Regioselective Haloaromatization of 1,2-Bis(ethynyl)benzene via Halogen Acids and PtCl₂. Platinum-Catalyzed 6-π Electrocyclization of 1,2-Bis(1‘-haloethenyl)benzene Intermediates

Abstract

[reaction: see text] Treatment of 1,2-bis(ethynyl)benzene (1) with aqueous HX (X = Br, I) in hot 3-pentanone (100-105 degrees C, 2 h) afforded 1,2-bis(1'-haloethenyl)benzene species 2-Br and 2-I in 98% and 95% yields, respectively. The hydrochlorination of endiyne 1 failed to proceed at elevated temperature but was implemented efficiently by PtCl2 (5 mol %) in hot 3-pentanone (100 degrees C, 2 h) to give 1,2-bis(1'-chloroethenyl)benzene 2-Cl in 80% yield. In the presence of PtCl2 (5 mol %), these halides 2-Cl,2-Br, and 2-I were subsequently converted to 1-halonaphthalenes 3-Cl, 3-Br, and 3-I in the mother solution via sequential 6-pi electrocyclization and dehalogenation reactions. PtCl2 (5 mol %) also effected direct haloaromatization of endiyne 1 with HX (X = Cl, Br, I) and gave 1-halonaphthalenes 3-Cl, 3-Br, and 3-I in 64-71% yields. This investigation reports the scope and the regioselectivity of haloaromatization of various enediynes catalyzed by PtCl2.

Synthetic and Mechanistic Investigations on the Rearrangement of 2,3‐Unsaturated 1,4‐Bis(alkylidene)carbenes to Enediynes

“…for C 12 H 10 O 2 [M] + 186.0681; found 186.0660. These data are in agreement with literature data 40c…”

Transition metals in organic synthesis: Highlights for the year 2005

“…Ruthenium, palladium, iridium, and gold catalyzed Markovnikov hydroiodinations of 1,2-bis(ethynyl)benzene [1397]. In contrast, platinum catalyzed a hydroiodination cyclization.…”

PtCl2-catalyzed benzannulation of nickel(II) 2,3-dialkynylporphyrins to form unusual phenanthroporphyrins

“…In marked contrast, heating of 2b (85e90 C) in the presence of PtCl 2 (1 equiv) for 1 h, leads to 100% consumption of starting material and formation of the unusual phenanthroporphyrin 4b (68%) and benzoporphyrin 5 (6%) (entry 1, Table 1). The absence of picenoporphyrin products suggests that initial p-and subsequent s-bonding of Pt(II) to the alkyne carbons is operative in the mechanism as proposed for simple enediyne motifs [15]. Reaction of 2b with 0.5 equiv of Pt(II) generates nearly identical results (entry 2, Table 1).…”

“…Like several established organometallic coupling strategies (e.g., Stille, Suzuki, Sonogashira), transformations of alkyne functionalities are extremely versatile for formation of cyclized hydrocarbon constructs [11], and thus can serve as an effective route to unusual pextended porphyrinic structures [12,13]. Within this theme, we report the preparation of Ni(II) 2,3-dialkynylporphyins and the PtCl 2 -catalyzed cyclization of the diyne-ene motif [14,15] to form unusual phenanthroporphyrin constructs [16].…”