Phosphine oxide elimination

Similarly phosphine oxide eliminations (Wittig reaction) occur very readily to give olefins. 

In a more direct comparison of the phosphine oxide elimination with the sulfone, Schreiber employed a identical system to Danishefsky, but used the phosphine oxide (422). Reaction with isobutyralde-hyde and subsequent elimination resulted in a 1 1 mixture of the ( )- and (Z)-alkenes (421 equation 98). It appears from the more complex example of the Merck synthesis and from this example, that the Julia coupling proceeds with higher ( )-selectivity, in similar yield. 

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

Quantitative yields of allenes were obtained from keten and the stable phosphoranes (48). Addition of similar phosphoranes occurred at the j3-position of the allenic ketones (49) to give the phosphoranes (50). The compound (50 = COPh) eliminated phosphine oxide under the... 

Oxazoles (191) are producedwhen triphenylphosphine is treated simultaneously with an a-azidocarbonyl compound and an acyl halide. The intermediate iminophosphoranes (189) react with the acyl halide before they can react with themselves to give pyrazines. Elimination of phosphine oxide from the resulting salts may give the intermediate halo-genoimines (190), or the oxazoles may be formed via the betaines (192). 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

Carbanions derived from phosphine oxides also add to carbonyl compounds. The adducts are stable but undergo elimination to form alkene on heating with a base such as sodium hydride. This reaction is known as the Horner-Wittig reaction.268... 

Malacria and coworkers reported a vinylation sequence of epoxides by employing vinyl phosphine oxides as a radical trap. The overall sequence relies on the facile elimination of phosphinoyl radicals. With vinyl phosphonates the THF derivatives were obtained (Scheme 22). The reaction works equally well under stoichiometric or catalytic conditions [102,103],... 

The benzotriazolyl derivative of acrolein acetal, compound 882, is lithiated, treated with chlorodiphenylphosphine, and the obtained intermediate is oxidized with hydrogen peroxide to phosphine oxide 883 (Scheme 145). The relatively acidic proton in derivative 883 is easily removed by a base, and the obtained anion adds to a carbonyl group of aldehyde or ketone. Subsequent rearrangement and elimination of the phosphorane group generates diene 884. For the derivatives of aldehydes (884, R2 = H), (E)-(E) stereoselectivity of the elimination is observed. Acidic alcoholysis of dienes 884 affords esters of P,y-unsaturated carboxylic acids 885 < 1997JOC4131>. 

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... 

Formate esters behave as typical carbonyl compounds in reactions with a number of ylides, eliminating phosphine oxide and forming vinyl ethers, e.g. (33).35 Stabilized phosphoranes are able to condense with the carbonyl group of cyclic thioanhydrides (34).38 Quinoline derivatives, e.g. (35), are obtained from the condensation of dicar-boalkoxy-ylides with isocyanates.37 Benzoyl isothiocyanates and keto-phosphoranes give quantitative yields of (36), which are unreactive in Wittig reactions but can be readily oxidized by selenous acid.38 The products obtained from reactions (Scheme 9) with the triazolinedione (37) depend upon the stability of the ylide used.39... 

From Azides and x-Acylphosphorus ylids Addition of azides to a-acylphosphorus ylids takes place at room temperature in dichloromethane or at 80°C in benzene, giving triazolines from which a phosphine oxide is spontaneously eliminated. " The ylids exist almost exclusively in the cis-enolate configuration, and a mechanism involving concerted 1,3-dipolar addition has been proposed (Scheme 12) on the basis that there is a low entropy of activation for the reaction, and that the reaction rate is insensitive to changes in solvent polarity. ... 

The unique feature of the Horner-Wittig reaction is that the addition intermediate can be isolated and purified. This provides a means for control of the stereochemistry of the reaction. It is possible to separate the two diastereomeric adducts in order to prepare the pure alkenes. The elimination process is syn so that the stereochemistry of the alkene depends on the stereochemistry of the adduct. Usually, the anti adduct is the major product, so it is the Z-alkene which is favored. The syn adduct is most easily obtained by reduction of /i-keto phosphine oxides.160... 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

Oxophilic phosphomum compound 47 forms from tributylphosphine (46) and the phenylselenide 45, and this then suffers nucleophilic attack by alcohol 18. The resulting free selenium nucleophile 48 displaces phosphine oxide 50 with the formation of phenylsele-nide 51. Phenylselenide 51 is oxidi/ed by hydrogen peroxide to phenylselenoxide 52, which at room temperature undergoes an elimination reaction to (-)-A9tl2)-capnellene (1). The mechanism is similar to that of the Cope elimination, proceeding via a cyclic transition state. 

This problem has been partially overcome by elimination of the phosphorus-oxygen bonds, as, for example, in the poly(phosphinoisocyanates), which have the structure shown in 6.47.42 It is also possible to form poly(metal phosphinates) with repeat unit -M(0PR20)2- by allowing a metal alkoxide to react with a phosphinic acid.43 Typical metal atoms are aluminum, cobalt, chromium, nickel, titanium, and zinc.43 Polymeric phosphine oxides can be prepared by the reactions... 

A convenient route to trivinylphosphine has been developed by thiol elimination from tris[2-(phenylthio)ethyl]phosphine oxide.56 The reaction mechanism involves a phosphoryl-stabilized carbanion, from which benzene thiolate anion is eliminated. 

Chan has reported that substitution of the 1,3-propa-dioxy ether linkage with a (25,35)-buta-dioxy ether linkage, as in 114, resulted in complete atropdiastereoselective coupling of the aryl units (Scheme 12.42). Reduction of the phosphine oxide (/ )-115 resulted in (R)-116 in 96% yield. This procedure eliminates a resolution step.140... 

---