Tetradecyltrimethylammonium permanganate : a novel potassium permanganate derived reagent for trans-dichlorination, trans-dibromination and cis-dihydroxylation of olefins t

Reactivity of KMn0 4 has been modified by converting it into long chain quaternary ammonium permanganate salt, viz., tetradecytrimethylammonium permanganate (TDTAP). This new versatile, stable reagent has been utilised for cbemoselective and stereoselective tra11s-dichlorination and dibromination of a variety of alkenes. Use of this reagent for vicinal dihydroxylation of alkenes in a two phase solvent system has also been demonstrated. The mechanism for the tra/ls dichlorination reaction and the nature of the reacting species has been proposed.

Potassium permanganate exhibits unique reactivity towards olefins. However its utility in organic synthesis has been severely limited by solubility problems. In an attempt to improve and modify the reactivity of potassium permanganate, several quaternary ammonium permanganates have been prepared, characterised and used. Solubility studies indicate that these compounds exist in solution as either ion pair or aggregates, with aggregation being promoted by high concentration, low temperature and solvents of low polarity. Detonation of quaternary ammonium pennangantaes during drying at elevated temperatures have been reported 1 • 2 .
A systematic study has shown that salts containing unsaturated groups in the cation (PhCH 2 N+Et 3 , PhCH 2 N+Me 3 , Ph 3 N+Me, C 5 H 5 N+Me) decomposed with explosive violence when the temperature is reached 80-90°. Compounds with a saturated alkyl chain (Et 4 N+, Bu 4 N+, CH 3 (CH 2 ) 15 N+Me 3 ), on the other hand, showed no explosive tendency, but they did decompose passively between 80° and 100°.
We have recently prepared tetradecyltrimethylammonium permanganate (TDTAP) salt and demonstrated its use as an excellent reagent for the chemoselective and stereoselective trans-dichlorination 3 and dibromination 4 of a variety of alkenes. Vicinal dihydroxylation 5 of alkenes with TDTAP and potassium hydroxide in a two phase solvent system has also been reported by us.
TDTAP, a violet crystalline solid, stable at room temperature for a few days, can be stored at 0° in a brown bottle for months and is highly soluble in methylene dichloride.

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This new reagent is readily prepared as follows : To a stirred solution of potassium permanganate (7.9 g, 50 mmol) in water (250 ml) at 25° was added dropwise over 30 min a solution of tetradecyltrimethylammonium bromide ( 17.5 g, 52 mmol) in water (250 ml) when a violet coloured precipitate formed immediately. The mixture was stirred for further 30 min the violet precipitate was filtered off, washed thoroughly with water and dried in vacuo over P 2 0 5 to furnish TDTAP (17.25 g, 92%), m.p. 165-170°.
The main disadvantage of the related reagent benzyltrimethylammonium permanganate is its instability 1 ·2, arises because of the easily formed benzyl radical initiates a chain reaction during drying or when this reagent is handled neat. In view of this it was expected that replacement of benzyl group by a long chain hydrocarbon radical, e.g. tetradecyl, would give rise to increased stability and solubility.
Differential thermal analysis of TDTAP has shown its decomposition to be stepwise exothermic process which starts at 102.3°, with the thermogram reaching a maxima at 119.5°. This clearly indicates that TDTAP decomposes passively at a relative higher temperature compared with earlier reported quaternary permanganate salts. In fact, as mentioned earlier, this salt being stable for months at 00 permits its ready access, whenever required.
Manganese-mediated stereoselective and chemoselective trans -dichlorinaton of alkenes with tetradecyltrimethylammonium permanganate-trimethylchlorosilane Formation of vic-dichlorides by addition of molecular chlorine to olefins has limited synthetic utility due to the occurrence of side reactions 6 . Moreover use of gaseous chlorine presents a potential environmental hazard and its quantitative utilization is hard to work out. Several other reagents add chlorine to double bonds. Sulfuryl chloride 7 reacts readily with most of the ethylenic compounds to yield saturated dichloro derivatives with evolution of sulfur dioxide. Reaction of trichloroamine 8 with ole fins provides a simple means for the preparation of vic-dichlorides. Phosphorous pentachloride 9 , antimony pentachloride 10 , iodobenzene dichloride 11 , tetrabutylammonium iodotetra-chloride 12 and copper(II) chloride 13 have been examined as chlorinating agents with good results in certain cases. cis-Vicinal dichlorinations of ole fins by molybdenum(VI)-acetyl chlo-ride14 and manganese(III) acetate-calcium chloride 15 have been reported. trans-Vicinal dichlorination of olefins with manganese dioxide-trimethychlorosilane (TMCS) 1 6 and Mn02"MnCI2"acetyl chloride 17 and acetal chlorination with Mn02"TMCS l8 have been documented. However, non-homogeneity of these inorganic based reagents in commonly used organic solvents limits their wider synthetic applications. In order to overcome this difficulty Marco et a/. used 1 9.20 benzyltriethylammonium perrnanganate and oxalyl chloride in methylene dichloride. Recently these authors also reported 21 the use ofKMn0 4 -trimethyl-chlorosilane in the presence ofbenzyltriethylammonium chloride to dichlorinate alkenes, open the epoxides and chemoselectively oxidize sulfides to sulfoxides. vic-Dichlorination with hexachloro-ethane22 in the presence of RuCl 2 (PPh 3 ) 3 as a catalyst and sodium chlorite 23 in the presence of Mn(acac) 3 and moist alumina have also been reported. As mentioned above there are severe drawbacks in terms of safety due to decomposition of the reagent when reagent-containing benzyl radical is used. We have reported 3 the use of TDTAP in combination with trimethylchlorosilane (TMCS) to lead chemoselective and stereoselective trans-dichlorination of alkenes (Table 1)  The reaction of 16-dehydropregnenolone acetate 3 and carvone 5 in methylene dichloride with excess of chlorine gas at 0 to 3° for 1.5 h furnished a complex reaction mixture of polychlorinated products. Addition of chlorine to both double bonds took place in both the cases. Column chromatograhic purification of polychlorinated products afforded 3/:J-acetoxy-5,6, 16, 17-tetrachloro-5a-pregnan-20one, 3/:J-acetoxy-5,6, 16,17 ,21-pentachloro-5a-pregnan-20one from 16-DPA 3 and two more unidentified compounds. The desired 5,6-dichlorinated product 11 could not be isolated from this reaction. In case of carvone 5 also hepta and nona chlorinated products were isolated by column chromatography. No trace of the desired 9,10-dichlorocarvonc 13 was found in this chlorination reaction. On the other hand treatment of compounds 16-DPA 3 and carvone 5 with excess ofTDTAP-TMCS in methylene dichloride under identical conditions led to the desired dichlorides 11 and 13 as the only products in high yield comparable with those obtained when a single equivalent ofTDTAP had employed. This excellent chemoselectivity clearly ruled out the possibility of generation of molecular chlorine in the reaction mixture and strongly suggest the formation of an oxochloro manganese intermediate as a chlorinating species.
A violet-coloured solution of TDTAP in methylene dichloride at 0 to 3° changed immediately to brown on treatment with TMCS. A solution of olefin in methylene dichloride was added to this mixture which was then stirred at 0 to 3° for l.S h. During this period the reaction mixture turned green. Along with rrans-dichlorinated products, we isolated hexamethyldisiloxane as one of the end products in this reaction.
In order to identify the nature of the actual reacting species in the trans-dichlorination reaction, we have carried out UV-visible absorption and EPR spectroscopic studies of the starting violet coloured solution of TDTAP in methylene dichloride, the brown intermediate (reacting species) formed after addition ofTMCS and the final green coloured methylene dichloride solution. The UV -visible spectrum of the starting homogeneous violet coloured solution ofTDTAP in methylene dichloride showed similar pattern as that of a solution of KMn0 4 in methylene dichloride and acetonitrile · mixture.
EPR spectrum of solution ofTDTAP in methylene dichloride, shows a straight line (EPR inactive). This confirms the (+VII) oxidation state of manganese in quaternary ammonium permanganate salt with no unpaired electrons. All these spectral data suggest that Mn (VII) in TDTAP is reduced by addition ofTMCS to a lower valent Mn-chlorine species. This species in turn delivers two chlorine atoms in a stereoselective manner to the olefin and thus furnishing the trans-dichlorinated product. There is a two step change in the oxidation state of permanganate in the course of the reaction. The preparation and spectral properties of some of the oxychlorides of manganes~ are reported 24 . Among these oxychlorides, manganese oxide trichloride (MnOCI 3 ) showed UV-visible maxima at 400 nm. Similar UV -visible absorption at 400 nm has been recorded for the brown species generated by the addition ofTMCS to violet coloured solution of TDTAP in methylene dichloride. We failed to isolate any of the dichlorinated products when the chlorination reaction was carried out at a temperature higher than S 0 • The brown species seem<; to be unstable and slowly ( 10 minutes) turns to green above so and immediately turns green on addition of TMCS to TDTAP solution at 2S 0 • The close resemblance ofUV-visible spectrum and stability pattern of the brown species with that of MnOCI 3 leads us to propose the formation of MnOCI 3 in the reaction medium, which inserts two chlorine atoms in a stepwise manner to the alkene (Scheme I). Olefins are usually treated with molecular bromine in solvents such as carbon tetrachloride, chloroform, carbon disulfide, acetic acid, ether or ethyl acetate to form dibromides 25 . For small scale preparations pyridinium bromide perbromide is the reagent of choice 26 . Copper( II) bromide also react with olefins in the presence of acetonitrile, methanol or triphenyl phosphine to furnish 13 exclusively vicinal dibromoalkanes in high yields. Anion exchange resins act as a bromine canier 27 and hydrobromic acid, hydrogen peroxide and benzyltrimethylammonium chloride in carabon tetrachloride have been used 28 to brominate alkenes. TDT AP in combination with trimethylbromosilane (TMBS) pro-vides4 a simple and mild method for stereoselective and chemoselective trans-dibromination of alkenes (Table 2).  The reaction of 16-dehydropregnenolone acetate 3 in methylene dichloride at 0 to 3° with I mol equiv. of bromine in methylene dichloride was instantaneous. The product isolated after 3 min was 5a,6J3, I6J3, 17 a-tetrabromide (21% ), starting pregnenolone acetate (76%) with no trace of Sa,6,B-dibromide 20. The stigamasterol derivative 2,22dien-6-one 2 on reaction with I mol equiv. of bromine in methylene dichloride at 0 to 3° for 5 min furnished a complex mixture of products from which a small amount of 2J3,3a,22,23~tetrabromide (6%) was isolated. With two mol equiv. of bromine at 0 to 3° for 1.5 h, a similar complex mixture formed containing small amount of same tetrabromide. With TDTAP-TMBS formation of dibrominated compound 19 as a single product in very good yield strongly suggests that trans-dibromination occurs by a different pathway and clearly rules out the possibility of generation of molecular bromine in the reaction medium as the brominating species.
Vicinal dihydroxylation of alkenes with tetradecyltri· methylammonium permanganate and potassium hydrox· ide in a two-phase solvent system Potassi urn perrnanganate is a very strong oxidizing agent. It has been established beyond doubt that the lowtemperature oxidation of alkenes with aqueous alkaline potassium permanganate yields mainly 1,2-glycols, by cishydroxylation. The cyclic manganese(V) ester intermediate is unstable, and is rapidly hydrolysed at both C-0-Mn bonds, more or less simultaneously to yield diols 29 -31 . The yields of diols, however are seldom above 50% though they can be improved with phase transfer catalysis 32 -34 or increased stirring35. Preparation and absorption spectrum of a stable solution of manganese(V) diester by reacting alkenes with pulverised potassium permanganate in dichloromethane and benzyltriethylammonium chloride as a phase transfer agent has been reported 36 · 37 by Ogino and his group. It is probable that the intermediate manganate(V) diester could be more stable under these conditions because it would be complexed with the quaternary ammonium ion and the hydrolysis reaction would be suppressed in the absence of water. This pro-cedure36 provided a simple route to a number of dials from alkenes in good yields. The first example of the use of stable cetyltrimethylammonium permanganate salt for the cisdihydroxylation of alkenes has been published38 by Chandrasekaran and co-workers.
As an extension ofChandrasekaran's and to some extent Ogino's work, we have reported 5 the first practical and acceptable yields of vicinal cis-dials from ~lkenes with tetradecyltrimethylammonium permanganate (TDTAP) salt and addition of potassium hydroxide at the beginning in tert-BuOH-CH 2 Cl 2 -H 2 0 as the solvent system. Treatment of alkene (1 mmol) in CH 2 CI 2 with solution ofTDTAP (1.2 mmol) in CH 2 CI 2 for 2 hat 30°followed by anhydrous or alkaline work-up38 (Method A), the yields of dial from the alkenes we used are poor to moderate (Table  3). Carrying out the reaction in aqueous tert-butanol followed by treatment with alkali even lowers the yield of the dials. Continuing the reaction for a longer period does not improve the yield of dials. In all the cases varying amounts of starting materials recovered unchanged.
. We found that when the reaction was conducted in a two phase solvent system of tert-butanol, CH 2 CI 2 and water in the ratio 50 : l 0 : 1.25 in the presence of 0.1 mmol of KOH (Method C), dihydroxylation occurs in good yield. In the beginning, the pH of the reaction mixture is 7.5, which changes to 9.5 after the adctition of aqueous KOH andremains the same throughout the entire reaction. The beneficial effect of addition of alkali at the beginning on the yields of dials in KMn0 4 oxidation of olefins is well known 3 9.40. A delicate balance for the formation of intermediate cyclic manganese(v) ester, its life time and its instant hydrolysis in alkaline condition (present from the beginning) in a two phase solvent system account for the increased dial formation.
Using bezyltrimethylammonium hydroxide as a base at the beginning in non aqueous solvent system of tert-butanol and CH 2 Cl 2 (Method B), reasonably good yields of the dials are realised (Entry l-4 in Table 3). Here again the delicate balance of the formation of cyclic ester and its hydrolysis with base soluble in the organic medium 41 are respon-  sible for the formation of the dials in respectable yields.

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TDTAP is a reagent derived from potassium permanganate. It is very much expected that this reagent will add two -OH groups to a double bond like alkaline KMnQ 4 or Os0 4 to give cis-dial from the less hindered side of the double bond. The exo, cis-diol35 is the product of selective oxidation of endo-dicyclopentadiene 26 (Table 3). It is known 42 .43 that the non bornane double bond of dicyclopentadiene 26 is the more reactive of the two. Again, the dihydroxylation of 16-dehydropregnenolone acetate 3, takes place on the electron deficient a,/3-unsaturated ketone to afford 3{3-acetoxy-16a, 17 a-dihydroxypregna-5-en-20-one 32. Potassium permanganate hydroxylation 44 and continuous permanganate oxidation 4 5 of 16-DPA, introduced the two cis-OH group at 16, 17 position from the less hindered side of the double bond to give same compound 32, leaving the 5,6-double bond unaffected.
cis-Diols were obtained in high yields from the corresponding alkenes using TDTAP, in a biphase solvent system with inorganic base, or a homogenous system with organic base present from the beginning of the reaction.
Conclusion : Reactivity of KMn0 4 has been modified by converting it into long chain quaternary ammonium salt, retradecyltrimethylammonium permanganate. This novel reagent has been utilised forchemoselective and stcreoselective trans-dichlorination, dibromination and vicinal dihydroxylation of a variety of alkenes. The UV -visible spectrum, EPR studies and stability patrem of the reacting brown species with that of MnOCI 3 lead us to propose the formation of MnOCI 3 in the reaction medium, which inserts two chlorine atoms in a stepwise manner to the alkene. Formation of molecular chlorine in the reaction medium has been 1070 completely ruled out.