Metal ynolates

F. Formal [3 + 2] Cycloaddition of Metal Ynolates with Oxiranes and... 

In this chapter synthetic methods, chemical properties and synthetic applications of metal ynolates are described. In the last part, heteroatom analogues of metal ynolates are briefly discussed. The following acronyms are in use ... 

Metal ynolates are not as easy to prepare in a similar fashion as metal enolates, because the intermediates may be labile monosubstituted ketenes. Several preparative methods for alkali metal ynolates have been reported, among which some have been used as intermediate steps in one-pot organic syntheses. Silyl ynolates have been prepared from lithium ynolates. There have been few reports on the other metal ynolates. Since there is no universal method to determine the yield of metal ynolates, the efficiency of preparation is estimated from the results of some of the following reactions. 

The a-chloro-a-sultinyl ketone 20 was prepared from methyl benzoate and chloromethyl phenyl sulfoxide 19 after in situ a-lithiation. Compound 20 is dimetallated by KH and f-BuLi to give the keto dianion 21, which is converted into a potassium/lithium ynolate 22 (equation 7). The resulting metal ynolates are converted into thioesters, carboxylic acids, amides and esters (Section V). 

Since metal ynolates are metallated ketene equivalents, metallation of ketenes is expected to afford metal ynolates. Direct metallation of monoalkylketenes is, however, fairly difficult due to the high lability of these ketenes (e.g. dimerization) and the strong elec-trophilicity of the carbonyl carbon . In contrast, silylketenes are so stable that lithiation of trimethylsilylketene (45) with BuLi at —100 °C provides the lithium ynolate 46 in good yield (equation 18). The f-butyldimethylsilylketene (47) is also lithiated to afford the lithium ynolate 48 (equation 19). ... 

Since unsubstituted metal ynolates have a terminal aUtyne, their metallation gives ynolate dianions. Schollkopf s method affords an ynolate dianion, starting from 3-phenylisox-azole (equation 3, Section II.A). ... 

The NMR spectrum of silyl-substituted potassium ynolates in THF-rfg shows signals at 132.8 and 33.4 ppm, which are close to those of alkynyl ethers. The IR spectrum in THF shows a strong absorption at 2229 cm , which is typical for alkyne . These results point to a metal ynolate rather than a metallated ketene. 

Since metal ynolates are ambient nucleophiles, with oxygen being a hard nucleophilic center and carbon a soft one, the position of attack at O or C would therefore depend on the hard/soft character of the electrophile (equation 24). When metal ynolates react as C-nucleophiles, they are regarded as metallated ketenes 59. Ynolates are also considered as electron-rich alkynes. In this section, the unique reactions of metal ynolates induced by these characteristic features are summarized. 

There has been only a few reports on reactions of small rings with metal ynolates. Oxiranes are much less electrophilic than carbonyls and sometimes need activation by Lewis acids or Lewis-acidic organometals. The lithium-trimethylaluminum ate complex of i/ZyZ-substituted ynolate 105 reacts with the oxirane 106 to give the y-lactone 107 (equation 44), while lithium silyl-substituted ynolates are inert to oxiranes. There have been no reports using carbon-substituted metal ynolates. 

Although the Michael addition of metal ynolates to a,/ -unsaturated carbonyl compounds is expected to give six-membered cycloadducts, 1,2-addition to carbonyl groups usually precedes 1,4-addition. The cycloaddition of the lithium-aluminum ate complex of silyl-substimted ynolate 112 with ethyl benzylideneacetoacetate (113), which is doubly activated by the ester and keto functions, gives the y-lactone 114 via a [4 4- 2] type cycloaddition (equation 46). Diethyl benzylidenemalonate (115) affords the uncyclized ketene 116 by reaction with 112 (equation 47). This could be taken as evidence for a stepwise mechanism for equation 46. ... 

Metal ynamines (metal ynamides, 180) are aza-analogues of metal ynolates and have not been studied as well as the ynamines (181), in spite of being much more reactive than the latter. l,4-Diphenyl-l,2,3-triazolyllithium (183), prepared by lithiation of 1,4-diphenyl-1,2,3-triazole (182), is converted into lithium ynamine (lithium ynamide) (184) on thermal elimination of nitrogen (equation 72). This ynamine (184) is methylated in moderate yields either by methyl iodide to give a ketenimine (185) and a dimerization product (186), or... 

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