Synthesis And Isolation Of Reactive Cations With Weakly Coordinating Carborane Anions

Author : Manish Khandelwal
Publisher :
ISBN 13 :
Total Pages : 420 pages
Book Rating : 4.:/5 (869 download)

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Download or read book Synthesis, Isolation and Reactivity of Low-coordinated Aluminum Cations written by Manish Khandelwal and published by . This book was released on 2012 with total page 420 pages. Available in PDF, EPUB and Kindle. Book excerpt: The role of weakly coordinating carborane based anions to isolate reactive aluminum and silylium cations is of extreme importance. The synthetic procedure of these salts was improved. A direct conversion of [Cs][CHB11H5X6] (X = Cl and I) to [Ph3C][CHB11H5X6] without using the intermediate silver salt ([Ag][CHB11H5X6]) is described. The existing literature method to synthesize [Cs][closo-1-CHB11H12] from B10H14 is tedious and non-reproducible. The work up procedure for the synthesis of [NEt4][arachno-6-H2CB9H12] from B10H14 was modified, which resulted in better yields and reproducibility. The conversion of [NEt4][closo-HCB11H11] to [Cs][closo-HCB11H11] did not work as reported even with excess of CsCl. So the [closo-1-CHB11H11] was isolated by precipitation using NMe3·HCl to get [NHMe3][closo-HCB11H11] in 87% yield. The corresponding Cs salt was synthesized by dissolving [NHMe3][closo-HCB11H11] in NaOH solution and precipitated by using excess CsCl in high yields. Furthermore, the synthesis of [Et3Si]+ salt from [Ph3C][CHB11H5Cl6] was improved by using a 9:1 mixture of hexanes:benzene. This solvent system resulted in lowering the reaction time from several days to a few hours (typically 2-3 h with sonication). The resulting [Et3Si][CHB11H5Cl6] was obtained as a beige solid and did not form sticky clumps. This process is suitable for small scale reactions where freshly prepared silylium salt is desired. In chapter 2, the reactivity of cationic dialkylaluminum and m-terphenylalkylaluminum compounds towards intramolecular hydroamination of primary and secondary aminopentenes is reported. The role of bulky ligands is emphasized by employing terphenyl ligands which increased the conversion rate by several folds. The reaction rates are strongly dependent on the substrate and the catalyst substituents. The aluminum compound [Dipp*AlEt][CHB11H5I6] (Dipp* = 2,6-Dipp2C6H3-, Dipp = 2,6-iPr2C6H3-), 4, bearing bulky ligand was the most active catalyst. Although the neutral species DcpAlEt2 (Dcp = 2,6-(2,6-Cl2C6H3)2C6H3-), 7, and Dipp*AlEt2, 8, showed some catalytic activity, they were more than 25 times less reactive than their cationic counterparts [DcpAlEt][CHB11H5Cl6], 3, and 4. The cyclization of secondary benzylaminopentenes with [Et2Al][CHB11H5I6], 1, was strongly dependent on the substitution of the C2 olefinic carbon. The ability of [Et2Al]+ to activate carbon dioxide is demonstrated in chapter 3. This report is the first example of a Lewis acid mediated catalytic reduction of CO2. This transformation was studied by various alkyl and phenyl silanes in presence of catalytic amounts (ca. 10%) of [Et2Al]+, a strong Lewis acid. The reactivity of [Et2Al]+ was also compared with [Et3Si]+ and AlBr3 and it was found that the cationic aluminum species is by far more reactive than [Et3Si]+ in the presence of excess of a silane. AlBr3 reacts only stoichiometrically. The solvent effect was probed by comparing the reactivity of [Et2Al]+ in C6D6 and in C6D5Br. The more polar C6D5Br shows slightly higher activity then C6D6 and the formation of side products (methylated solvent) were less in case of bromobenzene. In order to understand the reaction mechanism, the reactivity [Et2Al]+ was studied with proposed intermediates like silyl ester, silyl ethers, silanol and siloxane. From the reaction of [Et2Al]+ and siloxane, the formation of SiEt4 and polysiloxanes was confirmed as side products of this chemical transformation. The bulky ligands bearing the electronegative substituents such as amides (R2N-) can further enhance the Lewis acidity of the aluminum center. In order to synthesize such cationic aluminum amides, several monomeric aluminum amides were synthesized as potential prercursors (chapter 4). The aluminum amides R2AlN(Ar)SiMe3 (R = Et, Ar = Dipp, 1; R = i-Bu, Ar = Dipp, 2; R = Et, Ar = Mes, 3 (Mes = 2,4,6-Me3C6H2-); R = i-Bu, Ar = Mes, 4) were prepared by ethane or hydrogen elimination reactions. The LiCl salt elimination route afforded the aluminum amide Et2AlN(Mes)SiPh3 (5) in monomeric form. In addition, we report the synthesis and characterization of the dinuclear Ph2Si{N(Mes)AlEt2}2 (6) and the hydride-bridged eight-membered-ring compound {MesN(SiMe3)Al(i-Bu)([mu]-H)}2([mu]-LiH)([mu]-i-Bu2AlH) (7). All new compounds were characterized by 1H and 13C{1H} NMR spectroscopy, and compounds 5, 6, and 7 have also been characterized by single-crystal X-ray crystallography. The attempts to convert these neutral amides into cationic species are presented in chapter 5. The effect of counter cation ([Ph3C]+ vs [Et3Si]+) and counter anion ([CHB11H5I6] vs [CHB11H5Cl6]) on the isolation of cationic aluminum amides was also studied.The reaction of 1 with hexa-iodo carborane [Et3Si][CHB11H5I6] resulted in pure [1][CHB11H5I6]. The product was isolated as fine colorless crystalline solid. This is the first example of an isolated cationic aluminum amide. The formation of [5][CHB11H5Cl6] was observed by NMR spectroscopy. These compounds along with simple [Et2Al]+ were tested towards several hydrosilylation and olefin activation reactions: (i) the reactivity of [EtAlN(Mes)SiPh3][CHB11H5Cl6] [5+] towards 1-hexene polymerization, (ii) [AlEt2]+ mediated hydrosilylation of cyclohexene and 1-hexene, (iii) reduction of lactide and benzophenone by Et3SiH in presence of [AlEt2]+. The activation of H2 and CO was also attempted by using neutral and cationic aluminum amides (1 and 5) but without success. Attempts were aslo made to isolate cationic aluminum compounds featuring Al-[pi] interaction. None of these adducts formed single crystals either because of poor solubility or because of their tendency to form ionic liquids.