Bromine with phosphoranes

The phosphoranes, e.g. (69), which are intermediates in the Arbusov reactions of the cyclic phosphite (68) with chlorine, bromine, and benzenesulphenyl chloride have been detected at low temperatures by 31P n.m.r. spectroscopy.47 A similar technique revealed the intermediate (71) in the reaction of the thiophosphate (70) with chlorine.48... 

The phosphites 3 and 4 are oxidized by chlorine and bromine to furnish stable covalent phosphoranes. No Arbuzov reaction was observed. The trichlorophosphorane 5 can be reacted with again to form a spirophosphorane 7 which is hydrolyzed without ring cleavage to give an extremely stable hydroxyphosphorane or a phosphoraneoxide anion . . ... 

When the experiment was performed, however, not the least trace of optical activity was found for the resulting phosphorane 66). A similar result was obtained in the decomposition with bromine 68), where the optically inactive phosphorane 59 was formed. 

The synthesis of tedanin 589, an isoprenoid pigment of the marine sponge Tedanis digitata, starts with the Wittig reaction of phosphorane 506 with the carbonyl compound 587, giving 588. Bromination of the latter followed by treatment with KOH/methanol and twofold oxidation yield tedanin 589 280) (Scheme 99). 

Treatment of 2-acetyl-5-bromothiophene with excess methylidenetriphenylphosphorane has yielded some unexpected results <2000T7573>. The major product (46%) was the thienofuran 180 along with the phosphorane 181 (22%). The suggested mechanism for the formation of these two products involves initial nucleophilic displacement of the bromine, followed by addition of a second molecule of the ylide to the conjugated double bond of the 2-acetylthiophene unit (Scheme 46). 

The synthesized compounds were prepared by means of the Wittig reaction. The appropriate phosphoranes were obtained from commercially available aldehydes (through reduction, bromination, reaction with triphenylphosphine, and treatment with bases) and reacted with the required aldehydes. Mixtures of the E and Z stereoisomers in variable ratios were obtained and separated by chromatography. 

Polymer-supported Wittig reagents were first prepared more than 20 years ago [32]. It has been shown that the success of the reaction depends strongly upon (i) the preparation of the reagent by bromination and phosphination of cross-linked polystyrene rather than by co-polymerization using styryldi-phenyl phosphine, and (ii) the generation of the phosphorane with a base/ solvent system that swells the phosphonium sites in the polymer network (Scheme 6) [33]. Thus, bromination of polystyrene 1 yielded phenyl bromide 32, and this was followed by phosphination with n-butyUithium and chlor-odiphenylphosphine or with Hthium diphenylphosphide to give 33, a compound which is commercially available (Scheme 6). 

The routes and have both been considered for the reaction between propenal (R = H) and trimethyl phosphite, and hydrolysis of the oxyphosphorane yielded 3-(dimethoxyphosphinoyl)propanal. The phosphorane, 580 (R = Et), obtained from triethyl phosphite and but-3-en-2-one has been mentioned earlier in connection with the synthesis of phosphonic diesters possessing both oxo and hydroxy groups" as an example of its further usefulness through treatment with an electrophile, its reaction with bromine yields the bromo ester 581 and this, when heated, undergoes dehydrobromination to give diethyl (3-oxobutenyl)phosphonate (582), also obtained when 580 is heated with nbs. ... 

Reactions of fp-Polvstvrylmethvlltriphenvlphosphonium Ions. In these reactions the polymer-bound phosphorane reacts with an aldehyde or ketone to produce an alkene substituent on the polymer and leave the by-product triphenylphosphine oxide in solution, where it can be washed away from the polymer. The results are in Table II. Many of the same methods of phosphorane generation used to produce soluble alkenes were used. The yields in Table II are less accurate than the yields of micromolecular alkenes in Table 1 because of the difficulty of analysis of insoluble polymers. The yields have been based on weight changes of the polymer, analysis of the polymer for an element introduced during the reaction, the yield of triphenylphosphine oxide, or the amount of bromine consumed by the modified polymer. 

Halogenation at C-2. As above described monobromo exo-glycals are suitable substrates for palladium catalysed cross-coupling reactions. The direct reaction of monobrominated phosphoranes with lactones is not straightforward. However, the facile addition of chlorine to dichloroolefins 1 has been described some years ago. Thus halogenation of anomeric double seemed a viable process and bromi-nation of compounds 3 was examined. Considerable experimentation led to find the proper conditions which allowed the formation of brominated eoco-glycals from 3 (Scheme 5). 

---