Cation Charge Density and Precatalyst Selection in Group 2-Catalyzed Aminoalkene Hydroamination
Cation Charge Density and Precatalyst Selection in Group 2-Catalyzed Aminoalkene Hydroamination
复制标题
DOI：
10.1021/om101063m
复制
发表时间：
2011-03-28
期刊：
影响因子：
2.8
通讯作者：
Procopiou, Panayiotis A.
中科院分区：
文献类型：
--
作者：
Arrowsmith, Merle;Crimmin, Mark R.;Procopiou, Panayiotis A.
甲基镁和硅基酰胺-二酮酸钙、锶衍生物[{ArNC(Me)CHC(Me)NAr}MX-(THF)(n)] (M = Mg, X = CH3, n = 0; M = Ca, X = n (SiMe3)(2), n = 0或1；研究了M = Sr, X = N（SiMe3)(2), N = 1, Ar = 2,6-二异丙基苯基），硅酰胺[M{N(SiMe3)(2)}(2)](2)和[M{N(SiMe3)(2)}(2)(THF)(2)] （M = Mg, Ca, Sr, Ba）和烷基[M{CH(SiMe3)(2)}(2)(THF)(2)] （M = Mg, Ca, Sr, Ba）作为氨基烯烃氢胺化/环化的预催化剂。除钡外，所有预催化剂的氢胺化反应都能以接近定量的产量得到一系列五元和六元吡咯烷和哌啶衍生物，即使是有利的1-氨基-2,2-二苯基-4-戊烯底物，也明显限制在最多两次周转。六氢氮卓类化合物仅在镁酰胺类化合物中有明显的生成，而第二基团二烷基衍生物在高反应温度下分解，且范围更有限。总的来说，钙预催化剂被证明比它们的锶类似物更具活性，而锶类似物又被证明比镁预催化剂更具活性。在β -二氯胺酸衍生物中，与四氢呋喃溶剂化的衍生物相比，无四氢呋喃钙衍生物的配位不饱和程度越高，其对初级氨基烯烃底物的环化活性越高。相比之下，未溶剂化的双胺类化合物对这些底物的活性低于其溶剂化的类似物。使用硅酰胺预催化剂提供潜在可逆进入催化歧管。平衡的位置受到底物的性质和2族元素的特性的干扰，突出的是[{ArNC(Me)CHC(Me)NAr}Sr[N(SiMe3)(2)(THF)]与2-甲氧基乙胺的反应，这导致了初始硅酰胺（一种可分离的胺加合物）与胺/硅酰胺转氨化产物之间的平衡。与钙预催化剂进行相同的反应，提供了后者产品的类似物作为唯一可观察的物种。（1-烯丙基环己基）甲烷胺与每种钙和锶硅酰胺预催化剂的环化动力学研究提供了[催化剂]中明显的一级依赖性，而激活屏障的测定和Eyring分析提供了定量证据，表明本文报道的2族催化剂提供的活性至少与先前报道的镧系催化剂相当。在基于钙和锶的系统的特殊情况下，由于较大的二价碱土阳离子的电荷密度降低以及相应的较少限制速率决定的烯烃插入过渡态，提出了增强的催化性能，这是由明显的熵优势引起的。随着底物浓度的增加，环化速率也随之降低。后一种观察结果，以及大的动力学同位素效应（bbb4）的观察结果，被认为是与决定速率的烯烃插入相关的有益和协调的质子转移步骤的结果，被认为与michaelis - menten型动力学相一致。
The magnesium methyl and the calcium and strontium silylamide beta-diketiminate derivatives [{ArNC(Me)CHC(Me)NAr}MX-(THF)(n)] (M = Mg, X = CH3, n = 0; M = Ca, X = N(SiMe3)(2), n = 0 or 1; M = Sr, X = N(SiMe3)(2), n = 1, Ar = 2,6-diisopropylphenyl), the silylamides [M{N(SiMe3)(2)}(2)](2) and [M{N(SiMe3)(2)}(2)(THF)(2)] (M = Mg, Ca, Sr, Ba), and the alkyl species [M{CH(SiMe3)(2)}(2)(THF)(2)] (M = Mg, Ca, Sr, Ba) have been studied as precatalysts for the hydroamination/cyclization of aminoalkenes. Hydroamination afforded a series of five- and six-membered pyrrolidine and piperidine derivatives in near quantitative yields with all precatalysts, apart from the barium species, which were apparently limited to a maximum of two turnovers with even the favorable 1-amino-2,2-diphenyl-4-pentene substrate. Significant formation of hexahydroazepines was observed only with the magnesium amide species, while the group 2 dialkyl derivatives decomposed at high reaction temperatures and proved to be more limited in scope. In general, the calcium precatalysts proved to be more reactive than their strontium analogues, which, in turn, proved to be far more reactive than the magnesium species. Among the beta-diketiminate derivatives, the greater coordinathe unsaturation of the THF-free calcium derivative provided increased activity for the cyclization of primary aminoalkene substrates in comparison to its THF-solvated counterpart. In contrast, the unsolvated bis(amides) displayed lower activity with these substrates than their THF-solvated analogues. Use of silylamide precatalysts provides potentially reversible entry into the catalytic manifold. The position of the equilibrium is perturbed by both the nature of the substrate and the identity of the group 2 element, highlighted by the reaction of [{ArNC(Me)CHC(Me)NAr}Sr[N(SiMe3)(2)(THF)] with 2-methoxyethylamine, which results in equilibration between the starting silylamide, an isolable amine adduct, and the product of amine/silylamide transamination. Performing the same reaction with the calcium precatalyst provided the analogue of the latter product as the only observable species. A kinetic study of the cyclization of (1-allylcyclohexyl)methanamine with each of the calcium and strontium silylamide precatalysts provided an apparent first-order dependence in [catalyst], while determination of the activation barriers and Eyring analyses provided quantitative evidence that the group 2 catalysts reported herein provide activities at least commensurate with previously reported lanthanide-based catalyses. In the particular cases of systems based upon calcium and strontium, an enhanced catalytic performance is proposed to arise from a tangible entropic advantage resulting from the reduced charge density of the larger divalent alkaline earth cations and consequentially less constrained rate-determining alkene insertion transition states. The rate of cyclization was also found to decrease with increasing substrate concentration. This latter observation, along with the observation of large kinetic isotope effects (> 4), proposed to be a result of a beneficial and concerted proton transfer step associated with rate-determining alkene insertion, are reasoned to be consistent with Michaelis-Menten-type kinetics.