¹³C lithiation shifts in aldimines and ketimines. Evidence on the configuration of lithiated N-tert-butylimines

Abstract

ROBERT R. FRASER and NOEMI CHUAQUI-OFFERMANNS. Can. J. Chem. 59,3007 (1981). The "C shieldings for a series of aldimines and ketimines have been measured along with the shifts for their 1-lithio derivatives. For those aldimines with a primary or secondary alkyl group attached to nitrogen, the "lithiation shifts" for the attached carbon (C-4) were all upfield due to the change from anti to syn configuration on lithiation. In the ketimines and in the N-tert-butyl aldimines and ketimines. the lithiation shifts for the C-4 carbon failed to provide stereochemical information. The shifts at C-1 and C-2 proved similar to those previously observed for lithiated alkenes.ROBERT R. FRASER et NOEMI CHUAQUI-OFFERMANNS. Can. . I . Chem. 59, 3007 (1981). On a mesure les blindages du I3C d'une serie d'aldimines et de cetimines ainsi que les deplacements chimiques de leurs derives lithies en position 1. Pour les aldimines ayant un groupe alkyle primaire ou secondaire fixe sur I'azote, les deplacements conduits par la lithiation du carbone (C-4) se font tous vers les champs forts par suite d'un changement de configuration de anti a syn lors de la lithiation. Dans le cas des cetimines, des N-butylaldimines et cetimines, les deplacements provoques par la lithiation sur le carbone en position 4 n'ont pas apporte d'information sur la stereochimie. Les deplacements chimiques des carbones en positions 1 et 2 sont identiques a ceux observes anterieurement pour les derives lithies des aicenes.[Traduit par ie journal]In a series of studies of the stereochemistry of lithiation and alkylation of aldimines and ketimines, we have described evidence for a very marked energetic preference for the formation of a synl lithiated intermediate on the basis of formation of syn alkylation pmducts (1, 21, a b initio calculations (31, and the observation of extraordinarily large barriers to rotation about the N-C-4 single bond of several 1-lithio-3-azapent-2-enides (4). Distinction between configurations at the C=N bond of these imines having the formulae shown below is readily provided by the large syn-anti differential shielding which appears as preferential upfield shifts of 5 to 11 ppm ( 5 ) for either C-1 or C-4 when syn to each other.In this paper we wish to describe the 13C shielding parameters for a variety of lithiated imines. Comparison with the shieldings of their neutral precursors provides a measure of the "lithiation shifts" whose magnitudes appear to be significantly influenced by substituent effects and by the stereochemistry at the C=N bond. A more unique class of sterically congested aldimines and ketimines bearing an N-tert-butyl substituent was also studied. One of these, acetone N-tert-butylimine was observed to undergo lithiation and methylation to give only syn product, indicative of a syrz lithiated intermediate. Unfortunately, similar alkylation experiments on N-tert-butylaldimines were i n c~n c l u s i v e .~ Measurement of the I3C spectra of these aldimines and lithio derivatives gave "lithiation shift" data which failed to p...

1-Azaallylic Anions in Heterocyclic Chemistry

“…9 As mentioned before, metalated 1-azaallylic anions have a syn (Z)-C-N stereochemistry preference. This configuration has been described and confirmed by ab initio calculations, [10][11][12][13] NMR studies, [14][15][16][17][18][19][20] X-ray analysis, 21,22 and experimental evidence. [23][24][25][26] The experimental evidence was rationalized via ab initio computational studies.…”

“…As mentioned before, metalated 1-azaallylic anions have a syn ( Z )-C−N stereochemistry preference. This configuration has been described and confirmed by ab initio calculations, − NMR studies, − X-ray analysis, , and experimental evidence. − The experimental evidence was rationalized via ab initio computational studies . Energy barriers of 4.7 and 6.2 kcal/mol were estimated with a 4-31G basis set in favor of the syn isomer, calculated for acetaldimine and N -methylacetaldimine anions, respectively.…”

“…Studies on the 13 C NMR shielding for a series of aldimine and ketimine anions confirmed the syn geometry. 17 Deprotonation of a series of aldimines, which are anti configurated at the imino form, received an upfield shift for C-4, the one attached to the nitrogen atom, rationalized with the change from the anti to the syn configuration (Chart 2; R ) H; R 1 ) CH 2 Ph; R 2 ) H, CH 3 ). The latter is explained by a γ-effect of steric origin, when the R 1 group and the C-3 are situated on the same side of the space.…”

“…These experimental observations were explained by the NCC angle strain in the syn endo anions 9, 13 measured as 120°or less, in contrast with the exo angle, which is reliable to the ideal NCC ) 133°for acyclic imine anions. 10,11,17 It is worth noting how well the 1-azaallyl moiety approaches the angles for allyl anions (CCC ) 132°) and propene (CCC ) 125°).…”

Oligomerization and Hydroamination of Terminal Alkynes Promoted by the Cationic Organoactinide Compound [(Et2N)3U][BPh4]

“…The organic products were vacuum‐transferred (10 −6 mmHg) into a Schlenk tube and both residue and volatiles were characterized by 1 H, 13 C, and 2D NMR spectroscopy, GC‐MS, and by comparison with known compounds. For spectroscopic data of compounds, see the corresponding references: 1, 2 ,28 3 ,14b 4 ,29a 5 ,29b 8 ,29c, 29d 10 ,19 13 ,12 14 ,14 and 19 12…”