No abstract

NMR experiments for undergraduate organic chemistry laboratories are difficult to find. In an earlier paper we presented a laboratory exercise in which NMR spectroscopy and molecular mechanics are used in tandem for conformational analysis.1 In this paper we provide a laboratory exercise in which NMR is used to monitor enantiomeric excess in asymmetric reductions.

Detection and Isolation of 30I and 7 during the Solvolysis of 3 in pH 7.00 0.2 M Phosphate Buffer. A solution of 50 mg (0.148 mmol) of 3-HBr in 60 mb of pH 7.00 0.20 M phosphate buffer (µ = 1.0, KC1) was prepared under strictly anaerobic conditions and then stirred for 1.5 h at 30 °C. Opening of the reaction to the air was followed by extraction 5x with 50-mL portions of chloroform. These extracts were dried over MgS04, filtered, and concentrated to afford a green-yellow solid, 6.6-mg yield. TLC in 10% ethanol in chloroform indicated 7 as the major product with a trace amount of 30x-Thus, the crude product represents ~25% yield based on 3-HBr. Separation of the two quiñones was carried out by using a 25-g silica gel column prepared with chloroform. Elution with chloroform removed 30I followed

Aceanthrylene is reduced by sodium/ammonia solution to its 2,6-dihydro derivative (4), which indicates a protonation site in the dianion in contrast to the MNDO-calculated position of highest electron density. It is concluded that monoanion stability must be the prevailing feature. Compound 4 is also reduced by sodium/ammonia to provide 1,9-ethano-9,10-dihydroanthracene, which allowed the first proton NMR study of a rigid, boat-shaped dihydroanthracene. Especially interesting are the long range, five-bond homoallylic couplings between the 9and 10-positions, and the values measured are 4.7 Hz for 123456=/9i10 (dipseudoaxial) and 1.4 Hz for sJ9|1(y (pseudoaxial/pseudoequatorial). The protonation and alkylation of (3-phenylindenyl)lithium is also considered so as to better understand the behavior of the intermediates derived from aceanthrylene.

Metal-ammonia ring reductions have been carried out with 9-acetyl-10-alkylanthracenes, 10-alkylanthroate esters, l-acetyl-4-alkylnaphthalenes, 4-alkylnaphthoate esters, and p-tert-butylbenzoic acid. The stereochemistry of these reactions is determined by the protonation of the final monoanion. A model is suggested which involves considerable enolate character, as well as a variation in the extent of ring folding. In general, dihydroanthracene anions are the most highly folded and provide cis products, dihydronaphthalenes have intermediate folding and produce cis/trans mixtures, and p-tert-butyldihydrobenzoate dianion is planar and produces trans products. NMR methods for isomer assignment and conformational analyses are also provided.The protonation (and alkylation) of monoanions derived from dihydroanthracenes, -naphthalenes, and -benzenes has received considerable attention.1 These anions may be generated by deprotonation of the dihydro aromatics (1) or by electron addition to the fully aromatic compounds (2) . In the latter case, these monoanions represent the final intermediate in dissolving metal reduction (or reductive alkylation). In either case, it is the protonation (alkylation) of this intermediate that determines stereochemical (cis/trans) outcome.The most heavily studied case has been the dihydroanthracene system, and early observations1®-0 of trans products from the protonation of 9-metallo-9,10-dialkyl-9,10-dihydroanthracenes (DHA's) led to thé model shown as anion 3. This seemed reasonable, since (1) the DHA (1) (a)