Die durch Silberionen katalysierte Umlagerung von Propargyl‐phenyläther

Abstract

AbstractIt is known that propargyl‐phenylethers rearrange at about 200° to 2 H‐chromenes [1–4]. It is shown that this rearrangement occurs in benzene or chloroform at lower temperatures (20–80°) in the presence of silver‐tetrafluoroborate (or‐trifluoracetate). The ethers examined are presented in Scheme 1. Thus in chloroform at 61° in the presence of AgBF4, phenyl‐propargylether (3) yields 2 H‐chromene (13). With 0.78 molar equivalents AgBF4 in benzene at 80° the same ether 3 yields a 3:1 mixture of 2‐methyl‐cumaron (14) and 2 H‐chromene (13). From 1′‐methylpropargyl‐phenylether (4) and 2′‐butinyl‐3,5‐dimethylphenylether (5) under similar conditions the corresponding chromenes 16 and 17 resp. are obtained. Rearrangement of propargyl‐ and 2′‐butinyl‐1‐methyl‐2‐naphthylether (6 and 7 resp.) in benzene at 80° in the presence of AgBF4 gives the corresponding allenyl‐naphthalenones 18 and 19 resp. Treatment of propargyl‐ and 2′‐butinyl‐mesityl‐ether (8 and 9 resp.), and propargyl‐ and l′‐methylpropargyl‐2,6‐dimethyl‐phenylether (10 and 11 resp.) in benzene at 80° with AgRF, yields as the only product the corresponding 3‐allenyl‐phenols 21, 22,24 and 25 (Scheme 3). It is shown that in the presence of μ‐dichlor‐dirhodiuni (1)‐tetracarbonyl in benzene a t 80° the ether 4 rearranges to 2‐methyl‐2H‐chromene (16). However with this catalyst the predominant reaction is a cleavage to phenol. No reaction was observed when ethers 3 and 12, (Scheme 7 ) were treated with the tris‐(trimethylsily1)‐ester of vanadic acid in benzene a t 80° (see also [8]).By analogy with the known mechanism for thc silver catalysis of the reversible propargylesterl/allenylester rearrangement [S], the silver (1)ion is assumed to form a pre‐equilibrium π‐complex with the C, C‐triplebond of the substrate. This complex then undergoes a [3s, 3s]‐sigmatropic rearrangement (Scheme 2). In the case of the others 6, 7 and 12 the resulting allenyldienones were isolated. The 2,G‐dimethyl substituted ethers 8, 9, 10 and 11 resp. first give the usual allenyl‐ dienones (Scheme 3). These then undergo a novel silver catalysed dienon‐phenol‐rearrangement (Sclzenzu4) to give the 3‐allenylphenols 21, 22, 24 and 25. Thc others 3, 4 and 5 with free ortho positions presumably rearrange first to the non‐isolated 2‐allenyl‐phenols 15, 42 and 43 resp.(Scheme 7). These then rearrange, either thermally or by silver (1)ion catalysis to the 2H‐chromenes 13,16 and 17 resp. The rate of the rearrangement of 2‐allenylphenol (15) to 13 at room temperature in benzene or chloroform is approximately doubled when silver ions are present as catalyst.