Tris(trimethylsilyl)methane has excellent properties for an internal chemical shift thermometer to measure temperatures in 13C DNMR spectroscopy [*d between the 13C signals of and CH carbons]. It is Si(CH 3) 3 chemically inert, soluble in most organic solvents and has an almost linear dependence of *d on temperature. The sensitivity (approximately 1 ¡C Hz~1 at 90 MHz) allows the measurement of temperature to within 1 ¡C. The chemical shift is solvent dependent, so each solvent mixture must be calibrated. Calibrations are presented for seven solvents.

Low-temperature rapid-injection NMR (RINMR) experiments were performed on tris(trimethylsilyl)methyllithium. In THF/Me2O solutions, the separated ion (1S) reacted faster than can be measured at -130 degrees C with MeI and substituted benzaldehydes (k >/= 2 s -1), whereas the contact ion (1C) dissociated to 1S before reacting. Unexpectedly, the triple ion reacted faster with electron-rich benzaldehydes relative to electron-deficient ones. The addition of HMPA had no effect on the rate of reaction of the triple ion with p-diethylaminobenzaldehyde, and the immediate product of the reaction was the HMPA-solvated separated ion 1S, with the Peterson product forming only slowly. Thus, the aldehyde is catalyzing the dissociation of the triple ion. HMPA greatly decelerated the reaction of 1S (<10 -10), providing an estimate of the Lewis acid activating effect of a THF-solvated lithium cation in an organolithium addition to an aldehyde.

The role of polar solvents (particularly HMPA) in controlling the ratio of 1,2 to 1,4 addition of sulfur-substituted organolithium reagents to cyclohexenones and hexenal was studied. Low-temperature, multinuclear NMR studies provided quantitative information about the ratio of contact (CIP) and solvent-separated (SIP) ion pairs in solutions of dithianyllithiums and phenylthiobenzyllithiums in THF-HMPA solutions. The ratio of contact and separated ion pairs was manipulated by changes in the strength of solvation (generally through the addition of HMPA). Although the results are consistent with the CIP/SIP distribution being an important factor in determining the regioselectivity of these additions (Curtin-Hammett limitations prevent a direct correlation), it cannot be the only one. Changes in diastereomeric product ratios upon addition of HMPA suggest that complexation of HMPA to lithium has two effects. First, it causes ion pair separation, which enhances 1,4 addition. Second, it lowers the Lewis acidity and catalytic effectiveness of the lithium cation, which also favors 1,4 addition. For most sulfur-stabilized lithium reagents, 2 equiv of HMPA suffice to achieve >95% 1,4 addition, whereas 4 equiv of DMPU are required to achieve identical regiochemical and stereochemical results.

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Ether-solvated contact and separated ion pairs (CIP and SIP) for two lithium reagents, tris(trimethylsilyl)methyllithium (1) and bis(3,5-bistrifluoromethylphenylthio)methyllithium (2), have been characterized and observed for the first time under conditions of slow exchange by NMR spectroscopy, and barriers to interconversion have been measured. A Saunders isotope perturbation experiment was used to support identification of the CIP and SIP species for 2.