Enol Ethers as Substrates for Efficient <i>Z</i>- and Enantioselective Ring-Opening/Cross-Metathesis Reactions Promoted by Stereogenic-at-Mo Complexes: Utility in Chemical Synthesis and Mechanistic Attributes

The first examples of catalytic enantioselective ring-opening/cross-metathesis (EROCM) reactions that involve enol ethers are reported. Specifically, we demonstrate that catalytic EROCM of several oxa- and azabicycles, cyclobutenes and a cyclopropene with an alkyl- or aryl-substituted enol ether proceed readily in the presence of a stereogenic-at-Mo monopyrrolide-monoaryloxide. In some instances, as little as 0.15 mol % of the catalytically active alkylidene is sufficient to promote complete conversion within 10 min. The desired products are formed in up to 90% yield and >99:1 enantiomeric ratio (er) with the disubstituted enol ether generated in >90% Z selectivity. The enol ether of the enantiomerically enriched products can be easily differentiated from the terminal alkene through a number of functionalization procedures that lead to the formation of useful intermediates for chemical synthesis (e.g., efficient acid hydrolysis to afford the enantiomerically enriched carboxaldehyde). In certain cases, enantioselectivity is strongly dependent on enol ether concentration: larger equivalents of the cross partner leads to the formation of products of high enantiomeric purity (versus near racemic products with one equivalent). The length of reaction time can be critical to product enantiomeric purity; high enantioselectivity in reactions that proceed to >98% conversion in as brief a reaction time as 30 s can be nearly entirely eroded within 30 min. Mechanistic rationale that accounts for the above characteristics of the catalytic process is provided.本研究首次报道了涉及烯醇醚（enol ethers）的催化对映选择性开环/交叉复分解（EROCM）反应。具体而言，本研究证实，数种氧杂、氮杂双环化合物、环丁烯类以及一种环丙烯与烷基或芳基取代烯醇醚的催化EROCM反应，可在中心钼手性的单吡咯烷-单芳氧基钼亚烷基催化剂存在下顺利进行。在部分反应中，仅需0.15 mol%的活性亚烷基催化剂即可在10分钟内实现完全转化。目标产物的最高产率可达90%，对映体比（enantiomeric ratio, er）大于99:1，且生成的双取代烯醇醚的Z选择性超过90%。对映体富集产物中的烯醇醚基团，可通过多种官能化操作与末端烯烃轻松区分，这些操作可得到化学合成中有用的中间体（例如，通过高效酸性水解得到对映体富集的醛类化合物）。在部分反应中，对映选择性强烈依赖于烯醇醚的浓度：增加交叉偶联底物的当量数，可得到高对映体纯度的产物（而仅使用1当量底物时，产物近乎外消旋）。反应时长对产物的对映体纯度至关重要：在仅30秒内即可实现>98%转化率的反应中，其初始的高对映选择性可在30分钟内几乎完全丧失。本研究还提出了可解释该催化过程上述特征的机理解释。