Alkynes, addition reaction phosphine

A Michael-type addition reaction of phosphine generated from red phosphorus in concentrated aqueous KOH solution has been noted to provide moderate isolable yields of pure organophosphorus products.27 For example, tris-(2-cyanoethyl)phosphine is produced in 45% isolable yield from acrylonitrile, and tris-(2-[y-pyridyl]ethyl) phosphine oxide is isolated in 40% yield from 4-vinylpyridine under these conditions. Excellent yields of the tertiary phosphine oxide, tris-(2-cyanoethyl)phosphine oxide, have been reported using white phosphorus in absolute ethanol with KOH at ice/salt-bath temperatures.28 A variety of solvent systems were examined for this reaction involving a Michael-type addition to acrylonitrile. Similarly, tris-(Z-styryl)phosphine is produced from phenylacetylene under these conditions in 55% isolated yield. It is noteworthy that this last cited reaction involves stereospecific syn- addition of the phosphine to the alkyne. 

We initially observed an addition reaction of tertiary phosphines to unactivated alkynes. The method was then applied to reactive alkenes, allenes and 1,3-dienes, and finally to unactivated alkenes (Scheme 4). Such a step-up methodology turned out to be effective in this study. 

Several reports have appeared on the effect of additives on the Pauson-Khand reaction employing an alkyne-Co2(CO)6 complex. For example, addition of phosphine oxide improves the yields of cyclopentenones 119], while addition of dimethyl sulfoxide accelerates the reaction considerably [20]. Furthermore, it has been reported that the Pauson-Khand reaction proceeds even at room temperature when a tertiary amine M-oxide, such as trimethylamine M-oxide or N-methylmorpholine M-oxide, is added to the alkyne-Co2(CO)6 complex in the presence of alkenes [21]. These results suggest that in the Pauson-Khand reaction generation of coordinatively unsaturated cobalt species by the attack of oxides on the carbonyl ligand of the alkyne-Co2(CO)6 complex [22] is the key step. With this knowledge in mind, we examined further the effect of various other additives on the reaction to obtain information on the mechanism of this rearrangement. 

The double phosphinylation of propargylic alcohols with diphenylphos-phine oxide to form 2,3-bis(diphenylphosphinyl)-1-propenes is catalyzed by a thiolate-bridged diruthenium complex (Scheme 28) [69]. It has been shown that the reaction proceeds via three ruthenium-catalyzed transformations propargylation of the phosphine oxide, alkyne to allene isomerization, and addition of phosphine oxide to the allene structure. 

The addition of a phosphine group to the organic fragment has been studied in some detail in compounds with cluster-bound vinyl ligands. The zwitterionic adducts which are formed can then undergo nucleophilic addition reactions (411, 461, 462). A reaction of this type also occurs with amine-substituted alkynes coordinated to osmium and ruthenium complexes (117). 

Dihydrofiirans have seen considerable use as substrates in the Pauson-Khand reaction. The parent compound reacts in excellent yield with acetylene, terminal and internal alkynes. Yields in this system respond very well to the use of catalytic reaction conditions (equation 4). Another unusual experimental modification has also been found by Pauson to be useful in this system addition of tri-n-butylphosphine oxide nearly doubles the product yield in certain cases (equation 37). The role of the added substance is unclear. Addition of phosphine oxide does not always improve reaction efficiency at this time there are no guidelines to indicate when its use might be beneficial. Substituted dihydrofurans give somewhat lower but still acceptable yields the poor regioselectivity in unsymmetrical cases is the more significant difficulty with these substrates (equation 38). 

Acids can also react vith ruthenium complexes by either protonation or oxidative addition. The catalytic addition of acidic compounds is also important for example, a divalent ruthenium complex Ru(>7 -cydooctadienyl)2 catalyzes the addition reaction of carboxylic acid to alkynes in the presence of tertiary phosphines and maleic anhydride (Eq. 14.5) [71]. 

Addition reactions of the Si-Si bonds across carbon-carbon triple bonds have been most extensively studied since the 1970s by means of palladium catalysts. In the early reports, palladium complexes bearing tertiary phosphine ligands, mostly PPh3, were exclusively employed as effective catalysts, enabling the alkyne bis-silylation with activated disilanes, i.e., disilanes with electronegative elements on the silicon atoms such as hydro [36], fluoro [37], chloro [38], and alkoxy-disilanes [39,40] and those with cyclic structure (Scheme 4) [41-44]. The bis-silylation reactions could be successfully applied to terminal alkynes and acetylenedicarboxylates to give (Z)-l,2-bis(silyl)alkenes, which are otherwise difficult to synthesize. 

Phosphines, as nucleophiles, add to many unsaturated substrates giving metallated ylides. Scheme 17 collects some representative examples of the addition of phosphines to carbyne complexes, giving (57) [132], to allenylidenes (58) [133], rr-alkenyls (59) [134], or CT-alkynyls (60) [135]. Moreover, reaction of phosphines with jt-aUcenes [136] and n-alkynes (61)-(64) [137-140] have also been reported. It is not possible to explain in depth each reaction, but the variety of resulting products provides an adequate perspective about the synthetic possibilities of this type of reactions. 

The most important lr(I) complexes are rra/ 5-IrCl(CO)(PPh3)2 (Vaska s complex) and its phosphine derivatives, since they provide clear examples of oxidative addition reactions. Scheme 11.5 summarizes the addition reactions to IrCl(CO)(PPh3)2 [72]. Alkyl halides, acyl halides. H.. and SnCl4 oxidatively add to the Ir center, and reactions with O2. alkyne. CO. and SO2 give their coordinated complexes without complete cleavage of the appropriate bond. 

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