2308-38-5Relevant academic research and scientific papers
'New' catalysts for the ester-interchange reaction: The role of alkali-metal alkoxide clusters in achieving unprecedented reaction rates
Stanton, Matthew G.
, p. 5981 - 5989 (2007/10/03)
The catalytic effect of alkali-metal tert-butoxide clusters on the rate of ester interchange for several pairs of esters has been determined in nonpolar and weakly polar solvents. Reactivities increase in the order (Li+ + + + +) with the fastest rates reaching 107 catalytic turnovers per hour (TO/h). Ester interchange rates were sensitive to the size of both the transferring OR groups and the ester substituent. Phenyl esters did not exchange with aliphatic esters due to nonstatistical breakdown patterns in the tetrahedral intermediate. A first-order equilibration analysis on the interchange between tert-butyl acetate (tBuAc) and methyl benzoate (MeBz) (5 mol % NaOtBu) indicated enhanced reaction rates as the reaction proceeded. Isolation and quenching (DCl/D2O) of precipitated catalyst points to a mechanism whereby sequential methoxy incorporation into the catalyst cluster increases activity, but eventually precipitates out of solution as a 3:1 OMe:OtBu cluster. The rate law was determined to be k(obs)[MeBz]1[tBuAc]0[NaOtBu](x), where x = 1.2(1), 1.4(1), and 0.85(1) in hexane, ether, and THF, respectively, under conditions where tetrameric catalyst aggregates are expected. Reaction rates were generally observed to be higher in nonpolar solvents (hexane > toluene, ether > THF). Eyring analysis over a 40°C range yielded ΔH(≠) = 10.0(1) kcal mol-1 and ΔS(≠) = -32(3) eu. A Hammett (σ) plot generated with para-substituted methyl benzoates gave ρ +2.35 (R 0.996). These results are interpreted in terms of a catalytic cycle composed of two coupled transesterification reactions with a turnover-limiting addition of a tert-butoxy-containing cluster (tetramer) to methyl benzoate. Catalyst relative reactivities (Cs+ > Rb+ > K+ > Na+ > Li+) are interpreted in terms of competitive electrostatic interactions between the alkali-metal and ground-state and transition-state anions. This analysis predicts the observed linear dependence between log(k(obs)) and l/r(ionic).
Hydroboration. 57. Hydroboration with 9-Borabicyclononane of Alkenes Containing Representative Functional Groups
Brown, Herbert C.,Chen, Jackson C.
, p. 3978 - 3988 (2007/10/02)
The hydroboration of alkenes containing representative functional groups was examined with 9-borabicyclononane (9-BBN) in order to extend the hydroboration reaction for the preparation of functionally substituted organoboranes.Terminal alkenes containing a remote functional group are hydroborated with a remarkable regioselectivity (>=98percent terminal), producing the corresponding stable organoboranes. 9-BBN hydroborates the allylic derivatives so as to place boron essentially on the terminal carbon atom (>=97percent).The directive effect is further enhanced (>=99percent) in the case of β-methylallyl derivatives.The hydroboration of crotyl derivatives attaches boron predominantly at the 2-position, followed by an elimination-rehydroboration sequence.However, crotyl alcohol can be protected against elimination as the tert-butyl or tetrahydropyranyl ethers.The hydroboration-oxidation of ethyl crotonate involves a series of elimination, hydroboration, and condensation processes.In the vinyl, crotyl, and isobutenyl systems, the mesomeric effect of the substituent favors the placement of boron at the β-position, while the inductive effect favors the α-position, with the former effect predominating in most cases.Acyclic β-substituted organoboranes undergo rapid elimination.Nonpolar solvents and lower reaction temperatures decrease the rate of elimination.However, those derived from cyclic vinyl derivatives are relatively stable under neutral conditions, undergoing facile elimination in the presence of a base.
