Modifying alkylzinc reactivity with 2,2'-dipyridylamide: activation of tBu-Zn bonds for para-alkylation of benzophenone.
Modifying alkylzinc reactivity with 2,2'-dipyridylamide: activation of tBu-Zn bonds for para-alkylation of benzophenone.
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DOI：
10.1002/anie.201302426
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发表时间：
2013-07-08
影响因子：
16.6
通讯作者：
Robertson, Stuart D.
中科院分区：
文献类型：
--
作者：
Armstrong, David R.;Garden, Jennifer A.;Kennedy, Alan R.;Mulvey, Robert E.;Robertson, Stuart D.
最近，锌酸盐金属化化学中许多引人注目的进展都集中在仲单酰胺 2,2,6,6-四甲基哌啶 (TMP) 上。由 Kondo、Uchiyama 及其同事开发的原始 TMP 锌酸盐试剂 [LiZn (TMP) tBu2] 是芳香族和杂芳香族底物的有效碱。 [1]此后，Knochel 及其同事编制了一个基于 TMP 的新型金属化剂库，其中包括新戊酸锌 [(TMP) Zn-(OPiv)·LiCl](OPiv= pivalate)，它与 [LiZn (TMP) tBu2] 一样，显示出强大的去质子能力和出色的官能团耐受性，但具有额外的优势，即其芳基锌化衍生物具有较长的空气稳定性。 [2]结构上最好定义的锌酸盐金属化化学领域是涉及钠 TMP 试剂 [(TMEDA) Na (TMP)(tBu) Zn (tBu)](1; TMEDA= N, N, N’, N’-四甲基乙二胺)。该试剂比上述锌酸锂表现出更强的反应性，其许多芳基锌化衍生物采用类似的结构，这些结构是在带有末端配体的Na-阴离子-Zn-阴离子环的结构模板上设计的，即Na和Zn上分别是中性供体和阴离子（图1）。为了打破这个模板并开发可以刺激新反应性的新结构基序，我们研究了用多功能 2, 2'-联吡啶酰胺 [dpa,(2-NC5H4) 2N] 替代单功能 TMP。这种 N 桥双（N 杂环）提供三个潜在的连接 N 位点，可用于多种应用，包括医学、[3] 催化、[4] 光致发光、[5] 和超分子化学。 [6]在此，我们报告了一组引人注目的新颖结构，将 dpa 纳入含有叔丁基配体的中性锌、锌酸钠和锌酸钾修饰中。初步反应性研究表明，混合的钠-锌 dpa 配合物可以在对位（1, 6-加成）位置对酮二苯甲酮进行叔丁基化，这是具有挑战性的，并暗示该反应可以在钠组分中进行亚化学计量。通过 tBu2Zn 和胺 dpa (H) 在己烷溶液中按 1:1 化学计量组合直接合成中性杂配锌络合物 [{(dpa) Zn (tBu)} 2](2)。滤液的 NMR 研究表明绝对收率高于分离晶体的 70% 收率。 2（方案1）的中心对称分子结构具有有吸引力的沙漏形核心，是二聚体，扭曲的四面体锌原子[7]与一个酰胺单元的去质子化N键合，占据另一个酰胺单元的联吡啶口袋并完成与末端tBu配体的配位。在沙漏描述中（参见支持信息），每个灯泡包含一个褶皱六原子 (NCNCNZn) 环，其中 Zn 位于距离最近的 NCNCN 平面 1.0991 (3) 处，距离第二个 NCNCN 平面 1.7143 (3) 处。该基序与等电子中性锌二聚体 [{MeZnC (H)-Py2} 2](Py= 2-pyridyl) 非常相似。 [8]据我们所知，2 代表第一个具有 ZnÀ（桥头）N 键的结晶学表征的 Zn/dpa 复合物，尽管它在烷基化二聚衍生物中已被注意到，其中二聚化模式与 2 中的不同。[9] 2 内的反/反构象 [10]（即两个吡啶基 N 原子都远离桥头 N）也与 dpa (H)[11] 和上述烷基化 dpa 衍生物的多晶型物中发现的顺/反构象形成对比。预计在己烷溶液中 TMP/锌酸盐 1 和 dpa (H) 之间会发生简单的转氨反应 [12]，但反而是分离的结晶产物…
Many of the recent eye-catching advances in zincate executed metallation chemistry have centered on the secondary monoamide 2, 2, 6, 6-tetramethylpiperidide (TMP). Developed by Kondo, Uchiyama, and co-workers, the original TMP zincate reagent [LiZn (TMP) tBu2] is an effective base for both aromatic and heteroaromatic substrates.[1] Knochel and coworkers have since compiled a library of new TMP-based metalating agents, including the zinc pivalate [(TMP) Zn-(OPiv)· LiCl](OPiv= pivalate), which like [LiZn (TMP) tBu2], displays strong deprotonating power and exceptional functional-group tolerance but has the added advantage that its arylzincated derivatives boast prolonged air stability.[2] The area of zincate metalation chemistry that is best structurally defined is that involving the sodium TMP reagent [(TMEDA) Na (TMP)(tBu) Zn (tBu)](1; TMEDA= N, N, N’, N’-tetramethylethylenediamine). This reagent, which exhibits enhanced reactivity over the aforementioned lithium zincates, and many of its arylzincated derivatives adopt similar structures, which are designed on an architectural template of a Na–anion–Zn–anion ring carrying terminal ligands, namely a neutral donor and an anion on Na and Zn, respectively (Figure 1). To break this template and develop new structural motifs that could stimulate new reactivity we have investigated replacing monofunctional TMP with multifunctional 2, 2’-dipyridylamide [dpa,(2-NC5H4) 2N]. Offering three potential ligating N sites, this N-bridged bis (N heterocycle) finds utility in a diverse range of applications including medicine,[3] catalysis,[4] photoluminescence,[5] and supramolecular chemistry.[6] Herein we report a remarkable set of novel structures incorporating dpa into neutral zinc, sodium zincate, and potassium zincate modifications containing tert-butyl ligands. Preliminary reactivity studies reveal that the mixed sodium-zinc dpa complexes can tert-butylate the ketone benzophenone at the para (1, 6-addition) position, which is challenging, and hint that the reaction can be made substoichiometric in the sodium component. The neutral, heteroleptic zinc complex [{(dpa) Zn (tBu)} 2](2) was synthesized straightforwardly through the 1: 1 stoichiometric combination of tBu2Zn and the amine dpa (H) in a hexane solution. NMR studies of the filtrate revealed the absolute yield was higher than the 70% yield of the isolated crystals. Possessing an attractive hour-glass-shaped core, the centrosymmetric molecular structure of 2 (Scheme 1) is dimeric with the distorted tetrahedral zinc atom [7] bonded to the deprotonated N of one amido unit, which occupies the dipyridyl pocket of the other and completes its coordination with a terminal tBu ligand. In the hour-glass description (see the Supporting Information), each bulb comprises a puckered, six-atom (NCNCNZn) ring with Zn situated 1.0991 (3) from the nearest NCNCN plane and 1.7143 (3) from the second NCNCN plane. This motif bears a strong resemblance to that of the isoelectronic neutral zinc dimer [{MeZnC (H)-Py2} 2](Py= 2-pyridyl).[8] To our knowledge, 2 represents the first crystallographically characterized Zn/dpa complex showing a ZnÀ (bridgehead) N bond, although it has been noted in alkylated dimeric derivatives wherein the mode of dimerization is distinct from that in 2.[9] The anti/anti conformation [10](ie, both pyridyl N atoms directed away from the bridgehead N) within 2 also contrasts with the syn/anti conformations found in polymorphs of dpa (H)[11] and the aforementioned alkylated dpa derivatives.A simple transamination reaction [12] between TMP/zincate 1 and dpa (H) in a hexane solution was anticipated, but instead the isolated crystalline product …
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