Nucleophilicity Parameters for Phosphoryi-Stabilized Carbanions and Phosphorus Ylides: Implications for Wittig and Related Olefination Reactions
Nucleophilicity Parameters for Phosphoryi-Stabilized Carbanions and Phosphorus Ylides: Implications for Wittig and Related Olefination Reactions
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DOI：
10.1021/ja8056216
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发表时间：
2009-01-21
影响因子：
15
通讯作者：
Mayr, Herbert
中科院分区：
文献类型：
--
作者：
Appel, Roland;Loos, Robert;Mayr, Herbert
用紫外-可见光谱法测定了四种磷酰基稳定的碳负离子1a-d和四种磷叶立德1 e-h与二苯甲基鎓离子2a-h和结构相关的醌甲基化物2 i-m的反应动力学。二级速率常数（k）与2a-m的亲电性参数E线性相关，如相关性log k=s（N+E）（J. Am. Soc.2001，123，9500-9521），允许我们计算磷酰基取代的碳负离子和磷叶立德的亲核特异性参数N和s。这样，Horner-Wadsworth-Emmons碳负离子和Wittig叶立德的亲核反应性的直接比较成为可能。Ph_2PO和（EtO）（2）PO取代的碳负离子对Michael受体的反应活性相似，比类似取代的磷叶立德高10（4）~ 10（5）倍。这些亲核试剂对苯甲醛的相对反应性与对碳阳离子和迈克尔受体的反应性显著不同，这与这些烯化反应的初始步骤是一致的[2 + 2]环加成一致。本文研究了平衡离子（K ~+、Na ~+、Li ~+）对磷酰基稳定碳负离子在DMSO中亲核性的影响。而K+和Na+的影响几乎可以忽略不计的所有类型的碳负离子的研究，锂+配位降低了膦酸取代的乙酸酯阴离子（1a）的反应性的10倍（2），而膦酸取代的乙腈阴离子（1b）的反应性几乎不受影响。
The kinetics of the reactions of four phosphoryl-stabilized carbanions 1a-d and four phosphorus ylides le h with benzhydrylium ions 2a-h and structurally related quinone methides 2i-m have been determined by UV-vis spectroscopy. The second-order rate constants (k) correlated linearly with the electrophilicity parameters E of 2a-m, as required by the correlation log k=s(N+E) (J. Am. Chem. Soc. 2001, 123, 9500-9521), allowing us to calculate the nucleophile-specific parameters N and s for phosphoryl-substituted carbanions and phosphorus ylides. In this way, a direct comparison of the nucleophilic reactivities of Horner-Wadsworth-Emmons carbanions and Wittig ylides became possible. Ph2PO- and (EtO)(2)PO-substituted carbanions are found to show similar reactivities toward Michael acceptors, which are 10(4)-10(5) times higher than those of analogously substituted phosphorus ylides. The relative reactivities of these nucleophiles toward benzaldehydes differ significantly from those toward carbocations and Michael acceptors, in accordance with a concerted [2 + 2] cycloaddition being the initial step of these olefinations reactions. Effects of the counterion (K+, Na+, or Li+) on the nucleophilicities of the phosphoryl-stabilized carbanions in DMSO have been studied. Whereas the effects of K+ and Na+ are almost negligible for all types of carbanions investigated, Li+ coordination reduces the reactivities of phosphonate-substituted acetic ester anions (1a) by a factor of 10(2) while the reactivities of phosphonate-substituted acetonitrile anions (1b) remain almost unaffected.