Carbocationic n-endo-trig Cyclizations

✍ Scribed by Lei Shi; Markus Horn; Shinjiro Kobayashi; Herbert Mayr

• Book ID: 101835074
• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 350 KB
• Volume: 15
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200901246

✦ Synopsis

Abstract

Unsaturated benzyl cations (4‐MeOC~6~H~4~)CH^+^(CH~2~)~n~CHCH~2~ (1) have been generated laser‐flash photolytically in acetonitrile in the presence of enol ethers or 2‐methylfuran. The reactions of the cations 1 (n=2 and 4) with these π‐nucleophiles follow second‐order rate laws with rate constants comparable to those of the analogous saturated species (4‐MeOC~6~H~4~)CH^+^(CH~2~)~3~CH~3~. Product studies show the absence of cyclization products. In contrast, the carbocation (4‐MeOC~6~H~4~)CH^+^(CH~2~)~3~CHCH~2~ (1 d) undergoes a highly reversible 6‐endo‐trig cyclization which is approximately 10^7^ times faster than the corresponding intermolecular reaction of (4‐MeOC~6~H~4~)CH^+^(CH~2~)~3~CH~3~ with hex‐1‐ene. This cyclization yields a highly electrophilic, partially bridged carbocation, which accounts for the finding that 1 d is consumed eight times faster in trifluoroethanol solution than all other carbocations of this series. Quantum‐chemical calculations (B3LYP/6‐311G(d,p) and MP2/6‐31+G(2d,p)) have been performed to elucidate the structures of the involved carbocations. Consequences of these findings on the role of π‐participation in solvolysis reactions are discussed.