Direct Observation of a Cationic Gold(I)–Bicyclo[3.2.0]hept‐1(7)‐ene Complex Generated in the Cycloisomerization of a 7‐Phenyl‐1,6‐enyne

Abstract

The cycloisomerization of enynes catalyzed by electrophilic noble-metal complexes, [1] in particular Au and Pt, [2] has attracted considerable attention owing to the formation of skeletal rearrangement products, including vinylcyclopentenes (B and C), bicyclo[4.1.0]heptenes (D), and bicyclo-[3.2.0]hept-6-enes (E) from 1,6-enynes (A; Scheme 1). [3,4] Fürstner first posited that these outcomes were consistent with the intermediacy of metal-stabilized nonclassical cyclopropylmethyl-, cyclobutyl-, and homoallylic carbocations/ carbenes accessed through attack of the C=C moiety on a metal-complexed CC bond (Scheme 1), [5] and mechanistic thought in this area has evolved largely within this conceptual framework. The involvement of nonclassical carbocations/ carbenes is supported by a wealth of indirect experimental evidence, including trapping experiments, isotopic labeling studies, and stereochemical analyses, and through numerous computational studies. [1, 2] Absent, however, is direct experimental evidence regarding the structure and reactivity of these cationic complexes, as no organometallic intermediate has been observed spectroscopically in any of these transformations. [6][7][8][9][10] Platinum(II), [11] gold(I), [12,13] and rhodium(II) [14] complexes catalyze the cycloisomerization of 7-aryl-1,6-enynes (A, R = Ar) to form vinylcyclopentenes C and/or bicyclo-[3.2.0]hept-6-enes E (Scheme 1). Directly implicated in these and related [15,16] transformations is the strained bicyclo-[3.2.0]hept-1(7)-ene species I, presumably generated through 6-endo-cyclization followed by 1,2-alkyl migration from cyclopropyl carbene II and consumed either by ring opening to form B and/or 1,3-hydrogen migration to form E (Scheme 1). [11][12][13][14] Possible contributors to I include the metalated cyclobutyl carbenium ion I a, the p-alkene complex I b, and the metallacyclopropane complex I c. In this context, it appeared likely that contribution of the latter forms would engender stability to I not realized in the corresponding cyclopropyl carbene intermediates II or III, such that I might represent a local minimum on the reaction coordinate. Indeed, here we report the selective generation, spectroscopic characterization, and reactivity analysis of a gold-bicyclo-[3.2.0]hept-1(7)-ene complex formed in the gold-catalyzed cycloisomerization of a 7-phenyl-1,6-enyne.Toward detection of a reactive bicyclo[3.2.0]hept-1(7)-ene complex, we investigated the gold(I)-catalyzed conversion of the 7-phenyl-1,6-enyne 1 into the 6-phenylbicyclo[3.2.0]hept-6-ene 2 reported by Echavarren (Scheme 2). [12] In an initial experiment, a 1:1:1 mixture of 1, [LAuCl] (L = P(tBu) 2 (obiphenyl)), and AgSbF 6 in CD 2 Cl 2 was mixed thoroughly at À78 8C. 31 P and 1 H NMR analysis at À80 8C showed formation of an approximately 9:1 mixture of p-alkene and p-alkyne Scheme 1. Ligand-and substrate-dependent pathways for enyne cycloaddition catalyzed by electrophilic noble-metal complexes.Scheme 2. Gold-mediated conversion of 1 into 4.

Mechanistic Insight into the Dehydro-Diels–Alder Reaction of Styrene–Ynes

“…Characterization data is consistent with previously reported literature data. 49 (17 mL 7, 136.2, 136.1, 135.0, 129.6 (2C), 128.5-127.5 (m, 5C), 126.2 (t, J = 24.0 Hz, 2C), 123.0, 76.7, 73.9, 48.7, 36.0, 21.7. IR (film): 3289, 3034, 2923, 2276, 2118, 1598, 1494, 1346, 1161 5, 84.9, 84.4, 49.4, 36.2, 32.4, 21.6.…”

Intriguing mechanistic labyrinths in gold(i) catalysis

“…Only a few examples of highly strained bicyclo[3.2.0]hept-5-enes have been reported in gold( i ) catalysed isomerisations, 91,94,95 whereas products of formal [2+2] cycloaddition have been observed in the parent intermolecular reactions of alkynes with alkenes, 54 as well as in the cyclisation of 1,7-, 1,8- 46,96 and higher enynes. 55 Alternatively, 67 could give the product of alkene isomerisation 69 .…”

Gold(I)-catalyzed 6-endo hydroxycyclization of 7-substituted-1,6-enynes

“…In the case that MeOH is present a 5- exo methoxycyclization is observed, e.g., in the formation of X resembling the behaviour of I [16,20]. In addition, the gold-catalyzed reaction of 7-phenyl-1,6-enynes with a terminal double bond gives rise to bicyclo[3.2.0]heptene derivatives [19,21]. …”