ipso Halogenation. II. Bromination of phenols, isomerization and disproportionation of bromophenols, and dienone–phenol rearrangement of bromodienones

Abstract

. 1045 (1983). Bromination of p-crcsol, 2-bronlo-1,-crcsol. 3.4-dimcthylpl~enol. and mesitol in trifluororncthancsulfonic acid gives as the main product the bromo derivative with bromine rrreto to hydroxyl, a result attributed to thc interrncdiatc formation of a bromodicnonc and its rcarrangerncnt. Phenol does not give 111-bromophenol in tritluoron~cthanesulfor~ic acid. 4-Bromo-2.4.6-trin~cthylcyclohexa-2.5-dicnone rearranges to 3-bromomcsitol in tritluoromethanesulfonic acid and, sinlilarly, 2.4.6-tribromo-4-metl~ylcyclohexa-7.5-dicnonc rcarranges to 2,3,6-tribromo-4-methylphenol. Under appropriate conditions debromination of bromodienones is conipetitivc with rearrangement. Tetramcthylamnloniunl bromide in tritluoromethanesulfonic acid is an effective reagent for isomerization and disproportionation of bronlophenols. Tetramethylarnmoniun~ iodide in trifluoromethanesulfonic acid is an effective reagcnt for selective debromination of bromophenols at the ortlio and pclm positions.ALFRED FISCHER ct GEORGE NARAYANAN HENDERSON. Can. J. Chcni. 61, 1045 (1983). La bromation dep-cresol, du bromo-2-1,-cresol, du dimethyl-3,4 phCnol et du mesitol dans I'acide tritluoromCthanesulfonique donne conimc produit principal Ic dCrivC bromC portant un brome en position r~?c;trl par rapport i I'hydroxylc. On attribuc ce rCsultat i la formation intermkdiaire d'une bromodiCnone qui subirait ulterieurement unc transposition. Le phenol ne donne pas le tn-bromophenol dans I'acide tritluoromCthanesulfonique. La bromo-4 trimCthyl-2.4.6 cyclohexadiknc-2,5 one sc transpose en bronlo-3 mesitol dans I'acide tritluoromCthancsulfonique et d'une f a~o n analogue la tribronlo-2,4,6 methyl-4 cyclohexadikne-2,5 one sc transpose en tribromo-2,3.6 methyl-4 phinol. Dans des conditions approprikcs, la debromation des bromodiCnones entre en compCtition avec la transposition. Le bromure de tCtraniCthylammonium dans I'acide trifluoromCthanesulfonique est un rCactif efficace pour I'isoniCrisation et la disproportionation des bromophCnols. L'iodure dc tCtramCthylammonium dans I'acide trifluoromCthancsulfonique est un reactif efficace pour la debromation sClective des bromophCnols en position ortho et porn.[ Traduit par Ic journal]Aromatic electrophilic substitution reactions can lead to unexpected products as a result of ipso attack and subsequent reaction of the ipso intermediate so obtained (1). Chlorination of p-cresol gives 4-chloro-4-methylcyclohexa-2,5-dienone as well as the expected 2-chloro-4-methylphenol (2). Reaction of the chlorodienone in trifluoronlethanesulfonic (tritlic) acid gives 3-chloro-4-methylphenol (3). We have now investigated the bromination of D-cresol in an attemm to obtain the corresponding bromodienone, and have studied other reactions involving bromodienones as potential intermediates.Bromodienones have been known for a long time. Phenol, on reaction with bromine water, gives a tetrabromo derivative, first isolated b y Benedikt in 1879 (4). The debate on the structure of this product, alternatively identified as 2,4,6-tribromop...

Chlorination of oxybenzone and prediction of transformation products using non-equilibrium “forced” molecular dynamics

“…A similar mechanism involving ipso ring hydroxylation at the site of the substituent has been described by Gabriel et al [44]. Fischer and Henderson [45] reported bromination of para-cresol via a mechanism involving halogen addition ipso to the 2-methyl group followed by migration of the bromine to the adjacent 3-position.…”

Bromophloroglucinols and their methyl ethers

“…To arrive at reliable peak assignments as an essential prerequisite for subsequent correlations, we first recorded the spectra of the bromine-free parent compounds and found good agreement with published literature values for phloroglucinol (17,28,40) and its three methyl ethers (3a, 4a, 5a) (37,41). High molecular symmetry and the large chemical shift differences between oxygenated, brominated, and unsubstituted ring carbons lead to facile identification of the peaks for tribromophloroglucinol (2d), its trimethyl ether (Sd), and tribromoresorcinol(7f) as well as for 4,6-dibromoresorcinol (7e) and its dimethyl ether (8b).…”

“…Since three-bond coupling to the methoxy and two-bond coupling to the ortho proton are of approximately equal magnitude (4 Hz), the broad quintet patterns centered near 157.5 ppm established the resonance frequencies for C-5 of 3d and 4d, C-3 of 4e, and C-1 of 5c. This analysis leaves only the assignments for C-2 and C-4 of 2,4-dibromo-5-methoxyresorcinol (90.5 vs. 91.2 ppm) uncertain and suggests a reversal of the C-1 (157.4 ppm in CDC13) and C-5 (160.5 ppm) assignments for 5b published by Fischer and Henderson (37).…”

Ruthenium(II)‐Catalyzed Protocol for Preparation of Diverse α,β‐ and β,β‐Dihaloenones from Diazodicarbonyls

“…The diazodicarbonyl 1b first gives α,β‐dibromoenone 7 , which then undergoes enolization followed by HBr elimination to furnish the stable aromatic 4a . As evidence for this mechanism, the formation of intermediate 8 from 7 has been described by the Henderson’s group 30…”