Metal-mediated hydrodenitrogenation catalysis: Designing new models

Abstract

Reduction of Ta(DIPP)₂Cl·OEt₂(DIPP= 2,6-OC₆H₃ⁱPr₂) with 2 equiv. of NaHg in the presence of 3,5-lutidine results in cyclometalation of DIPP to give TaCl(DIPP)-(OC₆H₃ⁱPr-η²(C,C)-CMe =CH₂)(3,5-lutidine)₂ (10) in moderate yield. Metallacycle 10 was also isolated from the reaction of (η⁶-C₆Me₆)Ta(DIPP)₂Cl with 3,5-lutidine. Examination of both crude reaction mixtures by ¹H NMR revealed 10 to be the major product without any indication of the formation of η²-lutidine species. These observations suggest that η²(N,C)-coordination of 3,5-lutidine is kinetically incompetitive with respect to the cyclometalation of DIPP by d² tantalum. Such undesired reactivity of DIPP can be potentially inhibited by the use of linked aryloxide ligands to prevent close approach of metalatable C-H bonds of DIPP to the metal center. An efficient route to a family of silane-linked aryloxides was developed. Tris(2-hydroxy-3-isopropylphenyl)methylsilane (H₃TIPSI, 59), bis(2-hydroxy-3-isopropyl-phenyl)diphenylsilane (H₂BIPSI, 61), and bis(2-hydroxy-3-isopropylphenyl)dimethyl-silane (H₂BIPSI, 60) were obtained via deprotection of the parent silane-linked anisoles. The anisoles were prepared in high yields by treating 2-methoxy-3-isopropyl-phenyllithium·0.5TMEDA (27·0.5TMEDA) with an appropriate amount of chloroalkylsilanes. The deprotection was carried out employing BBr₃ in CH₂Cl₂ followed by hydrolysis of the intermediate boron ethers in the presence of a non-nucleophilic base to avoid protiodesilylation. Additionally, a significantly improved synthesis of 1,2-bis(3-isopropyl-2-hydroxyphenyl)ethane (H₂BIPP, 40) employing 27·0.5TMEDA as a starting reagent is reported. 2-Methoxyphenyllithium 27·0.5TMEDA was prepared via catalytic ortho-directed metalation of 2-isopropylanisole, and the mechanistic aspects of such metalations are presented. Trinuclear complex (AlBr₂)₃TIPSI (55) was isolated from the reaction of Me₃TISPI with 3 equiv. of AlBr₃ in benzene at 60°C. Preliminary reactivity studies show that Me₃TIPSI (49) and Me₂BIPSI (51) can be reacted with TaBr₅ under similar conditions to give Br₃Ta(MeTIPSI)(THF)₂ (62) and Br₃Ta(BIPP)OEt₂ (63), respectively, after appropriate reaction work ups.