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Conformational preferences in the l-(N,N-dialkylamino)-l-phenyl-2-propanol and 2-(N,N-dialkylamino)-1-phenyl-1-propanol series of amino alcohols and their benzoate esters have been studied by nuclear magnetic resonance and infrared spectroscopy. The threo amino alcohols reside almost exclusively in the intramolecularly hydrogen bonded form of the anti rotamers, while the erythro alcohols prefer the intramolecularly hydrogen bonded form of the gauche rotamers.Conformational equilibria in the 2-amino-1,2-diphenylethanol system have been studied in detail by NMR4 and infrared spectroscopy.l These earlier studies led to working hypotheses regarding the factors controlling conformation in that system. The present study was undertaken to test the applicability of these working hypotheses in related systems to further refine our understanding of the controlling factors. The compounds chosen for this study belong to the 1-amino-1-phenyl-2-propanol (I) and 2-amino-lphenyl-1-propanol (11) systems.The stereoisomeric N,N-dialkylamino alcohols in Table I were prepared by either nucleophilic scission of the C-0 bond of trans-1-phenyl-1-propene oxide, hydroboration of the appropriate enamine, or reduction of the corresponding amino ketone. The well-documented trans nature of the epoxide opening established the configuration of the erythro6 amino alcohols in these systems. In each case structure and homogeneity were established by NMR spectroscopy. The vicinal coupling constants, J a b , were conveniently obtained from the NMR spectra and reflect a weighted mean dependent on the relative populations of the three possible staggered rotamem6 This study, as the earlier one, is concerned with identifying trends and evaluating their conformational implicatrons. The calculations of approximate anti to gauche rotamer ratios using the previously suggested working values, J a n t i 10.3 and Jgauche = 2.6 H z ,~ are consistent with the objective. It is apparent from the values of J a b (Table I) that the threo amino alcohols 1 and 3 highly populate the anti rotamers tA and tA' (Charts I and 11), respectively. Intramolecular hydrogen bonding (OH-N) undoubtedly contributes to the stability of these rotamers, already favored on steric grounds. The intrinsic stablizing influence of the division of the four bulkiest groups into two pairs separated from one another by hydrogen atoms has been noted previously in the 2-amino-1,2-diphenylethanol ~y s t e m .~ At high dilution in carbon tetrachloride, only hydrogen bonded OH stretching bands appear in the infrared spectra of threo piperidino alcohols la and 3a (Table 11); therefore, the highly populated anti rotamers tA and tA', respectively, exist completely in the intramolecularly hydrogen bonded form. In addition to the strong intramolecular OH-N absorption bands, the corresponding threo pyrrolidino alcohols l b and 3b exhibit weak absorptions a t 3590 and 3621 cm-l, respectively. The band a t 3590 cm-l (lb) assigned to intramolecular OH-n (phenyl), is consistent with a small population of gauch...