Iron porphyrin-catalyzed olefination of ketenes with diazoacetate for the enantioselective synthesis of allenes.
نویسندگان
چکیده
The Wittig reaction and its variants are the most powerful approaches for constructing carbon-carbon double bonds in organic synthesis due to their unambiguous positioning and good stereoselectivity.1 Of the recent developments,2-4 much attention has been paid to ylide olefination of aldehydes3 and ketones4 under neutral conditions by transition metal complex-catalyzed decomposition of diazo compounds to generate ylide in situ. In our studies on ylide reactions in organic synthesis,2b-e,5 we are interested in developing an efficient method for the synthesis of allenes via olefination of ketenes, unstable carbonyl compounds,6 with diazoacetate in the presence of transition metal complex. Fortunately, we found that such an olefination process could be achieved smoothly in the presence of PPh3 and 0.5 mol % of tetra(pchlorophenyl)porphyrin iron chloride (Fe(TCP)Cl). In particular, when chiral phosphine was used, optically active allenes could be prepared with high enantioselectivities in high yields. In this communication, we wish to report the preliminary results. Allenes are of great importance due to their occurrence7 in natural products and biologically active compounds, and they are valuable intermediates8 in organic synthesis. Although many synthetic methods have been developed, a practical and mild process, in particular, for the synthesis of optically active ones,9 is still a challenge. Gratifyingly, it was found that allenic ester 1a could be synthesized in nearly quantitative yield when the reaction of 2-phenylprop-1-en-1-one with ethyl diazoacetate was carried out in toluene at 0 °C in the presence of PPh3 and 0.5 mol % of Fe(TCP)Cl (Table 1, entry 1). Further studies showed that a variety of ketenes with different structures are good substrates for this olefination. As shown in Table 1, pure disubstituted ketenes gave good to excellent yields (entries 1-8). Disubstituted ketenes, prepared in situ from acyl chlorides without further purification, also worked well to afford the trisubstituted allenes in good yields (entries 9 and 10). A one-pot strategy has also been developed for some unstable ketenes. For example, 4-monosubstituted and 4,4disubstituted allenic esters 1k-1o could be obtained in good yields when Et3N and acyl chloride were added sequentially10 into the ylide generated in situ from phosphine and diazoacetate (entries 11-15). Thus, various 4-monosubstituted and 4,4-disubstituted allenic esters could be synthesized (even in gram scale, entry 2 in Table 1) by the current protocol. For the olefination of aldehydes with diazo compounds catalyzed by transition metal complex, there are two mechanistic pathways in the literature,3,4 as shown in Scheme 1, depending on the metal complex. In the case of Fe(TCP)Cl employed, Woo and his coworkers3g,4a documented that the mechanism involved the formation of a free ylide (Path B in Scheme 1). On the basis of this mechanistic insight, it is envisaged that chiral allenes could be prepared enantioselectively just by the use of chiral phosphine instead of triphenylphosphine. To our delight, a variety of optically active allenic esters could be synthesized with excellent enantioselectivities (93-98% ee) in high yields by employing chiral diphosphine compound 4 under the optimal conditions, as shown in Table 2.11 These results also proved clearly that the mechanism of the present olefination involves the generation of an ylide through catalytic transfer of an iron(II) carbene ligand to phosphine. Compound 4 is a diphosphine, and the molar ratio of EDA to 4 used in the current reaction is 2:1. Therefore, both monoylide 6a and diylide 6b are possible intermediates of this reaction. To elucidate the real intermediate, ylides 6a and 6b were synthesized † Shanghai Institute of Organic Chemistry. ‡ Dalian Institute of Chemical Physics. Table 1. Olefination of Ketenes with EDA Catalyzed by Fe(TCP)Cla
منابع مشابه
A new and efficient method for the selective olefination of aldehydes with ethyl diazoacetate catalyzed by an iron(II) porphyrin complex.
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ورودعنوان ژورنال:
- Journal of the American Chemical Society
دوره 129 6 شماره
صفحات -
تاریخ انتشار 2007