Trimethyl phosphite as a trap for alkoxy radicals formed from the ring opening of oxiranylcarbinyl radicals. Conversion to alkenes. Mechanistic applications to the study of C-C versus C-O ring cleavage
Trimethyl phosphite as a trap for alkoxy radicals formed from the ring opening of oxiranylcarbinyl radicals. Conversion to alkenes. Mechanistic applications to the study of C-C versus C-O ring cleavage
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DOI：
10.1021/ja020761x
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发表时间：
2003-03-19
影响因子：
15
通讯作者：
Bentrude, WG
中科院分区：
文献类型：
--
作者：
Ding, BW;Bentrude, WG
亚磷酸三甲酯(MeO)(3)P是在80℃左右的AIBN/n-BU3SnH热条件下，由卤代环氧化物8-13生成的氧杂环丙基(2)体系中的一种有效的选择性捕捉剂。最初，在没有亚磷酸盐的情况下，定量地测量了8-13向烯丙醇7和/或乙烯基醚5的转化(表1)。中间体环氧基(2)、烯丙氧基(3)和乙烯氧基(4)自由基的结构变化涉及C-O(2->；3，k(1))和C-C(2->；4，k(2))自由基断裂过程的热力学和动力学的影响，并很容易解释产物乙烯基醚(5)和烯丙醇(7)生成量的变化。加成的(MeO)(3)P是惰性的乙烯氧基碳基4，选择性和快速地捕获烯丙氧基3，将其转移到磷酸三甲酯和烯丙基6。烯丙基自由基(6)二聚或被n-BU3SnH捕获得到烯烃，由卤代环氧化物8，9和13形成，产率69-95%。在(MeO)3P存在或不存在的情况下，中间体乙烯氧基自由基(4)被n-Bu3SnH捕获得到乙烯基醚(5)。(MeO)(3)P和n-EU3SnH的浓度是独立变化的，卤代环氧化物10、11和13中的磷酸盐、乙烯基醚(5)和/或烯烃的量被仔细监测。结果反映了自由基2-4结构变化的影响，特别是当它们影响中间体氧基酮2的C-O(k(1))和C-C(k(2))裂解以及它们的反向(k(-1)，k(-2))时。通过(MeO)(3)P从卤代环氧化物11和12转移烯丙氧基(3)，符合先前的预测，即在更接近动力学控制的条件下，可能会产生C-O断裂的产物，而不仅仅是C-C断裂的产物。因此，对于卤代环氧化物11、12和13中的氧杂环丙基自由基，C-O裂解(k(1)，2-gt；3)很容易与C-C裂解(k(2)，2-->；4)竞争，尽管在热力学上C-C裂解是有利的。
Trimethyl phosphite, (MeO)(3)P, is introduced as an efficient and selective trap in oxiranylcarbinyl radical (2) systems, formed from haloepoxides 8-13 under thermal AIBN/n-BU3SnH conditions at about 80 degreesC. Initially, the transformations of 8-13, in the absence of phosphite, to allyl alcohol 7 and/or vinyl ether 5 were measured quantitatively (Table 1). Structural variations in the intermediate oxiranylcarbinyl (2), allyloxy (3), and vinyloxycarbinyl (4) radicals involve influences of the thermodynamics and kinetics of the C-O (2 --> 3, k(1)) and C-C (2 --> 4, k(2)) radical scission processes and readily account for the changes in the amounts of product vinyl ether (5) and allyl alcohol (7) formed. Added (MeO)(3)P is inert to vinyloxycarbinyl radical 4 and selectively and rapidly traps allyloxy radical 3, diverting it to trimethyl phosphate and allyl radical 6. Allyl radicals (6) dimerize or are trapped by n-BU3SnH to give alkenes, formed from haloepoxides 8, 9, and 13 in 69-95% yields. Intermediate vinyloxycarbinyl radicals (4), in the presence or absence of (MeO)3P, are trapped by n-Bu3SnH to give vinyl ethers (5). The concentrations of (MeO)(3)P and n-EU3SnH were varied independently, and the amounts of phosphate, vinyl ether (5), and/or alkene from haloepoxides 10, 11, and 13 were carefully monitored. The results reflect readily understood influences of changes in the structures of radicals 2-4, particularly as they influence the C-O (k(1)) and C-C (k(2)) cleavages of intermediate oxiranylcarbinyl radical 2 and their reverse (k(-1), k(-2)). Diversion by (MeO)(3)P of allyloxy radicals (3) from haloepoxides 11 and 12 fulfills a prior prediction that under conditions closer to kinetic control, products of C-O scission, not just those of C-C scission, may result. Thus, for oxiranylcarbinyl radicals from haloepoxides 11, 12, and 13, C-O scission (k(1), 2 --> 3) competes readily with C-C cleavage (k(2), 2 --> 4), even though C-C scission is favored thermodynamically.