Triphenylphosphine oxide, elimination

The solvated phosphorane adds to the polarized carbonyl with the incipient C-21 methyl group pointing away from the bulk of the steroid nucleus. The newly formed carbon-carbon bond must then rotate in order for the tri-phenylphosphine group and oxygen atom to have the proper orientation for the elimination of triphenylphosphine oxide. This places the C-21 methyl in the CIS configuration. 

Finally, Cristau and coworkers have reported on a quite efficient preparation of triphenylphosphine oxide (Figure 2.13) by a similar addition-elimination reaction of red phosphorus with iodobenzene in the presence of a Lewis acid catalyst followed by oxidation of an intermediate tetraarylphosphonium salt.42 This approach holds the potential for the preparation of a variety of triarylphosphine oxides without proceeding through the normally used Grignard reagent. Of course, a variety of approaches is available for the efficient reduction of phosphine oxides and quaternary phosphonium salts to the parent phosphine, including the use of lithium aluminum hydride,43 meth-ylpolysiloxane,44 trichlorosilane,45 and hexachlorodisilane.46... 

Reaction of the iminotriphenylphosphorane 27 with dimethyl acetyl-enedicarboxylate gives a mixture of the quinoxalines 28 and 29. The formation of compound 28 can be rationalized as shown, ring closure involving Wittig-like elimination of triphenylphosphine oxide. The formation of compound 29 is thought to involve a phosphazacyclo-butene intermediate.35... 

The 1,7-electrocyclization of nitrile imines 47 has been proposed as a key step in the conversion of the stable phosphorus ylides 45 to pyrazolo[4,3-d] [2,3]benzodiazepines 48, upon refluxing in xylene (Scheme 16). Ringopening of the triazoles 45 and recyclization is postulated to give the pyra-zoles 46. Migration of the triphenylphosphine group, followed by the elimination of triphenylphosphine oxide, would then give the nitrile imine 47 (95TL5637). 

Analogous to the Wit tig olefmation, triphenylphosphine and carbon tetrabromide react to give an intermediate dibrominated phospho-nium ylide. Ylide 33 extends the chain of aldehyde 11 with elimination of triphenylphosphine oxide to give 1,1-dibromoalkene 12.17... 

Quaternisation of triphenylphosphine with an alkyl halide gives a quaternary phosphonium halide (4) which under the influence of a strong base eliminates hydrogen halide to give an alkylidenephosphorane [(5), an ylide]. The latter reacts with an aldehyde or ketone to give first an intermediate betaine (6), which rearranges to the oxaphosphetane (7), which then under the reaction conditions eliminates triphenylphosphine oxide to form an alkene. 

On the other hand the stability of 57 causes the reaction leading to a reversible oxaphosphetane where the isomers 63 and 65 can interconvert via the starting material. The stereoselectivity in this step is thermodynamically controlled. The more stable four-membered ring is anti 65, with the bulky groups on opposite sides of the ring. The product of this reaction after elimination of triphenylphosphine oxide is only the E-alkene 66. 

Interaction of 4,5 6,7-di-0-cyclohexylidene-2,3-dideoxy-l-C-phe-nyl-L-arafeino-hept-2-enose (65) with phenylmethylenetriphenylphos-phorane was accompanied9 6 by the formation of triphenylphosphine, instead of the expected triphenylphosphine oxide, thus indicating the abnormal character of this reaction. This result may be interpreted as involving possible addition of the phosphonium ylide to the alkenic bond, with subsequent stabilization of the intermediate betaine 82 through elimination of triphenylphosphine, and closure of the three-membered ring2(f) with formation of the cyclopropane derivative 83, as shown in equation 5. 

Betaines 208a and 208c are incapable of formation of furan derivatives, because of an unfavorable orientation of the carbonyl fragment and the nearest hydroxyl group. These betaines may, however, be sources of anhydro derivatives, as well as 208b, if it is assumed that elimination of triphenylphosphine oxide is accompanied by intramolecular attack of a distant hydroxyl group on the /3-carbon atom, as shown. 

This intermediate reacts very rapidly with aldehydes and ketones to give zwitterionic compounds (10-XXXa), which eliminate triphenylphosphine oxide under mild conditions to give olefins (10-XXXb) ... 

Phosphorus ylide reacts with aldehyde or ketone to form an intermediate betaine A, which collapsed into oxaphosphetane B. Elimination of triphenylphosphine oxide yields alkene (Scheme 3.31) (also see section 4.3.1). 

In Step [2], Ph3P=0 (triphenylphosphine oxide) is eliminated, forming two new n bonds. The formation of the very strong P—O double bond provides the driving force for the Wittig reaction. 

In 1955 Wittig et al. found that triphenylphosphine could induce the deoxygenation of epoxides at 200 Mechanistically, this process probably involves anti opening of the epoxide followed by syn elimination of triphenylphosphine oxide from a betaine intermediate. Accordingly, the reaction proceeds with inversion of stereochemistry, which means that franj-ejjoxides give ci5-alkenes. The systems of bis(di-methylamino)phosphorous acid/butyllithium, o and lithium diphenylphosphide have been examined... 

The process involves Wittig reaction type attack by the ylide at the carbonyl group followed by preferential elimination of triphenylphosphine oxide instead of triphenylar-sine oxide. 

A related triazole synthesis utilizes phosphorous ylids, such as 264. The initially formed triazenes cyclize with elimination of triphenylphosphine oxide. The reaction proceeded sluggishly with phenyl azide, but good results have been obtained with acyl or sulphonyl azides. Tosyl azide and 264 yielded 98% of the 1-tosyl-triazole 265. The tosyl group could be removed by solvolysis in boiling ethanol... 

Generally, oxaphosphetanes are thermodynamically unstable and fragment into alkenes and triphenylphosphine oxide. This elimination step is stereospecific with oxygen and phosphorus departing in a 5jn-periplanar mode to produce (Z)-alkenes, the driving force being formation of the very stable P = O bond (130-140 kcal/mol, 544-586 kJ/mol bond dissociation energy). 

Treatment of P-oxido ylides with electrophiles other than proton donors provides a route to stereospecific trisubstituted alkenes. For example, trapping the P-oxido phosphonium ylide B with formaldehyde (generated from paraformaldehyde) leads to dioxido phosphonium derivative D to yield, after elimination of triphenylphosphine oxide, the trisubstituted allylic alcohol... 

The stereochemistry is inverted. The nucleophile triphenylphosphine must attack the epoxide in an anti fashion, yet the triphenylphosphine oxide must eliminate with syn geometry. 

Examples of substrates [23] with heteroatom double bonds (C = X) are presented in the rosette of Scheme 5. Here we feature the efficient denitrogenation of diazoalkanes, diazoquinones, the hydrazone of benzil, and benzophenone oxime into the respective carbonyl products [23]. On the other hand, the bis-oxime afforded furoxane, while the phosphorane led to methyl maleate on elimination of triphenylphosphine oxide [23], Thiones are oxidized to the corresponding S-oxides and the iodonium ylide yields the labile w c-trione [23],... 

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