Sodium Alkyl(trimethylsilyl)amides:\nSubstituent- and\nSolvent-dependent Solution Structures and Reactivities

The preparation of sodium isopropyl­(tri­meth­yl­silyl)­amide\n(NaPTA), sodium (1-phenyl­ethyl)­(tri­meth­yl­silyl)amide\n(NaPETA), sodium <i>tert</i>-butyl­(tri­methyl­silyl)­amide\n(NaBTA), and isotopologues [<sup>15</sup>N]NaPTA and [<sup>15</sup>N]NaBTA are described. Solution structural studies using a combination\nof <sup>29</sup>Si NMR spectroscopy, the Method of Continuous Variations,\nand density functional theory computations provided insights into\naggregation and solvation in a range of solvents including toluene, <i>N</i>,<i>N</i>-dimethylethylamine, triethylamine,\nMTBE, THF, 1,2-dimethoxyethane (DME), diglyme, <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine\n(TMEDA), <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylcyclohexanediamine (TMCDA), <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-pentamethyldiethylenetriamine\n(PMDTA). 12-crown-4, 15-crown-5, and 18-crown-6 revealed solvent-\nand substituent-dependent dimer–monomer mixtures with affiliated\nsolvation numbers. Complexation of the three crown ethers documented\nboth crown and substituent dependencies. Qualitative studies of reactivity\nshowed a variety of reactions of NaPETA. Aminolysis of methyl benzoate\nwith dialkylamines mediated by NaPTA afforded high yields of benzamides.\nQuantitative rate studies of aminolysis of methyl benzoate by NaPTA\nrevealed a 47,000-fold range of rates. Detailed rate studies in toluene\nand THF showed dimer-based mechanisms. The role of primary- and secondary-shell\nsolvation by THF is discussed, including nuances of methods used to\nseparate the two contributions. PMDTA-solvated NaPTA monomer reacts\nas a monomer whereas bis-diglyme solvated monomer reacts as a dimer.\nRate studies exploring the structure–reactivity correlations\nof the three crown ethers show mono- and bis-crown-based pathways\nin which 15-crown-5the crown ether often said to be of choice\nfor sodiumwas decidedly inferior as an accelerant.