Enol ethers alkynic carbene complexes

The first examples of a [2 -i- 2] cycloaddition at an unsaturated substituent on a carbene ligand have been recently reported." In the example shown in Scheme 2 it can be seen that the cycloadditions can occur very rapidly between enol ethers and alkynic carbene complexes. Alkynic carbene complexes can serve as surrogates for alkynic esters since the cycloadducts can be very efficiently oxidized to their corresponding esters. While the [2 + 2] cycloaddition of methyl tetrolate and dihydropyran is unknown, a comparison can be made between the alkynic carbene complex (12b), which reacts at room temperature, and the doubly activated alkynic ester (15), which will only react at 180 C with the same substrate. 

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

In addition to reactions characteristic of carbonyl compounds, Fischer-type carbene complexes undergo a series of transformations which are unique to this class of compounds. These include olefin metathesis [206,265-267] (for the use as metathesis catalysts, see Section 3.2.5.3), alkyne insertion, benzannulation and other types of cyclization reaction. Generally, in most of these reactions electron-rich substrates (e.g. ynamines, enol ethers) react more readily than electron-poor compounds. Because many preparations with this type of complex take place under mild conditions, Fischer-type carbene complexes are being increasingly used for the synthesis [268-272] and modification [103,140,148,273] of sensitive natural products. 

Several examples have been reported, where cyclopropyl containing carbenes were used in [4 -H 2], [2 -f 2] and [3 -t- 2] cycloadditions giving monocyclic and polycyclic ring systems. Thus, the chromium cyclopropylethynyl(ethoxy)carbene complex 19 formed a [2-1-2] cycloadduct with silyl enol ethers 20 underwent further conversion with alkynes to form the bicyclo[4.2.0]oc-tadienone skeleton 21. ... 

Addition of phenols ArOH to symmetrical alkynes RC=CR, catalysed by [ Au(NHC) 2(//-OH)][BF4] (NHC=A-heterocyclic carbene), has been reported to be (Z)-stereoselective, affording enol ethers R(ArO)C=CHR. a -Coordination of Au(I) to the C=C bond and formation of the phenolate ArOAu has been suggested. An enantioselective domino process, involving the 5-exo-dig cyclization of (178), followed by a second cyclization, giving rise to (179), has been attained by employing a combination of the Au(I) complex (180) and the BINOL-derived phosphoric acid (181) as catalysts (Scheme 11). ... 

Z-Dienynes Phis Carbene Complexes Alternative benzannulation through reaction of dienynes (e.g., 173, Scheme 17.24) and carbene complexes provides a net [5 -I-1] approach for the construction of benzofurans (e.g., 176) [71-73]. In most of the examples reported, the carbene complex is relatively simple and the molecular complexity resides in the diene group. In this transformation, the critical dienylketene intermediate for benzannulation (174) is accessed through regioselec-tive alkyne insertion followed by CO insertion. The reaction was designed to produce the phenol-enol ether derivatives (e.g., 175) however, these complexes are unstable... 

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