METHYLENECYCLOPROPANE REARRANGEMENT AS A PROBE FOR FREE-RADICAL SUBSTITUENT EFFECTS - SIGMA.-VALUES FOR COMMONLY ENCOUNTERED CONJUGATING AND ORGANOMETALLIC GROUPS
METHYLENECYCLOPROPANE REARRANGEMENT AS A PROBE FOR FREE-RADICAL SUBSTITUENT EFFECTS - SIGMA.-VALUES FOR COMMONLY ENCOUNTERED CONJUGATING AND ORGANOMETALLIC GROUPS
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DOI：
10.1021/jo00391a015
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发表时间：
1987-07-24
影响因子：
3.6
通讯作者：
MCDONALD, S
中科院分区：
文献类型：
--
作者：
CREARY, X;MEHRSHEIKHMOHAMMADI, ME;MCDONALD, S
制备了CH2SiMe3、SiMe3、SnMe3、-B0CH2CH20和芳烃对位上的HgCl取代物。所有的都以取代基依赖的速率重新排列成相应的异丙基环丙烷9。与未被取代的类似物相比，这些常见的取代基可以提高重排速率，其中p-NMe2是最有效的。速率的增强可以用对取代基对双基中间体的稳定来解释。费率数据允许分配这些组的值，这些值以前没有确定过。对位中的硝基在增加重排速率方面也非常有效，这与该基团对许多其他自由基反应的作用形成对比。乙烯基作为自由基稳定基团比异丙烯或苯基更有效，这可能是由于异丙烯或苯基的共轭稳定所必需的平面构象中的空间相互作用。三甲基硅基、三甲基锡基和盐酸均能提高甲基环丙烷的重排速率，但仅在中等程度上。含硼取代基，其中硼可以作为受体基团，是更有效的自由基稳定基团之一，从它们对8重排速率的影响可以看出。环丙基和CH2SiMe3基团也在一定程度上提高了8的重排速率，当与碳氧基结合时，它们成为更有效的自由基稳定基团。因此，这两个共轭基团能够在俘获性自由基稳定中充当供体基团。自由基是有机化学中最重要的活性中间体之一。因此，人们继续对促进其稳定或不稳定的因素感兴趣。评估这种对自由基的影响不是一个简单的过程，因为极性效应经常在自由基反应中起作用，并且可以压倒真正的自由基效应。最近，我们和其他人试图定量描述取代基对自由基的影响。Timberlake等人对1及相关体系的热解研究对于深入了解某些基团稳定自由基的相对能力具有重要意义。各种各样的尺度也被开发出来，试图在没有极性效应的情况下测量取代基对苯基的影响。其中包括Jackson 's标度基于二苯汞的热解速率2.3 Fisher 's标度基于尽量减少自由基溴化中极性的贡献3.4 Jiang 's标度基于相对环二聚化速率4.5与许多以前的方法不同，Arnold 's标度是基于苯基中超精细耦合常数5.6的自由基稳定效应的非动力学测量Bordwell7最近的值基于pXa
CH2SiMe3, SiMe3, SnMe3,-B0CH2CH20, and HgCl substitution in the para position of the aromatic ring have been prepared. All rearrange thermally to the corresponding isopropylidenecyclopropanes 9 at rates that are substitutent dependent. These commonly encountered substituentsall enhance rearrangement rates relative to the unsubstituted analogue with p-NMe2 being the most effective. The rate enhancements are interpreted in terms of stabilization of the biradical intermediate by the para substituent. Rate data have allowed the assignment of'values for these groups, which have not been previously determined. The nitro group in the para position is also quite effective in increasing the rearrangement rate, which contrasts with the effect of this group on many other free radical reactions. Vinyl is somewhat more effective as a radical stabilizing group than is isopropenyl or phenyl, possibly due to steric interactions in the planar conformations necessary for conjugative stabilization by isopropenyl or phenyl. Trimethylsilyl, trimethylstannyl, and HgCl all enhance the methylenecyclopropane rearrangement rate, but only to a moderate extent. Boron containing substituents, where boron can act as an acceptor group, are among the more effective radical stabilizing groups, as implied by their effect on the rearrangement rate of 8. The cyclopropyl and CH2SiMe3 groups, which also enhance the rearrangement rate of 8 to a moderate extent, become even more effective radical stabilizing groups when present in conjunction with the carbethoxy group. These two conjugating groups are therefore capable of acting as donor groups in captodative radical stabilization.Free radicalsremain one of the reactive intermediates of fundamental importance in organic chemistry. As such, interest has continued in the factors that promote their stability or instability. 1 Evaluating such effects on free radicals is not a trivial process since polar effects often operate in free radical reactions and can overwhelm true free radical effects. Recently we and others have at-tempted to quantitatively describe substituent effects on free radicals. The pyrolysis studies of Timberlake and others2 on 1 and related systems have been of fundamental importance in providing insight into the relative abilities of certain groups to stabilize free radicals. Various' scales have also been developed that attempt to measure sub-stituent effects on benzylic type radicals in the absence of polareffects. Among these are Jackson’s' scale based on pyrolysis rates of the dibenzylmercurials 2.3 The'scale of Fisher is based on an attempt to minimize polar contributions in the free radical bromination of 3.4 Jiang’s' scaleis based on relative cyclodimerization rates of 4.5 Unlike many previous methods, Arnold’s* scale is a nonkinetic measure of radical stabilizing effects based on hyperfine coupling constants in the benzylic radical 5.6 The recent values of Bordwell7 are based on pXa