Tribromoisocyanuric Acid/Triphenylphosphine: a New System for Conversion of Alcohols into Alkyl Bromides

Foi desenvolvido um método fácil e eficiente para a conversão de alcoóis em brometos de alquila em meio neutro, utilizando ácido tribromoisocianúrico e trifenilfosfina (razão molar 1,0:0,7:2,0, álcool/ácido tribromoisocianúrico/trifenilfosfina) em diclorometano à temperatura ambiente. Esse método pode ser aplicado para a conversão de alcoóis primários, secundários, benzílicos e alílicos, sendo os brometos correspondentes obtidos em 67-82% de rendimento. Alcoóis terciários não são reativos sob estas condições.


Introduction
Alkyl bromides are versatile compounds from both academic and industrial points of view that can be easily converted into a variety number of other functional groups through well-known methodologies. 1 Furthermore, several marine organic bromides with interesting biological activity have also been described. 2Among the diverse methodologies described in the literature for the preparation of such compounds, the conversion of alcohols into the corresponding bromides is the most convenient. 3Several reagents can accomplish this transformation, including the harmful and corrosive HBr/H 2 SO 4 , 4 phosphorous (oxy)bromides 5,6 or SOBr 2 . 6However, a very attractive mild methodology is based on the Appel reaction that uses a combined system of triphenylphosphine (TPP) with an electrophilic source or bromine, such as CBr 4 , 7 2,4,4,6,-tetrabromo-1,5-cyclohexadienone, 8 (poly) bromocarbonyls, 9 and various N-bromo compounds. 3,10,11ribromoisocyanuric acid (TBCA, Figure 1) is a safe and stable electrophilic brominating reagent. 12We have studied its utilization for bromination of alkenes, 13 arenes, 14 β-dicarbonyl compounds, 15 as well as for the bromodecarboxylation of cinnamic acids. 16Furthermore, TBCA possesses some advantages compared to other N-bromo reagents, as it can easily be prepared from inexpensive materials (cyanuric acid, KBr and oxone) 17 and also presents high atom economy (i.e., maximizing the atomic mass from the reagents that can be incorporated into the product). 18ntinuing our interest on the chemistry of trihaloisocyanuric acids, 19 we present here our initial results on the utilization of the system TBCA/triphenylphosphine for conversion of alcohols into alkyl bromides.

Results and Discussion
In initial experiments, it was chosen 2-octanol as model substrate to test the methodology.Using an equimolar Tribromoisocyanuric Acid/Triphenylphosphine J. Braz.Chem.Soc.976 amount of the reagents (1.0:0.34:1.0,alcohol/TBCA/TPP), the reaction was slow.However, the reaction proceeded with complete substrate conversion using a molar ratio of 1.0:0.7:2.0 (alcohol/TBCA/TPP) within 1.5 h in CH 2 Cl 2 at room temperature.Therefore, the results of the reaction of alcohols with TBCA/TPP to produce the corresponding alkyl bromides (along with triphenylphosphine oxide and cyanuric acid) are shown in Table 1.In general, the reactions proceeded smoothly for the conversion of primary, secondary, benzylic and allylic alcohols to their corresponding alkyl bromides (67-82%).On the other hand, tertiary alcohols do not react, even in acetonitrile under reflux.Analysis of the crude reactions by gas chromatography (GC) indicated that the alkyl bromides were obtained pure, with exception of cyclohexyl bromide, which was formed as a mixture with cyclohexene (ca.2:3 by gas chromatography-mass spectrometry (GC-MS)).In addition, although TBCA is a useful reagent for the alcohol oxidation 20 and bromination of alkenes 13 and arenes, 14 none of these products were detected by the analytical techniques employed, showing that the method is quite general and suitable for the conversion of benzylic and allylic alcohols into their corresponding bromides.
A competitive reaction was designed for showing the chemoselectivity of the TBCA/TPP system toward alcohols (Scheme 1).Therefore, a mixture of equimolar amounts of 1-octanol and 2-octanol was subjected to the conditions described in Table 1 and it was observed that the primary alcohol is 12.5 more reactive than the secondary alcohol (determined by GC-MS).
Based on the above results, a S N 2 process could be involved in the reaction of alcohols with TBCA/TPP to produce alkyl bromides, as also observed in similar reactions with different N-bromo compounds. 3,11A plausible scheme for this transformation (Scheme 2) proceeds by TBCA bromination of TPP to produce a bromophosphonium salt (1) that is further converted into the oxyphosphonium intermediate (2).Bromide anion attack on the intermediate 2 gives the alkyl bromide and triphenylphosphine oxide.After two repetitions of the whole process, cyanuric acid is also formed and precipitates at the end of the reaction.

Conclusions
In summary, we have developed a very convenient route for the preparation of primary and secondary alkyl bromides from alcohols under neutral conditions.The reaction is easily reproducible, the conditions are mild and the reagents are easily available.Although the yields using our Scheme 1. methodology is lower than similar reactions (Table 2), the advantages of TBCA compared to other N-bromo reagents, such as easily preparation and high atom economy, turn our methodology very convenient.

General information
All chemicals and solvents were used as received.Tribromoisocyanuric acid was prepared as described. 17The spectra were recorded on a Bruker AC-200 spectrometer at 200 MHz ( 1 H) and 50 MHz ( 13 C) in CDCl 3 solutions with tetramethylsilane (TMS) as internal standard.High-resolution gas chromatography was performed on a HP-5890-II gas chromatograph with flame ionization detector (FID) using a 30 m (length), 0.25 mm internal diameter (ID), and 25 µm (phase thickness) RTX-5 capillary column and H 2 (flow rate 50 cm s -1 ) as carrier gas (split: 1:10).Infrared (IR) spectra were recorded on a Nicolet 740 FT-IR spectrometers (KBr film).GC-MS analyses were performed on a Shimadzu GCMS-QP2010S gas chromatograph with electron impact (70 eV) by using a 30 m DB-5 silica capillary column.
General procedure for the preparation of alkyl bromides TBCA (1.4 mmol) was added to a stirred solution of TPP (4 mmol) in CH 2 Cl 2 (50 cm 3 ).After 5 min, the alcohol (2 mmol) was added and the suspension was stirred at room temperature.After 1.5 h, cyanuric acid was filtered off, the liquid was washed with water (2 × 25 cm 3 ) and the organic phase was dried (Na 2 SO 4 ) and evaporated on a rotatory evaporator under reduced pressure.The residue was treated with pentane and filtered through a silica gel (70-230 mesh) pad.The pure alkyl bromide was obtained after evaporation of pentane.
based on alcohol.b Determined by GC-MS.