The regioselective deprotonation of pyridine in the gas phase has been investigated by using chemical reactivity studies. The mixture of regioisomers, trapped as carboxylates, formed in an equilibrium mixture is determined to result from 70-80% deprotonation in the 4-position, and 20-30% deprotonation at the 3-position. The ion formed by deprotonation in the 2-position is not measurably deprotonated at equilibrium because the ion is destabilized by lone-pair repulsion. From the composition of the mixture, the gas-phase acidities (DeltaH degrees acid) at the 4-, 3-, and 2-positions are determined to be 389.9 +/- 2.0, 391.2-391.5, and >391.5 kcal/mol, respectively. The relative acidities of the 4- and 3-positions are explained by using Hammett-Taft parameters, derived by using the measured gas-phase acidities of pyridine carboxylic acids. The values of sigmaF and sigmaR are -0.18 and 0.74, respectively, showing the infused nitrogen in pyridine to have a strong pi electron-withdrawing effect, but with little sigma-inductive effect.

Highly stereoselective boronic ester chemistry has been used for the synthesis of (4S,6S,7S)-7-hydroxy-4,6-dimethylnonanone (1), the pheromone of the cigarette beetle. 2-Bromo-1-butene (8) was made from 1-butyne via bromoboration and protodeboronation, and was converted to 1-ethylethenylmagnesium bromide. (R,R)-1,2-Dicyclohexyl-1,2-ethanediol [“(R)-DICHED”] methylboronate was treated with (dichloromethyl)lithium to yield (R)-DICHED (S)-1-chloroethylboronate (9), which with 1-ethylethenylmagnesium bromide yielded (R)-DICHED (R)−(2-ethyl-1-methyl-2-propenyl)boronate (10). Further chain extensions with (chloromethyl)lithium, (dichloromethyl)lithium followed by methylmagnesium bromide, and (dichloromethyl)lithium followed by ethylmagnesium bromide completed assembly of the carbon skeleton. Deboronation with hydrogen peroxide yielded (3S,5S,6S)-2-ethyl-3,5-dimethylocten-6-ol (14), which with osmium tetraoxide and sodium periodate yielded 1.

The reaction of catecholborane with 3-hexyne was found to produce more of the frans-addition product, catechol (£)-3-hexenyl-3-boronate, than the (2)-isomer, a result totally inconsistent with the stereospecific c/s-addition of catecholborane to internal alkynes previously reported. Reaction in the presence of azobis(isobutyronitrile), a free radical initiator, increased the proportion of (£)-isomer to >90%. Reaction in the presence of galvinoxyl, a free radical inhibitor, led to some increase in the amount of c/s-addition product, but the frans-isomer still predominated. Attempted addition of dichloroborane to 3-hexyne using boron trichloride and triethylsilane was apparently superseded by boron trichloride addition, but the use of diethylsilane with boron trichloride resulted in efficient stereospecific ris-hydroboration. The resulting (Z)-3-hexenyl-3-boron dichloride was easily converted to the methyl ester with methanol. It was found that heat or radical catalysts would isomerize the (Z)-3-hexenyl-3-boronic ester to the (£)-isomer.

Physical organic chemistry studies utilizing gas-phase ion chemistry in the past year are summarized. The studies reported range from thermochemical studies, illustrated by the isomeric cyclohexane acidity study and the study of alkyl cation reactions with ozone, to mechanistic investigations that find solvation to have very little influence in the transition state of S N 2 reactions. Also included are groundbreaking structural studies exemplified in the generation and characterization of the first example of an open-shell doublet triradical, and the integration of previously autonomous techniques in an empirical study of the reaction of previously difficult to generate neutral organic intermediates with ions. Studies of biological and organometallic reagents are also surveyed, as well as advances in photoelectron spectroscopy.

Asymmetric Synthesis of Serricornin via Boronic Esters.-The title compound (XIII), the pheromone of the cigarette beetle, is synthesized from chiral (II) in 59% overall yield. -(MATTESON, D. S.; SINGH, R. P.; SCHAFMAN, B.; YANG, J.; J. Org. Chem. 63 (1998) 13, 4466-4469; Dep. Chem., Wash. State Univ., Pullman, WA 99164, USA; EN)