Ligand Effects in Regio- and Enantioselective Cobalt-Catalyzed Intramolecular [4+2]- and [2+2]-Cycloaddition Reactions of Unactivated 1,3-Diene-8-ynes and 1,3-Diene-8-enes.

IF 16.9
Kiron Kumar Ghosh, Rajdip Chowdhury, Jonathan P Gordon, T V RajanBabu
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Abstract

Intramolecular Diels-Alder reactions have been extensively used for the syntheses of biologically active compounds including natural products. We have identified Co(I) complexes and optimal reaction conditions to accomplish highly diastereo- and enantioselective intramolecular [4+2]-cycloaddition reactions of 1,3-diene-ynes and 1,3-diene-enes. These catalysts, readily prepared from an earth-abundant metal and tunable ligands, serve as practical replacements for the prohibitively expensive Rh-complexes previously used for similar reactions. The cationic complexes are readily prepared by the reaction of LCoX2 with Zn in the presence of NaBARF. Both neutral and cationic Co(I)-complexes of (S)-BINAP and (S)-H8-BINAP ligands effect the cyclization of 1,3-diene-ynes giving the expected [4+2]-addition products in excellent selectivities. The [4+2]-cycloadditions of the less reactive 1,3-diene-enes proceed only with the cationic complexes and 1,2-bis-[(2S,5S)-2,5-diphenylphospholan-1-yl]ethane (PhBPE) and appropriately substituted phosphinooxazoline (PHOX) are the most optimal ligands for these reactions. Depending on the ligands, 1,3-diene-enes bearing an aromatic substituent at the terminal position undergo an intramolecular [2+2]-cycloaddition, giving cyclobutane products in very good yields and enantioselectivities. The absence of the corresponding [2+2]-cycloaddition in the 1,3-diene-yne substates may be related to the enhanced strain in the expected bicyclo[3.2.0]heptene products. A unified mechanism involving Co(I)/Co(III)-redox cycle, consistent with all available evidence, is proposed for these reactions.

Abstract Image

钴催化非活化1,3-二烯-8-炔和1,3-二烯-8-炔分子内[4+2]-和[2+2]-环加成反应中的配体效应
分子内Diels-Alder反应已广泛用于合成包括天然产物在内的生物活性化合物。我们已经确定了Co(I)配合物和最佳反应条件,以完成1,3-二烯炔和1,3-二烯炔的高非映对和对映选择性分子内[4+2]环加成反应。这些催化剂很容易从地球上丰富的金属和可调配体中制备出来,可以替代以前用于类似反应的昂贵的铑配合物。在NaBARF存在下,LCoX2与Zn反应制备出阳离子配合物。(S)-BINAP和(S)-H8-BINAP配体的中性和阳离子Co(I)-配合物都能影响1,3-二烯-炔的环化,从而产生预期的[4+2]加成产物,具有优异的选择性。活性较低的1,3-二烯烯的[4+2]环加成只与阳离子配合物进行,1,2-二-[(2S,5S)-2,5-二苯基磷-1-基]乙烷(PhBPE)和适当取代的磷酰喹啉(PHOX)是这些反应的最佳配体。根据配体的不同,在末端位置带有芳香取代基的1,3-二烯烯发生分子内[2+2]环加成反应,生成收率和对映选择性非常好的环丁烷产物。在1,3-二烯-炔底物中缺少相应的[2+2]-环加成可能与预期的双环[3.2.0]庚烯产物中的菌株增强有关。本文提出了一种涉及Co(I)/Co(III)-氧化还原循环的统一机制,与所有现有证据一致。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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