Ethynylation malonates

Under similar conditions, (2-oxoazetidinyl)malonates 129 can be alkynylated by (trimethylsilyl)ethynyl iodonium triflate (Scheme 53). In contrast to the previous reaction (Scheme 52), this alkynylation directly affords the desilylated terminal alkynes 130 as the final isolated products [112]. This reaction (Scheme 53) allows ethynylation of malonates under milder conditions compared to the reaction shown in Scheme 51. 

The reactions of the lithium enolate of diethyl 2-[(diphenylmethylene)amino]malonate with several alkynyliodonium triflates are rare examples of enolate alkynylations with iodonium species other than the ethynyl(phenyl)- and (phenylethynyl)phenyliodonium ions (equation 125)16. Two experimental protocols were followed, i.e. addition of the enolates to the iodonium salts and vice versa, the former procedure giving higher yields of alkynylmalonates. As with other enolate alkynylations, these reactions are thought to involve alkylidenecarbene intermediates. It has been proposed, however, that the carbenes rearrange with migration of the diethyl 2-[(diphenyl) amino] malonate anion 16. 

Phenyl[(trimethylsilyl)ethynyl]iodonium triflate has also been employed for alkynylations of diethyl 2-phthalimidomalonate and the (2-oxoazetidinyl)malonates shown in equation 126. However, unlike the result with the [(diphenyl)amino]malonate system (equation 125), the trimethylsilyl group is lost, and the ethynyl group is ultimately introduced. 

Cycloadditions of 1,6-diynes with 2-ethynyl-5,5-dimethyl-1,3,2-dioxaborinane (62) were also catalyzed by 6 to furnish stable arylboronates in good yields (Scheme 3.11) [48]. For example, a malonate-derived diyne underwent cycloaddition with 62 in the presence of 5 mol % of 6 at room temperature to afford the cycloadduct desired in 86% yield. However, the corresponding unsymmefrical diyne with one terminal methyl substituent required an increased catalyst loading of 10 mol %. Accordingly, regioisomers 63 and 64 were formed with a moderate selectivity at a 63/64 ratio of 1 2.4. 

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