On the enantioselectivity of transition metal-catalyzed 1,3-cycloadditions of 2-diazoeyclohexane-1,3-diones
On the enantioselectivity of transition metal-catalyzed 1,3-cycloadditions of 2-diazoeyclohexane-1,3-diones
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DOI：
10.1002/hlca.200390257
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发表时间：
2003-01-01
影响因子：
1.8
通讯作者：
Bernardinelli, G
中科院分区：
文献类型：
--
作者：
Müller, P;Allenbach, YF;Bernardinelli, G
本文研究了2-重氮环己二酮1a ~ 1d在Rh-Ⅱ催化剂存在下与无环和环状烯醇醚的1，3-环加成反应，得到二氢呋喃。与1-乙氧基丙-1-烯（13 a）的顺/反混合物反应得到顺/反比为85：15的二氢呋喃14 a，而与（Z）-1-乙氧基-3，3，3-三氟丙-1-烯（13 b）反应只得到顺式产物14 b。反应的立体化学结果是一致的，而不是逐步环加成机制。研究了2-重氮环己烷-1，3-二酮（1a）或2-重氮二甲酮（=2-重氮-5，5-二甲基环己烷-1，3-二酮; 1b）与呋喃和二氢呋喃的不对称环加成反应，但没有观察到明显的对映选择性，这些重氮化合物的报道的对映选择性环加成反应不能重现。用哈蒙德假设初步解释了2-重氮环己-1，3-二酮环加成反应中不存在对映选择性的现象。环加成反应的过渡态发生在反应坐标的早期，由于中间体二卡宾的高反应性。早期过渡态与低选择性相关。与此相反，稳定的双卡宾转移的过渡态出现较晚，反应表现出较高的选择性。
The formal 1,3-cycloaddition of 2-diazocyclohexane-1,3-diones 1a-1d to acyclic and cyclic enol ethers in the presence of Rh-II-catalysts to afford dihydrofurans has been investigated. Reaction with a cis/trans mixture of 1-ethoxyprop-1-ene (13a) yielded the dihydrofuran 14a with a cis/trans ratio of 85:15, while that with (Z)-1-ethoxy-3,3,3-trifluoroprop-1-ene (13b) gave the cis-product 14b exclusively. The stereochemical outcome of the reaction is consistent with a concerted rather than stepwise mechanism for cycloaddition. The asymmetric cycloaddition of 2-diazocyclohexane-1,3-dione (1a) or 2-diazodimedone (=2-diazo-5,5-dimethylcyclohexane-1,3-dione; 1b) to furan and dihydrofuran was investigated with a representative selection of chiral, nonracemic Rh-II catalysts, but no significant enantioselectivity was observed, and the reported enantioselective cycloadditions of these diazo compounds could not be reproduced. The absence of enantioselectivity in the cycloadditions of 2-diazocyclohexane-1,3-diones is tentatively explained in terms of the Hammond postulate. The transition state for the cycloaddition occurs early on the reaction coordinate owing to the high reactivity of the intermediate metallocarbene. An early transition state is associated with low selectivity. In contrast, the transition state for transfer of stabilized metallocarbenes occurs later, and the reactions exhibit higher selectivity.