Recent developments in homobimetallic reagents and catalysts for organic synthesis

WU Wei 0 GU DeLiang 0 WANG ShiMeng 0 NING YingNan 0 MAO GuoLiang ) 0 0 Provincial Key Laboratory of Oil and Gas Chemical Technology, College of Chemistry and Chemical Engineering, Northeast Petroleum University , Daqing 163318, China Organometallics are a family of useful organic chemicals because they play important roles in organic synthesis as reagents and as catalysts. They can be classified according to the number of metals they contain. Bimetallic compounds are important organometallics and they are either homobimetallic or heterobimetallic depending on whether the two metals are the same or different. In this paper, we focus on homobimetallic compounds. Homobimetallic compounds are generally used as dianions to react with electrophiles in organic synthesis. Recently, homobimetallics have also been used as catalysts in organic reactions such as in asymmetric reactions. 1 Homobimetallic reagents for organic synthesis Homobimetallic reagents can be classified according to the relative positions of the two carbon atoms that bear the metal atoms. The positions of the metals will affect their reactivity. The reactions of bimetallic compounds have been summarized previously [1]. Recently, the cyclization reactions of dianions were thoroughly reviewed by Langer et al. [2]. In this paper, we compile, analyze and discuss recent developments in homobimetallic reagent-mediated reactions. Some important results that have been summarized in previous reviews will also be included. Among bimetallics, organodilithium compounds are a family of very important intermediates. Many other organobimetallic compounds can be obtained by the transmetalation of corresponding dilithio compounds. Organodilithium compounds can be synthesized by several methods including hydrogen-lithium exchange (deprotonation), halogen-lithium exchange, transmetalation reactions, carbon-heteroatom bond cleavage and the lithiation of multiple carbon-carbon bonds, etc. [3]. 1,1-Bimetallic compounds 1,1-Bimetallic compounds are also referred to as geminal bimetallics. Because two metals are attached to the same carbon atom, geminal bimetallics exhibit very interesting reaction properties. Marek and Normant summarized the synthesis and reactions of both homo- and hetero-sp3geminal organodimetallics [4]. These included dilithioalkane, dimagnesioalkane, dialuminioalkane, diborioalkane and dizincoalkane reagents. As highly reactive dianion species geminal bimetallic compounds form open-chain products by reacting with monofunctional electrophiles followed by the addition of water. Cyclization reactions can also take place if the geminal bimetallic is treated with a dielectrophile [2]. (1) 1,1-Dilithio compounds. The lithiation of compounds containing a CH2 group at the ortho- position of a The Author(s) 2011. This article is published with open access at Springerlink.com sulfone or a nitrile can lead to the formation of a true ,-dianion, which has been confirmed by single crystal crystallography [5]. These dianions can undergo cyclization reactions with biselectrophiles. Langer and co-workers [6] carried out the first direct transformation of silyl enol ethers to the geminal lithiated allene 1 (Scheme 1), which was trapped by a ketone to give the corresponding diol 2. A subsequent domino reaction with nitriles gives the corresponding dihydropyrimidine 3 [7]. Synthetic equivalents of 1,1-dilithioethylene 4 [8] and 1,1-dilithiocyclopropane 5 [9] were prepared by sequential lithiation using two different methods. These methods provide a convenient way to introduce two different electrophiles (Scheme 2) . (2) 1,1-Dizinco compounds. The transition metal catalyzed cross-coupling reaction of an organometallic reagent with an organic electrophile is one of the most important skeleton-constructing methods in organic synthesis. By employing a geminal bimetallic reagent, sequential coupling reactions can be carried out to introduce two electrophiles onto one carbon atom. Matsubaras group realized this using bis(iodozincio)methane 6 and 1,1-bis(iodozincio)ethane (Scheme 3) [10]. 1,2-Bimetallic compounds Diphenylacetylene can be reduced by metallic lithium to form the cis-dilithium adduct 7, which was converted into cis-stilbene after treatment with methanol [11]. It has been found that trimethylsilyl substituted styrenes can also be reduced by metallic lithium to form the corresponding 1,2-dilithio intermediate 8 (Scheme 4) [12]. Yus et al. [13] successfully reduced methyl-substituted styrene with metallic lithium in the presence of a catalytic amount of 4,4-ditert-bytylbiphenyl (DTBB). The dilithio compounds 9 formed were captured in situ by carbonyl compounds or by chlorotrimethylsilane. A very special type of 1,2-dilithio compound originates from ortho-carborane. Many substituents can be introduced to the carborane skeleton through transmetalation or other reactions. The direct insertion of sulfur or selenium into the carbon-lithium bonds leads to corresponding ortho-carboranedithiolate and diselenolate complexes. (Pentamethylcyclopentadienyl)iridium (Cp*Ir) was introduced to the orthocarboranediselenolate dianion to form a 5-membered ring structure (compound 10 in Scheme 5) [14]. The interaction between orthocarboranedithiolate and diselenolate complexes with lanthanocene chlorides afforded a series of Scheme 1 Preparation of geminal lithiated allenes and their application in organic synthesis. Scheme 2 Reactions of geminal dilithio equivalents. Scheme 3 Sequential coupling reaction of 1,1-bis(iodozincio)methane. Scheme 4 Preparation and reactions of the 1,2-dilithio compounds. dinuclear organolanthanide complexes. Upon reaction with Me2GeCl2 or Me2SnBr2 [15], 1,2-bis(chlorogermyl)carborane or 1,2-bis(bromostannyl)carborane 11 is generated, respectively. These two compounds can be converted into a several cyclic compounds (12 in Scheme 5) [16]. Mercuracarborands were prepared using dilithio carborane and they formed the porous material 13 with a new bonding motif by supramolecular self-assembly [17]. The dimerization of a 1,2-dicuprio carborane that was prepared in situ led to the formation of 1,1-bis(o-carborane) 14 after hydrolysis [18]. Zirconium and hafnium substituted carboranes were also prepared and their structures confirmed by single crystal X-ray analysis [19]. A highly strained carborane-1,2-disilacyclobutene derivative 15 was prepared by a reaction between 1,2dichlorotetraalkyldisilane and dilithiated carborane. Exposure of this reactive compound to atmospheric oxygen led to the insertion of one oxygen atom between the Si-Si bond [20]. Upon catalysis by Pd(PPh3)4, compound 15 reacted with substituted alkynes to afford the 6-membered cyclic products 16 (Scheme 5) [21]. In their research into the structure/reactivity relationships of lanthanacarboranes, Xie and coworkers [22] synthesized compounds 17 bearing a 6-, 7- and 8- membered inner ring. They investigated CC bond cleavage during the reduc (...truncated)