Mixed Lithium Amide-Lithium Halide Compounds: Unusual Halide-Deficient Amido Metal Anionic Crowns
Mixed Lithium Amide-Lithium Halide Compounds: Unusual Halide-Deficient Amido Metal Anionic Crowns
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混合氨基锂-卤化锂化合物：异常的缺乏卤化物的氨基金属阴离子冠
DOI：
10.1002/ange.201102023
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发表时间：
2011
影响因子：
--
通讯作者：
Kennedy A
中科院分区：
文献类型：
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作者：
Kennedy A
碱金属卤化物盐对有机转化的反应性/选择性有很大的影响，无论是有益的还是有害的。[1]在许多情况下，在复分解反应中原位形成的金属卤化物盐被认为是一种无害的副产品。最近，越来越多的案例被曝光，卤化锂以一种非无害的、往往是主导的方式影响有机金属反应。诺切尔·埃塔尔。通过在传统的格氏试剂或豪泽试剂中添加化学计量比的氯化锂，以诱导比单金属镁试剂更强的反应性，利用了这一效应。提出了LiCl在一系列去质子化[3]和加成反应中所起的令人惊讶的和深刻的作用，[4]确立了LiCl催化可以用微量的LiCl检测到，并且对于二异丙胺与不饱和酯的1，4-加成反应，小于1.0mol%的LiCl可以引起“显著的加速”(70倍)。我们最近合成并表征了镁酸盐[(Thf)2Li(μ-Cl)2 mg(Tmp)(Thf)]，发现它的功能与诺切尔-S原位格氏体系(tmp=2，2，6，6-四甲基吡啶)相同。与这项工作相关的是，我们以前发现NaHMDS和(±)-司马豆碱的正己烷溶液可以与不定水反应生成含羟基的钠盐，[(±)-司马豆碱}Na(μ-HMDS)Na{(±)-司马豆碱}]+[Na4(μ-HMDS)4(OH)](1；方案1)，其中HMDS是1，1，1，3，3-六甲基二硅氮化物。[7]鉴于这个二胺-NaHMDS系统已经正式捕获了单体NaOH，我们设想类似的LiHMDS系统可以捕获亚化学计量的其他盐，特别是Lewis两性金属卤化物，对于金属盐增强反应来说，它们似乎比金属氢氧化物更重要。我们研究了几种实现这一目标的方法。首先，通过尝试LiHMDS和二胺与亚化学计量比LiX(其中X是氯、溴或碘)的直接结合(共络合)；其次，通过将nBuLi与NH4X结合(铵盐法[8])，然后在二胺存在的情况下引入超化学计量比LiHMDS；以及第三，通过以类似于先前方法的方式处理NEt4X(有机铵盐法)(方案2；辅助信息，方案S1)。令人满意的是，这些反应为我们提供了一个更好的结构洞察力，以了解Lix与LiHMDS的协调。为简洁起见，本文仅讨论共络合路线(对于2-4)和铵盐法(对于5)，尽管在支持信息中给出了其他路线的全部细节。我们的研究重点是生长适合于X射线分析的晶体，这可以为有机转化中使用的含锂卤化物溶液中潜在存在的物种提供洞察。第一个反应结合在一起
Alkali metal halide salts can dramatically influence the reactivity/selectivity of organic transformations in either beneficial or detrimental ways.[1] In many circumstances, the metal halide salt formed in situ in a metathesis reaction is dismissed as an innocent by-product. Recently, more cases have come to light where lithium halides affect organometallic reactions in a non-innocent, often dominant way. Knochel etal. has exploited this effect by adding stoichiometric amounts of LiCl to conventional Grignard or Hauser reagents to induce an enhanced reactivity with respect to that of monometallic magnesium reagents.[2] Collum et al. presented the surprising and profound role that LiCl plays in a series of deprotonation [3] and addition reactions,[4] establishing that LiCl catalysis is detectable with miniscule quantities of LiCl, and that “striking accelerations”(70 fold) are elicited by less than 1.0 mol% LiCl for 1, 4-addition reactions of lithium diisopropylamide to unsaturated esters.[4] Despite this, firm structural evidence of the crucial halide-incorporated species that may be involved in these reactions is rare.[1h, 5] In one example, we recently synthesized and characterized the magnesiate [(thf) 2Li (μ-Cl) 2Mg (TMP)(thf)] and found that it functions identically to Knochel s insitu Grignard system (TMP= 2, 2, 6, 6-tetramethylpiperidide).[6] Herein we start to deconvolute the complex chemistry at work when synthetically important lithium amides come into contact with a halide source. Pertinent to this work, we previously discovered that a hexane solution of NaHMDS and (À)-sparteine can react with adventitious water to yield the hydroxy-incorporated sodium sodiate,[{(À)-sparteine} Na (μ-HMDS) Na {(À)-sparteine}]+[Na4 (μ-HMDS) 4 (OH)] À (1; Scheme 1), where HMDS is 1, 1, 1, 3, 3, 3-hexamethyldisilazide.[7] Given that this diamine–NaHMDS system has formally captured monomeric NaOH, we envisaged that a similar LiHMDS system could capture substoichiometric quantities of other salts, and particularly the Lewis amphoteric metal halides, which appear far more important than metal hydroxides for metal salt-enhanced reactions.We have investigated several approaches in reaching this goal. Firstly, by attempting direct combination (co-complexation) of LiHMDS and a diamine with sub-stoichiometric LiX (where X is Cl, Br, or I); secondly, by combining nBuLi with NH4X (ammonium salt route [8]) and then introducing superstoichiometric LiHMDS in the presence of a diamine; and, thirdly, by treating NEt4X (organoammonium salt route) in a similar manner to the previous approach (Scheme 2; Supporting Information, Scheme S1). Gratifyingly, these reactions provide us with an enhanced structural insight into the coordination of LiX with LiHMDS. For brevity, only the cocomplexation route (for 2–4) and ammonium salt route (for 5) are discussed herein, although full details of the other routes are given in the Supporting Information. Our research focused on growing crystals suitable for X-ray analysis that could provide insight into species potentially present in lithium amide–halide-containing solutions used in organic transformations. The first reaction combined