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Organometallic Manganese Compounds in Organic Synthesis

Alina A. Grineva1,2, Noël Lugan1, and Dmitry A. Valyaev1

1LCC-CNRS, Université de Toulouse, CNRS, 31077 Toulouse cedex 4, France

2Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninsky Pr., Moscow, 119991 Russia

1.1 Introduction

Organometallic manganese chemistry emerged in 1937 with the in situ generation of the first Mn(II) species, PhMnI[1], followed by the preparation of two other emblematic Mn(0) and Mn(I) derivatives, Mn2(CO)10 [2] and CpMn(CO)3 [3] in 1949 and 1954, respectively. Despite a long history, this research field remained for many years a purely fundamental area with only sporadic applications in organic synthesis [4, 5] and homogeneous catalysis [6]. The situation started to change rapidly in the late 1970s, and Mn(II) s-complexes soon became valuable tools for organic synthesis acting as soft and chemoselective nucleophilic reagents. Recent excellent reviews by Gérard Cahiez, one of the pioneers and key players in the field, are available on this topic [7], and therefore it will not be covered in our contribution. Even if the chemistry of Mn(I) complexes directed to organic synthesis is much less developed, some interesting results have been obtained during the last 30?years, but they have never been systematically reviewed. The main goal of this chapter is to provide an overview of the multiple facets of manganese organometallic chemistry, from the information on various metallic precursors and basic reactivity patterns to the design of Mn-containing chiral ligands and application of Mn(I)-mediated processes in organic synthesis.

1.2 Basic Manganese Precursors Relevant to Organometallic Chemistry

Unlike more electropositive alkaline and alkaline earth metals, commercial manganese powder undergoes the insertion across C-halogen bond to form organometallic RMnX species uniquely for highly activated allylhalides and a-halogenated esters under Barbier conditions [8]. Still, the reduction of Mn(II) salts MnX2 or manganates MnX2?×?2LiX (X = Cl, Br, I) with metallic Mg [9], Li/naphthalene [10] or Li/2-phenylpyridine [11] systems, and potassium graphite KC8 [12] was shown to generate in situ a very reactive form of metallic manganese, capable of activating a variety of alkenyl-, aryl-, and heteroarylhalide substrates under mild conditions.

Commercially available anhydrous manganese salts MnX2 represent the simplest manganese-containing starting materials. While MnCl2 and MnBr2 are very stable, MnI2 is light sensitive, and its commercial form is often considered as too impure for given synthetic applications [7]. Fortunately, this compound can be easily prepared on demand upon reaction of Mn powder with iodine in ether [13]. The solubility of MnX2 salts in ethereal solvents is markedly different and strongly decreases in MnI2?>?MnBr2?»?MnCl2 order. Yet, when necessary, MnCl2 can be easily transformed by refluxing in tetrahydrofurane (THF) into the pink solvated complex MnCl2?×?2THF [14], which can be used for the synthesis of Mn(II) complexes, even in toluene [15].

Reactions of MnX2 with organolithium RLi or Grignard RMgX reagents remain the most convenient route to organometallic Mn(II) complexes. In particular, mixed RMnX, homoleptic R2Mn, and even manganate [R3Mn]- or [R4Mn]2- species can be obtained, depending on the reagents stoichiometry [7]. In general, their thermal stability follows the trend [R4Mn]2-?~?[R3Mn]-?>?RMnX?»?R2Mn, and the less stable dialkylmanganese compounds typically decompose by a ß-elimination process. However, some stable homoleptic Mn(II) s-complexes such as Mn3(Mes)6 [16], (Mes - 2,4,6-trimethylphenyl), or Mn4(CH2tBu)8 [17] can be conveniently produced on a multi-gram scale. In contrast to ubiquitous ferrocene, manganocene Cp2Mn [18] (Cp = ?5-C5H5 herein and throughout the chapter) is not stable under ambient conditions due to the strongly ionic character of the Mn-Cp bond and thus has found quite a limited use in synthetic chemistry [19]. We can also mention the highly sensitive complex [Mn2(N(SiMe3)2)4], yet accessible on a large scale [14], which has been applied recently to the preparation of some Mn(II) amidinate complexes [20] and well-defined clusters [21] using the ability of amide ligands to serve as internal base.

Manganese carbonyl Mn2(CO)10 is the foremost accessible Mn(0) compound, which can be easily transformed into Mn(I) halide complexes Mn(CO)5X (X = Cl, Br, I) upon the oxidation with the corresponding free halogen [22]. To date, commercially available air-stable Mn2(CO)10 and Mn(CO)5Br remain the most popular manganese precursors, which could be either directly used in catalysis [6, 23] or exploited for the synthesis of various Mn(I) precatalysts [24]. The chemistry of Mn(CO)5Cl and Mn(CO)5I is much less developed because of more difficult preparations and lower stability of these compounds. Half-sandwich Mn(I) complex CpMn(CO)3, also known as cymantrene, and its methylated analogue Cp´Mn(CO)3 (Cp´ = ?5-C5H4Me), belongs to a few examples of quite sophisticated transition metal organometallic compounds produced industrially at a scale of hundred tons per year as anti-knock gasoline additives. Although their use in such application is currently declining in many countries, technical grade CpMn(CO)3 or Cp´Mn(CO)3 are actually available at a cost 20-30 USD per kilogram from chosen suppliers, analytically pure samples suitable for laboratory use being eventually prepared upon recrystallization from hexane [25] or vacuum distillation, respectively. The availability and rich reactivity of these organometallic complexes discussed in Sections 1.4 and 1.5 make them attractive candidates for application in organic synthesis.

1.3 Overview of the Synthetic Chemistry for Main Classes of Mn(I) Complexes

Besides thermal CO ligand(s) substitution, the most important reactivity pattern of Mn2(CO)10 (1) deals with different types of activation of the metal-metal bond (Scheme 1.1). In particular, under UV (350?nm) or visible light (430?nm) irradiation, a smooth generation of the metal-centered radical 2 is achieved; this transformation has recently found numerous applications in photo-induced polymerization processes [26]. In addition to the oxidative Mn-Mn bond cleavage in 1, induced by halogens already mentioned in Section 1.2, the reduction of this compound with sodium amalgam [27], Na/K alloy [28], or commercial trialkylborohydrides M[BR3H] [29] (M = Li, K) leads to the formation of anionic metallocarbonylate species 3. Reactions of the latter with various electrophiles constitute a powerful approach to the synthesis of Mn(I) s-complexes Mn(CO)5E (4E). Upon protonation of 3 with aqueous H3PO4, the hydride complex 4H can be isolated as a volatile liquid [30]. However, due to its extreme air sensitivity and toxicity, 4H is preferably generated and used in situ [31]. Substitution of one or two CO groups in 4H by P-donor ligands strongly improves the stability of the resulting hydrides Mn(CO)5-n(L)nH [31, 32]. The alkylation of 3 with MeI proceeds easily to form air-stable 4Me, which has been recently applied for expedient preparation of 16-electron Mn(I) complexes of bifunctional pincer ligands [28]. Similarly, the acylation affords isolable s-acyl derivatives 4COAr, which upon heating undergo selective CO deinsertion, providing a viable route to Mn(I) aryl complexes 4Ar [33].

Carbonyl ligand substitution plays a central role in the chemistry of Mn(CO)5Br (5) in the context of the synthesis of various Mn(I) precatalysts (Scheme 1.2) [24]. While the first CO ligand can be easily replaced at room temperature by a donor ligand L to form cis-Mn(CO)4(L)Br (6) [34], heating at 50-60?°C is often required to remove the second CO group. For some monodentate ligands, besides the kinetically controlled formation of fac-7 species, formation of more thermodynamically stable mer,trans-7 products can eventually be observed at higher temperatures [35]. Chelating ligands necessarily lead to fac-isomers of complexes 8. The thermal reaction of 5 with L3 pincer-type systems can afford either the neutral dicarbonyl Mn(CO)2(L3)Br (9) or the cationic tricarbonyl [Mn(CO)3(L3)]Br (10) complexes upon substitution of three CO or two CO and Br- ligand combinations, respectively [24]. However, the formation of monocarbonyl Mn(I) species trans-Mn(P-P)2(CO)Br (11) from 5 and chelating diphosphines involving the substitution of four CO ligands can be achieved only under UV irradiation [36].

Scheme 1.1 Photo-induced and reductive cleavage of Mn-Mn...