Base-catalysed ring openings of 1,2-diphenylcycloalkanols having five-, six-, seven- and eight-membered rings
作者:Sayid M. Moosavi、Roy S. Beddoes、C. Ian F. Watt
DOI:10.1039/a607548g
日期:——
trans-Isomers of 1,2-diphenylcyclopentanol, 5,
1,2-diphenylcyclohexanol, 6, 1,2-diphenylcycloheptanol, 7, and
1,2-diphenylcyclooctanol, 8, have been prepared as have their acyclic
analogues, threo- and erythro-3,4-diphenylhexan-3-ol,
9. All structural assignments are confirmed by X-ray crystal structure
determinations and experimentally determined structures are compared
with the results of empirical force field calculations which also yield
strain energies for each of the compounds. With alkali metal
dimsyl–dimethyl sulfoxide or 1,3-diaminopropane with its potassium
salt as base, the cycloalkanols are isomerised to enolates of
corresponding 1,n-diphenylalkan-1-ones, and the acyclic
alkanols cleaved to propylbenzene and the enolate of propiophenone. The
products are consistent with a polar mechanism involving collapse of
alkoxide to expel a benzylic carbanion, followed by one or more proton
transfers to yield the observed products. Rates increase in the order,
6 < 5 7 < 9
< 8, with a spread in reactivities of ca.
106. Logarithms of relative rates correlate poorly with
estimates of strain release in the reactions. Correlations are improved
by incorporation of estimates of entropy changes associated with ring
opening or cleavage, but remain poor. The fate of isotopic labels in the
reactions of
2,n,n-trideuterio-1,2-diphenylcycloalkanols.
[2H3]-5, [2H3]-7 and
[2H3]-8, shows that protonation of the benzylic
carbanion is by solvent DMSO for the cyclooctanol,
[2H3]-8, and that competing intramolecular proton
transfer occurs in the cycloheptanol, [2H3]-7, and
cyclopentanol, [2H3]-5. Kinetic isotope effects
associated with the labelling patterns are consistent with a change in
rate-limiting step from the initial carbon–carbon bond cleavage in
the case of 8, to rate-limiting proton transfer in the case of
5.
我们制备出了 1,2-二苯基环戊醇 5、1,2-二苯基环己醇 6、1,2-二苯基环庚醇 7 和 1,2-二苯基环辛醇 8 的反式异构体,以及它们的无环类似物--3,4-二苯基己醇 9。所有的结构分配都得到了 X 射线晶体结构测定的证实,实验测定的结构与经验力场计算的结果进行了比较,后者还得出了每种化合物的应变能。以碱金属二甲基亚砜或 1,3-二氨基丙烷及其钾盐为碱,环烷醇异构化为相应的 1,n-二苯基烷-1-酮的烯醇盐,无环烷醇裂解为丙基苯和苯丙酮的烯醇盐。这些产物与极性机理相吻合,其中包括烷氧基崩解以排出苄基碳酰离子,然后进行一次或多次质子转移以产生所观察到的产物。反应速率依次为 6 < 5 7 < 9 < 8,反应活性相差约 106。相对速率的对数与反应中应变释放的估计值相关性较差。如果加入与开环或裂解相关的熵变估计值,相关性会有所改善,但仍然很差。同位素标签在 2,n,n-三氚基-1,2-二苯基环烷醇反应中的去向。
[2H3]-5、[2H3]-7 和 [2H3]-8,表明环辛醇 [2H3]-8 中苄基碳酰离子的质子化是通过溶剂 DMSO 进行的,而环庚醇 [2H3]-7 和环戊醇 [2H3]-5 中发生了竞争性分子内质子转移。与标记模式相关的动力学同位素效应与限速步骤的变化一致,即 8 的限速步骤由最初的碳碳键裂解转变为 5 的限速质子转移。