2.4.6- Triphenylphosphine, formation

If use is made of the five-membered cyclic phosphorus derivatives, the geometry of which is more suitable than that of the triphenylphosphine, formation of the phos-phorane intermediate is more favored and the yield of olefin increases. Deoxygenation with KSeCN in mildly alkaline medium can similarly occur by heteroatom exchange retention is observed as a consequence of double inversion. Open-chain compounds react with good yields, but cyclic oxiranes yields vary considerably with the number of ring atoms, due to steric hindrance. Alkali metal 0,0-diethyl-phosphorotellurates are very useful reagents for the conversion of terminal oxiranes, Eq. 104 but the reaction does work for other types. In the case of oxiranes formally derived from acyclic olefins, the (Z) compounds react more readily than the (E)-oxiranes. ... 

In the chlorination of 2,4-dichlorophenol it has been found that traces of amine (23), onium salts (24), or triphenylphosphine oxide (25) are excellent catalysts to further chlorination by chlorine ia the ortho position with respect to the hydroxyl function. During chlorination (80°C, without solvent) these catalysts cause traces of 2,4,5-trichlorophenol ( 500 1000 ppm) to be transformed iato tetrachlorophenol. Thus these techniques leave no 2,4,5-trichlorophenol ia the final product, yielding a 2,4,6-trichlorophenol of outstanding quaUty. The possibiUty of chlorination usiag SO2CI2 ia the presence of Lewis catalysts has been discussed (26), but no mention is made of 2,4,5-trichlorophenol formation or content. 

A number of reductive procedures have found general applicability. a-Azidoketones may be reduced catalytically to the dihydropyrazines (80OPP265) and a direct conversion of a-azidoketones to pyrazines by treatment with triphenylphosphine in benzene (Scheme 55) has been reported to proceed in moderate to good yields (69LA(727)23l). Similarly, a-nitroketones may be reduced to the a-aminoketones which dimerize spontaneously (69USP3453279). The products from this reaction are pyrazines and piperazines and an intermolecular redox reaction between the initially formed dihydropyrazines may explain their formation. Normally, if the reaction is carried out in aqueous acetic acid the pyrazine predominates, but in less polar solvents over-reduction results in extensive piperazine formation. 

The sulfur atom can be used to initiate C—C bond formation. 2-Thio- and 4-thio-6,7-diphenyllumazine (166) react with phenacyl halides to give the phenacylthio derivatives (167), which on heating in DMF in the presence of triphenylphosphine extrude sulfur to form the benzoylmethyl derivative (168) in its tautomeric vinylogous amide form (169 equation 51). 

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

There are some reports on reactions involving complete N—N cleavage in diazirine reactions such as formation of amidine (205) from chlorophenyldiazirine, or on formation of products containing only one nitrogen atom. Betaine (206) was described as a product from difluorodiazirine and triphenylphosphine. Compound (207) is formed from decomposing (204) and cyclohexane (79AHC(24)63). 

The orange-red [SsN] anion Xm2.x 465 nm) is obtained by the addition of triphenylphosphine to a solution of a [S4N] salt in acetonitrile.It can be isolated as a salt in combination with large counterions, e.g., [Ph4As] or [N(PPh3)2] , but it is unstable with respect to the formation of the blue [S4N] anion in solution or in the solid state under the influence of heat or pressure. 

Reaction of lithium 2,5-dimethylpyrrolate ion with [RhCl(CO)2]2 leads to formation of 84 (88PAC1193 90P1503). This is the first example of the mixed mode, when the ti N) and ti (C=C) coordination are realized simultaneously. Nucleophilic addition of triphenylphosphine and triphenylarsine gives 85 (E = P, As). The iridium analogs of 84 and 85 have also been synthesized. 

It was found [99JCS(PI )3713] that, in all cases, the formation of the deiodinated products 38 and 39 was accompanied by formation of the diynes 40 which were isolated in 60-90% yield. The authors believed that the mechanism of deiodination may be represented as an interaction ofbis(triphenylphosphine)phenylethynyl-palladium(II) hydride with the 4-iodopyrazole, giving rise to the bisftriphenylphos-phine)phenylethynyl palladium(II) iodide complex which, due to the reductive elimination of 1 -iodoalkyne and subsequent addition of alk-1 -yne, converts into the initial palladium complex. Furthermore, the interaction of 1-iodoalkynes with the initial alkyne in the presence of Cul and EtsN (the Cadiot-Chodkiewicz reaction) results in the formation of the observed disubstituted butadiynes 40 (Scheme 51). 

Similarly the active oxygen of oxaziranes can be transferred to triphenylphosphine with the formation of ]ihosphine oxide and to tertiary amines yielding amine oxides. ... 

Winterfeldt reported a triphenylphosphine-catalyzed lactone formation from benzaldehyde and dimethylacetylenedicarboxylate in less than 20% yield... 

The mechanistic pathway" " can be divided into three steps 1. formation of the activating agent from triphenylphosphine and diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxylate (DIAD) 2. activation of the substrate alcohol 1 3. a bimolecular nucleophilic substitution (Sn2) at the activated carbon center. 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

A structurally unrelated agent is tazadolene (40). The synthesis of tazadolene begins with P-keto ester 37 and subsequent enamine formation with 3-amino-1-propanol followed by hydrogenolysis to give 38. This phenylhydroxymethyl compound is then dehydrated with hydrochloride acid to form olefin 39. Treatment with bromine and triphenylphosphine effects cycliza-tion to form the azetidine ring of tazadolene [10]. 

In pyridinium chloride ionic liquids and in l,2-dimethyl-3-hexylimida2olium chloride ([HMMIMjCl), where the C(2) position is protected by a methyl group, only [PdClJ was observed, whereas in [HMIMjCl, the EXAFS showed the formation of a bis-carbene complex. In the presence of triphenylphosphine, Pd-P coordination was observed in all ionic liquids except where the carbene complex was formed. During the Heck reaction, the formation of palladium was found to be quicker than in the absence of reagents. Overall, the EXAFS showed the presence of small palladium clusters of approximately 1 nm diameter formed in solution. 

Scheme 3b). It is instructive at this point to reiterate that the furan nucleus can be used in synthesis as a progenitor for a 1,4-dicarbonyl. Whereas the action of aqueous acid on a furan is known to provide direct access to a 1,4-dicarbonyl compound, exposure of a furan to an alcohol and an acid catalyst should result in the formation of a 1,4-diketal. Indeed, when a solution of intermediate 15 in benzene is treated with excess ethylene glycol, a catalytic amount of / ara-toluenesulfonic acid, and a trace of hydroquinone at reflux, bisethylene ketal 14 is formed in a yield of 71 %. The azeotropic removal of water provides a driving force for the ketalization reaction, and the presence of a trace of hydroquinone suppresses the formation of polymeric material. Through a Finkelstein reaction,14 the action of sodium iodide on primary bromide 14 results in the formation of primary iodide 23, a substance which is then treated, in crude form, with triphenylphosphine to give crystalline phosphonium iodide 24 in a yield of 93 % from 14.

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

The successful implementation of this strategy is shown in Scheme 4. In the central double cyclization step, the combined action of palladium(n) acetate (10 mol %), triphenylphosphine (20 mol %), and silver carbonate (2 equiv.) on trienyl iodide 16 in refluxing THF results in the formation of tricycle 20 (ca. 83 % yield). Compound 20 is the only product formed in this spectacular transformation. It is noteworthy that the stereochemical course of the initial insertion (see 17—>18) is guided by an equatorially disposed /-butyldimethylsilyl ether at C-6 in a transition state having a preferred eclipsed orientation of the C-Pd a bond and the exocyclic double bond (see 17). Insertion of the trisubstituted cycloheptene double bond into the C-Pd bond in 18 then gives a new organopal-... 

The ability of triphenylphosphine to act as a reducing agent probably involves initial formation of Ph3PCl2, which then undergoes solvolysis. If the synthesis is carried out using a small volume of ethanol, an orange polymorph is formed [45]. 

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