2236-57-9Relevant academic research and scientific papers
The gas phase 1,2-Wittig rearrangement is an anion reaction. A joint experimental and theoretical study
Sheldon, John C.,Taylor, Mark S.,Bowie, John H.,Dua, Suresh,Chia, C.S. Brian,Eichinger, Peter C. H.
, p. 333 - 340 (1999)
The migratory aptitudes of alkyl groups in the gas phase 1,2-Wittig rearrangement have been determined experimentally as follows. An anion Ph- -C(OR1)(OR2), on collisional activation, competitively rearranges to the two 1,2-Wittig ions PhC(R1)(OR2)(O-) and PhC(R2)(OR1)(O-)[R1 and R2 = alkyl and R1 2]. These two ions respectively eliminate R2OH and R1OH. The smaller alkanol is eliminated preferentially, indicating that R2 (the larger alkyl group) is migrating preferentially (observed tert-Bu > iso-Pr > Et > Me): a trend generally taken to indicate a radical reaction. However, a Hammett investigation of the relative losses of MeOH from R-C6H4--C(OMe)2 shows this loss decreases markedly as R becomes more electron withdrawing, an observation not consistent with a radical reaction. Ab initio calculations [at the CISD/6-311 + + G**//RHF (and UHF)/6-311 + + G** levels of theory] have been used to construct potential surface maps for the model 1,2-Wittig systems -CH2OMe→ EtO-, and -CH2OEt→PrO-. Each of these exothermic reactions involves migration of an alkyl anion. There are no discrete intermediates in the reaction pathways. There is no indication of a radical pathway for either rearrangement. It is proposed that the gas phase 1,2-Wittig rearrangement involves an anionic migration, and that it is not the barrier to the early saddle point but the Arrhenius A factor (or the frequency factor of the QET), which controls the rate of the rearrangement. Weak H-bonding between the alkyl anion and the oxygen of the neutral carbonyl species acts as a pivot in holding the molecular complex together during the migration process. This electrostatic interaction increases with an increase in the number of hydrogens able to H-bond to oxygen and with the number of equivalent ways this H-bonding can occur. The relative migratory aptitude of alkyl anions bound within these molecular complexes is tert-Bu- > iso-Pr- > Et- ? Me-, an order quite different from the migratory aptitudes of anions expected from thermodynamic considerations. This conclusion indicates that great care must be exercised in utilising thermodynamically derived migratory aptitudes to explain the course of a kinetically controlled reaction in the gas phase.
Tetrabutylammonium tribromide (TBATB) as an efficient generator of HBr for an efficient chemoselective reagent for acetalization of carbonyl compounds
Gopinath, Rangam,Haque, Sk. Jiaul,Patel, Bhisma K.
, p. 5842 - 5845 (2007/10/03)
Acyclic and cyclic acetals of various carbonyl compounds were obtained in excellent yields under a mild reaction condition in the presence of trialkyl orthoformate and a catalytic amount of tetrabutylammonium tribromide (TBATB) in absolute alcohol. Chemoselective acetalization of an aldehyde in the presence of ketone, unsymmetrical acetal formation, shorter reaction times, mild reaction conditions, the stability of acid-sensitive protecting groups, high efficiencies, facile isolation of the desired products, and the catalytic nature of the reagent make the present methodology a practical alternative.
Thermal and photochemical fragmentation of α,α-dialkoxybenzyl radicals: A comparison of the thermal reactions with laser induced fragmentations by using laser flash and laser-jet photolyses
Banks,Scaiano,Adam, Waldemar,Oestrich, Rolf Schulte
, p. 2473 - 2477 (2007/10/02)
The thermal and photochemical cleavage of α,α-dialkoxybenzyl radicals has been examined using a combination of techniques, including two-laser two-color laser flash photolysis and the laser-jet technique. For the parent α,α-dimethoxybenzyl radical photofragmentation occurs with a quantum yield of 0.80. The study of several unsymmetrically substituted radicals (e.g., methoxyisopropoxy) leads to the conclusion that the photoinduced fragmentation shows no selectivity. In contrast, the thermal decomposition of the radicals shows that fragmentation follows the expected radical stabilities, i.e., isopropyl > ethyl > methyl, the differences being almost exclusively due to changes in the activation energy. By comparing with literature data for methyl elimination it is possible to estimate the rate constants for fragmentation at 56°C as 213, 1380, and 16 600 s-1 for methyl, ethyl, and isopropyl elimination.
Methoxyphenyldiazirine as Precursor to Methoxyphenylcarbene
Wlostowska, Joanna,Moss, Robert A.,Guo, Wenjeng,Chang, Ming Jing
, p. 432 - 433 (2007/10/02)
Reaction of bromo(phenyl)diazirine (2) with methoxide ion gives methoxy(phenyl)diazirine (3), which yields cyclopropanes (4) on photolysis in alkenes.
