Astruc D. - Modern arene chemistry (2002)(en)

394 11 Arenetricarbonylchromium Complexes: Ipso, Cine, Tele Nucleophilic Aromatic Substitutions

Scheme 42. Cine and tele-meta SNAr: CaCl bond cleavage.

be explained in terms of preferential addition of the stabilized carbanion to the carbon eclipsed by a CraCO bond in the most stable conformer [69e]. This tele-meta regioselectivity is also observed on adding LiEt3BD to o-chloro- or o-fluorotrialkylsilylbenzenetricarbonyl- chromium complexes having an anti-eclipsed conformation with respect to the SiR3 group, giving p-deuterated trialkylsilylbenzenetricarbonylchromium complexes [69d].

The e ect of the nature of the leaving group has been delineated in the case of 2,6- dimethylfluoro and chloro complexes. Complex 96a reacts with LiC(CH3)[S(CH2)3S] and CF3CO2D to give exclusively complex 97a by a tele-meta SNAr [70], whereas complex 96b reacts with LiC(Ph)[S(CH2)3S] and CF3CO2D to give deuterated complex 97b by a tele-para SNAr [72].

Scheme 43. Tele-meta and tele-para SNAr: CaCl bond cleavage.

11.3.2

Cleavage of CxO Bonds

Reaction of NuLi (Nu ¼ CMe2CN, CHMeCN, CH2CN, etc.) with 5-deuterio-2,3-dimethyldi- phenylethertricarbonylchromium complex 98 yields, after CF3CO2H(D) treatment, the deuterated complex 99. This reaction has been shown to involve a 1,5-deuteride migration from the C5 carbon to the C1 carbon (Scheme 44) [75a, 75b].

Scheme 44. Tele-meta SNAr: CaO bond cleavage.

11.3 Cine and Tele Nucleophilic Aromatic Substitutions 395

Reaction of LiEt3BD and CF3CO2H with o-trimethylsilyldiphenylethertricarbonylchro- mium complex 100 gives the tele-meta complex 101 in 77 % yield (Scheme 45) [69d].

Scheme 45. Tele-meta SNAr: CaO bond cleavage.

Treatment of veratroletricarbonylchromium complex 102 with LiCH2CN in THF at 40 C for 30 min and then with acid leads to the formation of (3-methoxyphenyl)acetonitrile complex 103a in 47 % yield (Scheme 46). From this experiment, it cannot be ascertained whether the carbanion adds to the carbon ortho to the methoxy group (cine SNAr) or to the carbon para to the methoxy group (tele-para SNAr) because in each case a meta-disubstituted complex is formed. Consequently, CF3CO2D is used, whereupon complex 103b deuterated at the C2 carbon is obtained, as befits a cine SNAr. When the positions ortho to the MeO groups are substituted, tele regioselectivity is observed [76]. The same reaction with 1,2,3-trimethoxy- benzenetricarbonylchromium complex 71 a ords complex 72 by a tele-meta SNAr as the major product (42 % yield), contaminated by the ipso complex 73 (22 % yield) (vide supra; Scheme 34) [58, 74].

Scheme 46. Cine SNAr: CaO bond cleavage.

11.3.3

Cleavage of CxN Bonds

An unprecedented cleavage of a carbon–nitrogen bond of a tertiary amine is observed on treating o-trimethylsilyldimethylanilinetricarbonylchromium complex 104a with LiEt3BD and then with CF3CO2H. Complexes 105a and 105b (ratio 20:80) are recovered as a result of tele-meta SNAr (20:80 ratio; 55 % yield; Scheme 47) [73]. When the same experiment is

Scheme 47. Cine and tele SNAr: CaN bond cleavage.

398 11 Arenetricarbonylchromium Complexes: Ipso, Cine, Tele Nucleophilic Aromatic Substitutions

400

12

Activation of Simple Arenes by the CpFeB Group and Applications to the Synthesis of Dendritic Molecular Batteries

Didier Astruc, Sylvain Nlate, and Jaime Ruiz

Abstract

The CpFeþ-induced polyfunctionalization of polymethylbenzenes under ambient conditions gives virtually quantitative yields of CpFeþ(h6-arene)-centered stars and dendritic cores on a large scale. From the chlorotoluene complex, mild nucleophilic reaction with ethanol in the presence of K2CO3 followed by mild reaction with allyl bromide in the presence of tBuOK gives a 60 % yield of the triallyl phenol dendron p-HOC6H4C(allyl)3. This dendron and its functionalized derivatives have been used as building blocks for the divergent synthesis of large dendrimers and for the convergent synthesis of nonafunctional dendrons including dendrons bearing ferrocenylsilyl termini on their branches. The various dendrimers, with branch numbers of up to 243 in theory, have been decorated with ferrocenyl groups, and polyaminodendrimers have been functionalized with ferrocenyl, cobaltocenyl, and CpFeþ- arene moieties. The generation or isolation of two redox states of these metallodendrimers and electron-transfer reactions illustrate their function as molecular electron reservoirs or molecular batteries.

12.1

Introduction

Molecular batteries should be useful as part of nanoscopic molecular electronic devices [1–4]. Ideally, nanoscopic assemblies containing a large number of redox centers, for which both redox forms are stable and which operate at the same redox potential, should be able to deliver or accept a large number of electrons at once at this redox potential. Such an assembly constitutes a battery – a molecular battery or a molecular electron reservoir if it is a molecular assembly. Metallodendrimers [5–14] are excellent candidates for such a function when at least two of their redox states are stable [15]. This property is realized with some late transition metal metallocenes and analogous sandwich systems. We describe herein the synthesis and properties of such nanoscopic metallo-assemblies that can be considered as molecular batteries. These syntheses are largely based on the activation of simple arenes by the electron-withdrawing group CpFeþ [16]. This metal group is thus useful for both

Modern Arene Chemistry. Edited by Didier Astruc

Copyright 8 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30489-4

40212 Activation of Simple Arenes by the CpFeþ Group

12.2.2

Solubility, Stability, and General Reactivity Trends

The yellow CpFeþ(h6-arene) salts (most commonly BF4 or PF6 ) are usually stable up to at least 200 C, are stable in concentrated sulfuric acid, and are very resistant towards oxidation

(until recently, it was believed that they could not be oxidized [23]; vide infra). They are not easy to reduce either [23] (vide infra). The chloride salts [CpFeþ(h6-arene)] Cl are water-soluble; they are formed upon hydrolysis following ligand-exchange reactions between ferrocene and the arene in the presence of aluminum chloride [21]. Such aqueous solutions may sometimes

be directly used for nucleophilic reactions [22] (vide infra). The BF4 salts are also sometimes quite soluble in water, but the PF6 salts are much less so. Electrophilic reactions that are readily undergone by the free arenes, such as Friedel–Crafts reactions, are no longer possible on the CpFeþ(h6-arene) complexes [19, 23]. On the other hand, a range of nucleophilic re-

actions that are impossible or very di cult to carry out with free arenes become possible under ambient or mild conditions with the CpFeþ(h6-arene) complexes (Scheme 2) [16–20].

Scheme 2. Main reactions of the [CpFe(h6-arene)]þ complexes.