Three-carbon Dowd-Beckwith ring expansion reaction versus intramolecular 1,5-hydrogen transfer reaction: A theoretical study
Three-carbon Dowd-Beckwith ring expansion reaction versus intramolecular 1,5-hydrogen transfer reaction: A theoretical study
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DOI：
10.1021/jo051551g
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发表时间：
2005-11-11
影响因子：
3.6
通讯作者：
Sordo, TL
中科院分区：
文献类型：
--
作者：
Ardura, D;Sordo, TL
研究了AIBN/HSnBu_3与1-甲基-1，3-二氢-1，3-二氧杂环己烷加成形成的伯自由基的演化规律。（1 R *，2 R *）-1-（3-碘丙基）-5-氧代环戊烷甲酸甲酯（3-碘丙基）-2-甲基-5-氧代环戊烷甲酸酯，和（1-R*，2S*）-l-采用ROMP 2/6-311++G（2d，2 p）//UB 3LYP/6- 31 G（d，p）通过PCM-UAHF模型考虑溶剂影响的计算。根据理论结果，对于甲基1-（3-碘丙基）-5-氧代环戊烷甲酸甲酯和（1 R *，2S*）-1-（3-碘丙基）-2-甲基-5-氧代环戊烷羧酸酯的主要产物是来自三碳环膨胀的环辛烷衍生物，而对于甲基-5-氧代环戊烷羧酸酯，（1 R *，2，R*）-1-（3-碘丙基）-2-甲基-5-氧代环戊烷羧酸酯的主要产物是对应于1，5-H转座的产物，与实验结果一致。这种不同的行为是决定相对能垒的几个因素的结果。甲基取代基由于空间排斥而使（1-R*，2 R *）-1-（3-碘丙基）-2-甲基-5-氧代环戊烷羧酸甲酯的扩环过程不稳定，但在P-反式取代的底物的情况下有利于扩环过程，因为它使体系沿沿着更有利的构象演化成为可能。甲基也有利于1，5-H转座，使转座产物成为叔自由基。环膨胀过程的第二阶段通过共振来稳定。
The evolution of the primary radicals formed by addition of AIBN/HSnBu3 to methyl 1-(3-iodopropyl)5-oxocyclopentanecarboxylate, methyl (1R*,2R*)-1-(3-iodopropyl)-2-methyl-5-oxocyclopentanecarboxylate, and methyl (1-R*,2S*)-l-(3-iodopropyl)-2-methyl-5-oxocyclopentanecarboxylate in benzene has been theoretically investigated by ROMP2/6-311++G(2d,2p)//UB3LYP/6-31G(d,p) calculations taking into account the effect of solvent through a PCM-UAHF model. According to the theoretical results, for methyl 1-(3-iodopropyl)-5-oxocyclopentanecarboxylate and methyl (1R*,2S*)-1-(3-iodopropyl)-2-methyl-5-oxocyclopentanecarboxylate the major product is the cyclooctane derivative from the three-carbon ring expansion, whereas for methyl (1R*,2,R*)-1-(3-iodopropyl)-2-methyl-5oxocyclopentanecarboxylate the major product is that corresponding to the 1,5-H transposition in agreement with the experimental findings. This different behavior is a consequence of several factors determining the relative energy barriers. The methyl substituent destabilizes the ring expansion process for methyl (1-R*,2R*)-1-(3-iodopropyl)-2-methyl-5-oxocyclopentanecarboxylate because of steric repulsion but favors it in the case of the P-trans-substituted substrate because it makes possible the evolution of the system along more favorable conformations. The methyl group also favors the 1,5-H transposition rendering the transposed product a tertiary radical. The second stage of the ring expansion process is stabilized by resonance.