Organometallic compounds electron deficiency

Structures of Main Group Organometallic Compounds Containing Electron-Deficient... 

In this review, emphasis will be placed first on definitive proof of structure for compounds of main group organometallic compounds that contain the electron-deficient or multicentered bonds described. Evidence for the occurrence of these systems in both liquid an gaseous states also will be given. Finally, this structural information will be used to elucidate the bonding in the molecules. 

Main-group organometallic compounds are versatile tools in organic synthesis, but their structures are complicated by the involvement of the multicenter, two-electron bonds and ion-dipole interactions that are involved in aggregate formation (5). Electron deficiency or Lewis acidity of the metallic center and nucleophilicity or basicity of the substituents are important considerations in synthesis. The complexity of the structures and interactions is, however, the origin of much of the unique behavior of these organometallic compounds. 

Photoaddition and substitution of electron-deficient aromatic compounds such as o-dicyanobenzene (o-DCNB), p-DCNB, and TCNB by use of group 14 organometallic compounds are classified to the reaction of the radical anions of electron-deficient aromatic compounds with carbon radical species generated... 

Arenes and heteroarenes which are particularly easy to metalate are tricarbo-nyl( 76-arene)chromium complexes [380, 381], ferrocenes [13, 382, 383], thiophenes [157, 158, 181, 370, 384], furans [370, 385], and most azoles [386-389]. Meta-lated oxazoles, indoles, or furans can, however, be unstable and undergo ring-opening reactions [179, 181, 388]. Pyridines and other six-membered, nitrogen-containing heterocycles can also be lithiated [59, 370, 390-398] or magnesiated [399], but because nucleophilic organometallic compounds readily add to electron-deficient heteroarenes, dimerization can occur, and alkylations of such metalated heteroarenes often require careful optimization of the reaction conditions [368, 400, 401] (Schemes 5.42 and 5.69). 

The reaction of a Co(I) nucleophile with an appropriate alkyl donor is used most frequently for the formation of a Co-C bond, which also can be formed readily by addition of a Co(I) complex to an acetylenic compound or an electron-deficient olefin (5). The nu-cleophilicity of Co(I) in Co(I)(BDHC) is expected to be similar to that in the corrinoid complex, as indicated by their redox potentials. The formation of Co-C a-bond is the attractive criterion for vitamin Bi2 models. Sodium hydroborate (NaBH4) was used for the reduction of Co(III)(CN)2(BDHC) in tetrahydrofuran-water (1 1 or 2 1 v/v). The univalent cobalt complex thus obtained, Co(I)(BDHC), was converted readily to an organometallic derivative in which the axial position of cobalt was alkylated on treatment with an alkyl iodide or bromide. As expected for organo-cobalt derivatives, the resulting alkylated complexes were photolabile (17). 

A couple of years ago we have disclosed a new mode of alkyne activation towards isomerization as a detouring outcome of the Sonogashira coupling. As a result of coupling electron deficient (hetero)aryl halides (or a,p-unsaturated p-halo carbonyl compounds) 11 and aryl propargyl alcohols 12 a new access to 1,3-di (hetero)aryl propenones 13, i.e., chalcones, was established (Scheme 9) [77, 78]. The scope for electron deficient (hetero)aromatic halides 11 is fairly broad and even organometallic complexes like 13c can be synthesized by this sequence. 

Trialkylaluminum and alkylaluminum hydrides associate with alkyl or hydride bridges. Since there are no available lone-pair electrons with which to form bridges by standard two-center two-electron interactions, multicenter bonding is invoked in the same manner as for electron-deficient boranes (see Boron Hydrides), alkyllithium (see Alkali Metals Organometallic Chemistry), dialkylberyllium and dialkylmagnesium compounds (see Beryllium Magnesium Organometallic Chemistry). 

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