Triphenylphosphine Dibromide1

Ph3PBr2

[1034-39-5]  · C15H18Br2P  · Triphenylphosphine Dibromide  · (MW 389.10)

(bromination of alcohols,3,4 phenols,22 and enols;3 cleavage of ethers23,32,33 and acetals25 to alkyl bromides; cyclization of amino alcohols34,35 to cyclic amines; conversion of carboxylic acid derivatives into acyl bromides;36,37 bromination39 or dehydration38 of carboxamide groups)

Alternate Names: bromotriphenylphosphonium bromide; dibromotriphenylphosphorane.

Physical Data: adduct:2 colorless crystalline solid; mp 235 °C (dec). Ph3P: mp 80.5 °C; bp 377 °C (in inert gas); d254 1.194 g cm-3; n80D 1.6358. Br2: mp -7.2 °C; bp 59.5 °C; d254 3.12 g cm-3.

Solubility: adduct: sol CH2Cl2, MeCN, PhCN, DMF; less sol PhH, PhCl. Ph3P: sol ether, PhH, CHCl3, AcOH; less sol alcohol; pract. insol. H2O. Br2: sol H2O (3.58 g/100 mL); very sol. alcohol, ether, CHCl3, CCl4, CS2.

Form Supplied in: adduct: hygroscopic solid; commercially available; purity 95-96%. Both parent compounds, Ph3P and Br2, are widely available. Ph3P: odorless platelets or prisms; purity ~99%; typical impurity Ph3PO ~1%. Br2: dark reddish liquid, volatile, with suffocating odor; vaporizes rapidly at rt; purity >99.5%.

Analysis of Reagent Purity: adduct: 31P NMR (CH2Cl2) +49 ppm (ionic form);2 Raman (solid phase) 239 cm-1 (P-Br). Typical impurities, Ph3P and Ph3PO: 31P NMR (various solvents) -5 to -8 ppm and +25 to +29 ppm, respectively.

Preparative Methods: several preparations are described,2,23b,32b but in most cases the hygroscopic reagent is prepared just before use by addition of Bromine to Triphenylphosphine in a dry solvent with cooling.

Handling, Storage, and Precautions: adduct: corrosive; very hygroscopic; moisture sensitive; incompatible with strong oxidizing agents and strong bases; may decompose on exposure to moist air or water; do not get in eyes, on skin, or on clothing; keep containers tightly closed and store in a cool dry place; these reagents should be handled in a fume hood.

Introduction.

The Ph3PBr2 adduct was first used in synthesis in 19593 for the preparation of alkyl and acyl bromides from alcohols and carboxylic acids, and for the dehydration of amides and oximes to nitriles. Since then it has been widely employed as a versatile reagent for a number of synthetic reactions.

Conversion of Alcohols into Alkyl Bromides.

It has been established that Ph3PX2 reagents present considerable advantages over phosphorus halides (i.e. PX3, PX5) for which only two replaceable groups would be necessary and desirable.4a The mechanism4b of the reaction of Ph3PBr2 with an alcohol involves initial rapid formation of an alkyloxyphosphonium bromide,4,5,7a which then collapses slowly to the phosphine oxide and an alkyl bromide (eq 1). SN2 substitution and high yields are generally observed with primary and secondary alcohols.3,4a This reagent shows little tendency to induce either carbonium ion rearrangement in the carbon skeleton or elimination reactions; this is illustrated by the preparation of neopentyl bromide from neopentyl alcohol (eq 2)4a and in the steroid field,6 in which cholesterol and 3b-cholestanol give respectively 3b-bromocholest-5-ene and 3a-bromocholestane in yields of about 80%. Inversion of configuration is found in the SN2 reactions of (+)-endo-norbornanol (eq 3)7a,8 and 7-norbornanol. However, a similar reaction with (-)-endo-norbornanol leads mainly to racemic bromide via a nonclassical norbornyl cation intermediate.7b Likewise, little rearrangement is observed in the bromination of 3-methyl-2-butanol (eq 4).9 Greater amounts of rearranged product are formed in the same reaction with Ph3PCNBr (3%), Bromine-Triphenyl Phosphite (55.6%), or Triphenylphosphine-Carbon Tetrabromide (6-10%).

Ph3PBr2 can be used with sensitive substrates which incorporate cyclopropyl rings (e.g. cyclopropyl carbinol11,12) or unsaturation10,13 (e.g. cinnamyl alcohol and alkynediols) with little if any side reactions. Cyclopropyl carbinols are transformed into bromides in good yield without ring opening or ring expansion, with the formation of only trace amounts of homoallylic bromides; no bromocyclobutane derivatives are detected (eq 5). Ph3PBr2 also proves superior to Phosphorus(III) Bromide for the bromination of alkynediols to dibromoalkynes (eq 6).13

Ph3PBr2 provides comparable yields to Ph3P/CBr4 in the bromination of diethyl 1-hydroxyalkylphosphonates.14 A number of other primary,15 secondary,11-14 and even tertiary alcohols are brominated with Ph3PBr2, but with preponderant elimination4a,6,16 in the latter case. A polymer-supported Ph3PBr2 reagent17 has been successfully applied to alcohol bromination in refluxing CHCl3; product isolation is facilited by the ready separation of supported from the nonsupported species. However, note that a polymer-supported Ph3P/CBr4 reagent leads to better bromination results than the Ph3PBr2 system in a number of cases.

Similar mono- and dibrominations have also been achieved in the carbohydrate field (eq 7).18 Configurational inversion at chiral centers is observed throughout. Ph3PBr2/Imidazole (ImH) or Ph3P/2,4,5-tribromoimidazole/ImH combinations in refluxing PhMe or in PhMe-pyridine mixtures are preferred in some cases. In the nucleoside field,19 double activation is achieved with Ph3PBr2 in pyridine, leading both to bromination of the 5-position of the sugar moiety and to substitution on the C-6 position of the adenine part (eq 8).19b

A variety of solvents such as CCl4, PhH, triglyme, CHCl3, PhMe, pyridine, NMP, MeCN, DMF, and mixtures of these have been employed in these bromination reactions, depending on substrate solubility. Both O- and C-formylations are encountered in some cases when DMF is the solvent. Ph3PBr2 and both Ph3PX2 analogs (X = Cl, I) can react with DMF according to the sequence in eq 9.20 The intermediate alkoxyimonium bromide (3) affords either the expected bromide by heating, or the formyl ester by hydrolytic workup with secondary20 or tertiary16 alcohols (e.g. eq 10). Vilsmeier-type formylation reactions on nucleophilic carbon atoms can also take place, as developed early38a and then encountered later as a side reaction in the steroid field.21 Such O-formylation of secondary alcohols is interestingly used in the differentiation of diols, leading to a new synthesis of bromo formates (eq 10).20b Under mild conditions, primary alcohols still afford bromides whereas secondary ones are converted almost exclusively to the corresponding formate esters.

Synthesis of Aryl Bromides from Phenols.

Already developed in the pioneering work,4a the preparation of aryl bromides from phenols with Ph3PBr2 was later extended to a number of substrates (eq 11).22

Alkyl Bromides from Ethers and Acetals.

Ph3PBr2 cleaves dialkyl ethers to give the two alkyl bromides under essentially neutral conditions.23 This reaction offers obvious advantages as it avoids both the strongly acidic and basic media which are usually employed for ether cleavage. Brominations are generally conducted at reflux in high boiling solvents such as PhCl, PhCN, DMF, or NMP. Primary and secondary alkyl groups provide good yields of bromides without rearrangement in PhCl or PhCN, at 60 to 120 °C,23a,b as illustrated in eq 12.23c Phenyl alkyl ethers initially afford aryloxyphosphonium bromides which collapse to bromobenzene only by heating at higher temperature (>230 °C as seen before). Alkyl t-butyl ethers are cleaved more easily in DMF23b or in MeCN,24 usually between 60 and 110 °C; the t-butyl group is converted into isobutene in this process. In a related reaction, endoxides are transformed into arenes28 by Ph3PBr2 treatment in PhCl. Aromatization takes place via HBr elimination from the initially formed dibromide.

Direct conversion of tetrahydropyranyl (THP)25 and tetrahydrofuranyl ethers26 (THF) to bromides can be achieved by Ph3PBr2 treatment under milder conditions (eq 13).25a THP and THF ethers afford good yields of acyclic, saturated, and unsaturated,25c primary, secondary, and even tertiary alkyl bromides; under the same conditions, cyclohexyl THF ethers provide mainly cyclohexene derivatives. In view of the fact that the preceding reaction of THP ethers can be stopped (at low temperature) at the stage of either the alkoxyphosphonium (see eq 1) or the pentacovalent ROPPh3Br intermediate, hydrolysis of the reaction mixture at -50 °C, leads to the corresponding alcohols in good yield (74-97%) (eq 14).29 The method is efficient for the deprotection of secondary and tertiary THP ethers, as well as for acyclic acetal ether, dialkyl acetals, and O-glucosides. Acetal functions can be removed with retention of stereochemistry, as illustrated in the conversion of (-)-menthol THP ether into (-)-menthol with full recovery of the optical activity. This procedure is not applicable to primary THP ethers, where the corresponding bromides are formed under these conditions.

Reaction of hindered trialkylsilyl ethers with Ph3PBr2 in CH2Cl2 at rt affords primary and secondary alkyl bromides in excellent yield (70-94%); the reaction is valuable in the b-lactam field.27a The reaction rate is increased by addition of a catalytic amount of Zinc Bromide. Silylated enol ethers, such as trimethylsilyl(1-phenylvinyloxy)silane, provide vinyl bromides such as a-bromostyrene with Ph3PBr2 in refluxing CCl4.27b

a-Alkynyl Ketones and b-Bromo-a-Vinyl Ketones from b-Diketones.

The previously described bromination of alcohols can be applied to the enol form of b-diketones. The first attempts in this field were conducted with the Ph3PBr2/Triethylamine system on dibenzoylmethane, leading to phenylethynyl phenyl ketone.30a Elimination at the oxyphosphonium stage or HBr elimination from the initially generated vinyl bromide is probably involved in this reaction. Analogously, reaction of unsymmetrical b-diketones30b with excess Ph3PBr2/Et3N proceeds with the formation of 3:1 mixtures of a,b-ynones in good overall yield (eq 15). Ethynyltriphenylphosphonium bromides are similarly obtained30c starting from (benzoylmethylene)- and (carbamoylmethylene)triphenylphosphoranes.

Similar treatment of unconjugated b-diketones31 in PhH or MeCN as solvent leads to b-bromo-a,b-unsaturated ketones with superior results relative to the same transformation with PBr3 as the brominating agent (eq 16).31c The reaction of acyclic 2,4-pentanedione with Ph3PBr2/Et3N in PhH-MeCN31c or in CH2Cl2,31d but in the absence of Et3N, appears not to be totally stereoselective, since a mixture of the geometrically isomeric bromo enones is produced, with 88% (E/Z ~ 87/13) and 85% (E/Z ~ 93/7) yields, respectively.

Epoxide Opening to Vicinal Dibromides or Bromohydrins.

Epoxide ring opening with Ph3PBr232a takes place in MeCN or PhH at 20-50 °C, affording vicinal dibromides (32-74%); cis-trans isomer mixtures are obtained in the case of cycloalkene epoxides. This reaction involves initial cleavage of the epoxide C-O bond at the most substituted epoxide carbon; bromoalkoxyphosphonium salts are thus formed, and these undergo subsequent substitution to give dibromides and Ph3PO. In further studies,32b reaction of cis-epoxides in PhH produced erythro-dibromides exclusively (eq 17); trans-epoxides exhibit less specificity, leading to a mixture of threo- and erythro-dibromides. Use of a more polar solvent such as CH2Cl2 or MeCN, instead of PhH, with cis-epoxides provides erythro-threo mixtures (60-40 to 50-50). By reacting epoxides first with HCl and then with Ph3PBr2, it is possible to obtain vicinal bromochlorides stereospecifically that are also products of two SN2 displacements.

The reaction can also be carried out so as to give bromohydrins. In the steroid field33a with conformationally rigid epoxides, oxirane cleavage appears to be quite stereoselective and leads only to the bromohydrin resulting from the usual anti opening of the ring in high yield (90-97%). Less hindered and rigid substrates afford regioisomeric mixtures of cyclic trans-bromohydrins. Equivalent results33b are obtained, in the steroid series, by use of polymeric Ph3PBr2 for the transformation of epoxides to bromohydrins under mild nonacidic conditions; oxirane ring opening remains regio- and stereoselective.

Cyclization of b- and g-Amino Alcohols to Aziridines and Azetidines.

b-Amino alcohols undergo cyclization upon Ph3PBr2/Et3N treatment,34 providing the corresponding aziridines along with substituted piperazines as dimeric byproducts (eq 18). Walden inversion is observed in the ring closure of both threo- and erythro-ephedrine and this supports the postulated 1,2-trans-elimination. Application of this procedure to the synthesis of 1-monosubstituted aziridines is unsuccessful, giving only the piperazine byproduct. N-Aryl g-amino alcohols are cyclized to azetidines by Ph3PBr2/Et3N in MeCN (eq 19).35a Under these conditions, some starting material is recovered unchanged along with mixed tetrahydroquinolines as side products and the expected N-arylazetidine. Comparable results are obtained (45-65%)35b with some other N-protecting groups.

Acyl Bromides from Carboxylic Acids, Anhydrides, and Esters.

Carboxylic acid bromides are prepared by reaction of Ph3PBr2 with various carboxylic acids and anhydrides in boiling PhCl.36a Milder conditions and better yields were later obtained in a comparative study36b by using CH2Cl2 at rt (eq 20). Further improvements are observed through the use of the corresponding trimethylsilyl esters; acyl bromides are thus obtained under mild and neutral conditions, allowing reactions with sensitive acid substrates without side reactions (eq 20). This transformation is also applicable to generate the acyl bromides of more hindered trialkylsilyl esters such as the TBDMS, TIPS, and TBDPS carboxylates; in CH2Cl2, the reaction rate increases in the presence of a catalytic amount of ZnBr2.27a

Direct reaction of less reactive alkyl esters and lactones37 with Ph3PBr2 affords both acyl and alkyl bromides. This reaction is achieved at MeCN reflux37a-c for a-halogenated esters such as CF3CO2R, but only at higher temperatures and with longer reaction times for unhalogenated ones. The reaction of an ester with Ph3PBr2 presumably forms an oxonium salt, which then undergoes cleavage through reaction with bromide ion. Prolonged heating (80-110 °C) of lactones with Ph3PBr2 gives the expected acyl alkyl dibromides, but in low yield (<=50%) due to a concomitant degradation.37d

Dehydration of Ureas and Amides to Carbodiimides, Nitriles, Isocyanides, and Ketenimines.

Dehydration of N,N-disubstituted ureas by heating with Ph3PBr2/Et3N in PhH or PhCl at 70-80 °C affords carbodiimides38a in good yields. Subsequent improvements38d involving milder conditions provides access to certain unstable derivatives from N,N-disubstituted ureas; this procedure compares favorably with previous ones and with another related approach using the Triphenylphosphine-Carbon Tetrachloride reagent system (eq 21). Reaction of N,N-dialkylidene ureas with Ph3PBr2 in PhH leads to a-haloalkyl carbodiimides.38e

By the same Ph3PBr2/Et3N procedure carried out in refluxing PhH, disubstituted cyanamides such as Me2NCN are obtained from the N,N-disubstituted urea Me2NCONH2 (67%); similarly, isocyanides result from monosubstituted formamides (56-73%),38a,40b and nitriles from primary amides and oximes (58-68%).3 Via the same route, ketenimines (R1R2C=C=NR3) are prepared38a,40b in CH2Cl2 at reflux by dehydration of secondary amides having a C-H bond adjacent to the carbonyl function (45-93%). Application of this procedure to sulfimides gives highly reactive ketenimines, which are used for further reactions without isolation; [2 + 2] cycloaddition of such species with Schiff bases provides, in good yield, N-(tosyl)azetidin-2-imines related to b-lactam derivatives (eq 22).38b The diphosphorylated ketenimine [(EtO)2(O)P]2C=C=NPh is obtained analogously in 87% yield with Ph3PBr2/Et3N from the a,a-diphosphorylated acetanilide.38c

Imidoyl Bromides from Secondary Amides.

In the case of the secondary diarylamide benzanilide, reaction with Ph3PBr2/Et3N in refluxing PhH provides N-phenylbenzimidoyl bromide in 65% yield, whereas a 70% yield is attained with the Ph3P/CBr4 reagent system.39 The closely related N-methylbenzamide affords a dimeric compound40b with Ph3PBr2 in boiling CH2Cl2 by intermolecular N-imidoylation (see below for a similar intramolecular process) between generated imidoyl bromide or oxyphosphonium species and the starting carboxamide. Cyclodehydration of secondary carboxylic diamides with Ph3PBr2/Et3N affords intramolecular O-imidoylation products with a benzoxadiazepine structure.40 Without Et3N, intramolecular N-imidoylation products with an o-imino lactam structure are obtained; O-imidoylated products rearrange to N-analogs under acidic conditions.

Iminophosphoranes from Amines, Hydrazines, and Related Derivatives.

Compounds containing a P=N bond, such as iminophosphoranes, are widely used as synthetic intermediates, especially in the heterocyclic field. Aromatic primary amines are generally transformed into iminotriphenylphosphoranes, and aliphatic amines into aminophosphonium bromides with Ph3PBr2/Et3N in PhH, PhCl, CCl4, or CH2Cl2 as solvent;41a,b more basic conditions are usually required to convert an alkylaminophosphonium salt into the corresponding iminophosphorane.41b As protected primary amines, these phosphinimines (iminophosphoranes) can then be subjected to monoalkylation, the Ph3P protecting group being cleaved in a subsequent step to provide secondary amines (eq 23).41a-c

Some aliphatic and aromatic phosphinimines (prepared as above) undergo a Wittig-like condensation with CO2 or CS2, giving rise to isocyanates and isothiocyanates, respectively (eq 23).41d Dehydration reactions promoted by Ph3PBr2/Et3N are used for intramolecular cyclizations involving an imide carbonyl group and an aromatic amine;42 an aza-Wittig reaction of a phosphinimide intermediate is assumed to occur. Iminophosphoranes derived from hydrazines43a and acylhydrazines43b,c are also used as intermediates in heterocyclic synthesis. Reactions of metalated amines RNHMgX with Ph3PBr2 in PhH-ether can also provide aminophosphonium salts Ph3+PNHR Br- in fair to good yields (33-74%).43d

Other Applications.

Reaction of Ph3PBr2 with benzoins in MeCN at rt provides diaryl a-diketones in excellent yields (75-98%).44 Ph3PBr2 is used as a precursor in the synthesis of dialkoxytriphenylphosphoranes such as (CF3CH2O)2PPh345 and (t-BuCH2O)2PPh3,46 which are used as alkylating or acylating and cyclodehydration agents, respectively. Direct synthesis of b-lactams by [2 + 2] cycloaddition between carboxylic acids and imides, thus avoiding the use of acid halides, is achieved with Ph3PBr2 (40-55%).47 Ph3PBr2, as a dehydrating agent, effects esterification reactions with tertiary alcohols,48a such as t-BuOH in PhH-HMPA, and with aromatic or aryl allylic acids and primary or secondary aliphatic alcohols in petroleum ether (27-85%).48b

Preparation of Bromotrimethylsilane in quantitative yield occurs by deoxygenation of (Me3Si)2O with Ph3PBr2 (in 1,2-dichlorobenzene at reflux) in the presence of a catalytic amount of Zn; a one-pot preparation of pseudohalogenosilanes, in excellent yield (80-95%), can follow by subsequent addition of the appropriate salt XM in DMF (X = N3, NCO, CN; M = K, Na,).49 Treatment of Ph3PBr2 with Iodotrimethylsilane in CH2Cl2 at 0 °C yields Ph3PI2 (86%).50 Reaction of Ph3PBr2/Et3N with EtO2CCH2CH=CHCO2Et in PhH leads to the corresponding alkylidenephosphonium ylide in 80% yield.51 Beckmann rearrangement of cycloalkanone oximes into lactams (74-81% yields) is effected by Ph3PBr2 in dry PhH at 50-60 °C.52 The Ph3PBr2-Et3N reagent system compares favorably with a number of other related phosphorus reagents for the rearrangement of N-allylamides into nitriles under mild conditions via 3-aza-Claisen reaction.53


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