Regioselective Electrophilic Substitution via Lithiation and Intramolecular Arene Exchange (Haptotropic Rearrangement) in (Naphthalene)tricarbonylchromium

Abstract

The sequential reaction of (η6-naphthalene)tricarbonylchromium (1) with (tetramethylpiperidyl)lithium (TMPLi) and electrophiles (D+, ClSiMe3, C02, Mel, Et0S02CF3) is highly regioselective and exclusively yields (2-substituted naphthalene)tricarbonylchromium complexes bearing the substituent in the coordinated ring. With n-BuLi, the same sequence gives mixtures of the 1- and 2-substituted naphthalene complexes in the ratio of 3:7. [l-4a,8a-η-l,3-(SiMe3)2C10H6]Cr(CO)3(5) is obtained in reactions with excess TMPLi and ClSiMe3. The selectively labeled complexes (l-4a,8a-η-[2-D]naphthalene)Cr(CO)3(3a) and (1-4a,8a-η-[l,3-8-D7]naphthalene)Cr(CO)3(3b) were used in a kinetic study of the haptotropic rearrangement of (naphthalene)tricarbonylchromium. The rate constants and activation parameters of the rearrangement in cyclohexane-d12and in benzene were determined bylH and2H NMR methods. In cyclohexane, the activation parameters for the intramolecular rearrangement in 3b were found to be ΔH* = 20.4 kcal mol-1and AS* = -23 cal mol-1K1. Haptotropic rearrangement was found to proceed by first-order kinetics and, within experimental error limits, at the same rate in benzene as in cyclohexane. Addition of THF accelerated intra- as well as intermolecular arene exchange. In benzene, at 100 °C, the intramolecular ring migration of 3b is 9 times faster than arene exchange. (l-4a,8a-η-2-Methylnaphthalene)Cr(CO)3(3e) rearranges (ΔH* = 26.2 kcal mol-1) on heating in cyclohexane to an equilibrium mixture (65:35) of the two isomers, the major isomer being the one in which the metal is coordinated to the substituted ring. © 1987, American Chemical Society. All rights reserved.