C-H Activation of Isobutylene Using Frustrated Lewis Pairs: Aluminum and Boron σ-Allyl Complexes
C-H Activation of Isobutylene Using Frustrated Lewis Pairs: Aluminum and Boron σ-Allyl Complexes
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DOI：
10.1002/anie.201200328
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发表时间：
2012-01-01
影响因子：
16.6
通讯作者：
Stephan, Douglas W.
中科院分区：
文献类型：
--
作者：
Menard, Gabriel;Stephan, Douglas W.
虽然小分子的活化在过去的半个世纪里一直是过渡金属化学的范畴，但近年来过渡金属和主族化合物的反应性之间的相似之处越来越普遍。[1]在涉及主族化合物的方法中，受抑刘易斯对（FLP），即在空间上被抑制形成经典加合物的刘易斯酸和碱的组合，引起了特别的关注。虽然FLP最初因其异源活化H2的能力而备受关注[2]，但它们也被证明可以活化小分子，如CO2，[3] N2 O，[4] NO，[5]烯烃，炔烃等。[1c]大多数关于FLP的公开工作利用硼基刘易斯酸，特别是B（C6 F5）3。[1c]最近，我们[3b，d，6]和其他人[3 i，7]已经开始探索铝在这种化学中的应用。尽管硼/膦（B/P）和铝/膦（Al/P）FLP能够捕集CO2，但只有衍生自Al/P FLP的加合物显示出经历进一步的反应，导致CO2还原。[3b烯丙基是有机合成中的重要片段;[8]然而，使用未活化的烯烃化合物安装此类基团通常需要使用苛刻的碱，如KOtBu/nBuLi。[9]此外，使用活化的烯烃，[10]如烯丙基卤化物，[11]乙烯缩酮，[12]烯丙基醇盐，[13]和三甲基甲硅烷基烯烃衍生物，[14]已用于安装烯丙基取代基。[8c虽然这些试剂可用于羰基官能团的烯丙基化[11，16]，但烯烃的烯丙基化仅在有限数量的情况下实现。[10a为此，我们已经探索了硼基和铝基FLP在异丁烯的C2 H4活化中的反应性，作为获得α-烯丙基阴离子盐的途径。这些产品的性质在本文中建立和对比。虽然硼酸烯丙酯在可逆反应中形成，但相应的铝酸烯丙酯随后经历乙烯插入，因此提供了未活化烯烃的烯丙基化的罕见实例。在258 ℃下，将1大气压的异丁烯加入到包含在J-Young NMR管中的tBu 3 P和Al（C6 F5）3在C6 D5 Br中的1：1溶液中，导致放热反应，产生新产物和tBu 3 P的1：1混合物，如分别在60和62 ppm处的31 P NMR共振所示。以1：2的tBu 3 P/Al（C6 F5）3比率重复反应导致单一产物的形成。通过用己烷研磨反应混合物从放大反应中分离出该物质1（方案1）。31 P {1H}和31 P NMR光谱显示在60 ppm处的共振，具有426 Hz的强P-H耦合常数，这与单键耦合的值一致，因此与tBu 3 PH+离子的形成一致。19 F {1H} NMR谱仅显示三个共振，因此表明Al（C6 F5）3碎片存在于等效环境中。1H NMR谱在3.91 ppm处显示宽单峰，在2.10 ppm处显示尖锐单峰，相对积分分别为4和3。当在低温（1030 ℃）下获得1H NMR谱时，在3.91ppm处的共振分裂成两个宽单峰。[18]总的来说，这些数据意味着在两个当量的Al（C6 F5）3片段之间形成含有桥接烯丙基部分的阴离子化合物以及相应的鏻阳离子，因此1的配方为[tBu 3 PH][{（C6 F5）3Al} 2 {CH 2C（CH 3）CH 2}]。NMR数据支持烯丙基几何结构，其可能是μ2-η1：η1，尽管μ2-η3：η3几何结构不能被排除。
Although the activation of small molecules has been the purview of transition-metal chemistry for the past halfcentury, parallels between the reactivity of transition-metal and main-group compounds have become increasingly prevalent in recent years.[1] Among the approaches that involve main-group compounds, frustrated Lewis pairs (FLPs), that is, the combination of Lewis acids and bases that are sterically inhibited from forming classical adducts, have drawn particular attention. While FLPs initially garnered much attention for their ability to heterolytically activate H2,[2] they have also been shown to activate small molecules such as CO2,[3] N2O,[4] NO,[5] alkenes, alkynes, among others.[1c] The majority of the published work on FLPs exploits boron-based Lewis acids, particularly B (C6F5) 3.[1c] More recently, we [3b, d, 6] and others,[3i, 7] have begun to explore the use of aluminum in such chemistry. Whereas boron/phosphine (B/P) and aluminum/phosphine (Al/P) FLPs are capable of CO2 capture, only the adducts derived from the Al/P FLPs have been shown to undergo further reactions, resulting in CO2 reduction.[3b, d] Allyl groups are important fragments in organic synthesis;[8] however, the installation of such groups using unactivated olefin compounds usually requires the use of harsh bases, such as KOtBu/nBuLi.[9] In addition, the use of activated olefins,[10] such as allyl halides,[11] ethylene ketals,[12] allyl alkoxides,[13] and trimethylsilyl olefin derivatives,[14] have been used to install allyl substituents.[8c, 9, 15] While such reagents can be employed for the allylation of carbonyl functionalities,[11, 16] the allylation of olefins has only been achieved in a limited number of cases.[10a, 17] To this end, we have explored the reactivity of boron-and aluminum-based FLPs in the CÀH activation of isobutylene as a route to σ-allyl anion salts. The nature of these products is established and contrasted herein. Whereas allyl borates form in a reversible reaction, the corresponding allyl aluminate undergoes subsequent ethylene insertion, thus providing a rare example of the allylation of an unactivated olefin. The addition of 1 atm of isobutylene to a 1: 1 solution of tBu3P and Al (C6F5) 3 in C6D5Br, contained in a J-Young NMR tube at 258C, led to an exothermic reaction with the generation of a 1: 1 mixture of a new product and tBu3P, as indictated by the 31P NMR resonances at 60 and 62 ppm, respectively. Repetition of the reaction with a 1: 2 ratio of tBu3P/Al (C6F5) 3 led to the formation of a single product. This species 1 was isolated from a scaled-up reaction by trituration of the reaction mixture with hexanes (Scheme 1). The 31P {1H} and 31P NMR spectra revealed a resonance at 60 ppm with a strong P–H coupling constant of 426 Hz, which is consistent with the value of a one-bond coupling and thus the formation of the tBu3PH+ ion. The 19F {1H} NMR spectrum exhibited only three resonances, thus suggesting that the Al (C6F5) 3 fragments exist in equivalent environments. The 1H NMR spectrum exhibited a broad singlet at 3.91 ppm and a sharp singlet at 2.10 ppm, with a relative integration of four and three, respectively. When the 1H NMR spectrum was acquired at low temperature (À308C), the resonance at 3.91 ppm split into two broad singlets.[18] Collectively, these data imply the formation of an anionic compound containing a bridging allyl moiety between two equivalent Al (C6F5) 3 fragments together with the corresponding phosphonium cation and thus the formulation of 1 as [tBu3PH][{(C6F5) 3Al} 2 {CH2C (CH3) CH2}]. The NMR data support an allyl geometry that is likely μ2-η1: η1, although the μ2-η3: η3 geometry could not be dismissed …