the use of N-heterocycles; for example, Echavarren et al [4] and England and Padwa [5] reported intramolecular cyclizations of indol derivatives in the presence of gold catalysts.…”

“…1,7-Dimethyl-4-p-tolyl-6,7-dihydropyrrolo[2,3-c]azepin-8(1H)-one (3a) and 1, 5-dimethyl-8-p-tolyl-5,6-dihydropyrrolo[3,2-c] 3(aryl) ). 13 3 J a,b = 2.9 Hz, 1 H, H-b); 6.61 (d, 3 J a,b = 2.9 Hz, 1 H, H-a); 6.09 (t, 3 J CH,CH2 = 7.5 Hz, 1 H, =CH); 3.71 (d, 3 J CH,CH2 = 7.5 Hz, 2 H, CH 2 ); 3.15 (s, 3 H, OCNCH 3 ); 3.07 (s, 3 H, NCH 3 ); 2.35 ppm (s, 3 H, aryl CH 3 ). 13 C NMR (125.8 MHz, CDCl 3 ): d = 166.0 (CO); 137.9, 137.7 (C-i',p); 136.6 (C-i); 131.6 (C-a'); 129.3 (C-m); 127.4 (C-o); 124.6 (C-a); 123.8 (=CH); 123.5 (C-b'); 109.6 (C-b); 47.5 (CH 2 ); 36.4 (NCH 3 ); 35.2 (OCNCH 3 ); 21.1 ppm (aryl CH 3 ).…”