TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases: lithium-mediated alumination or lithiation-alkylaluminium-trapping reagents?
TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases: lithium-mediated alumination or lithiation-alkylaluminium-trapping reagents?
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DOI：
10.1039/c4sc01108b
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发表时间：
2014-01-01
期刊：
影响因子：
8.4
通讯作者：
Robertson, Stuart D.
中科院分区：
文献类型：
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作者：
Armstrong, David R.;Crosbie, Elaine;Robertson, Stuart D.
TMP-铝酸锂碱“LiTMP中心点Al（Bu-i）（3）”1和“LiTMP中心点Al（TMP）（Bu-i）（2）”2（其中TMP是2，2，6，6-四甲基哌啶）最近作为“铝化”试剂受到关注，因为它们可以在各种芳族底物上进行铝-氢交换。先前的研究表明，1在THF溶液中作为单一物质存在，配制为[（THF）中心点Li（mu-TMP）（mu-Bu-i）Al（Bu-i）（2）]1中心点THF，具有接触离子对结构，如通过分离晶体的X射线晶体学研究所证明的。但在这里使用苯甲醚作为案例底物，揭示了预结晶的1中心点THF根本不能使苯甲醚去质子化，无论是在己烷中还是在THF溶液中，这与1的早期原位应用相矛盾，1的早期原位应用揭示了苯甲醚的接近定量的金属化。在THF溶液中原位制备的1的NMR光谱研究将这种反应性从1中心点THF归因于涉及LiTMP中心点THF、Al（Bu-i）（3）中心点THF、[{Li（THF）（4）}（+）{Al（TMP）（Bu-i）（3）}（-）] 1中心点（THF）（4）、[（THF）中心点Li（mu-TMP）（μ-OC 4 H7）-Al（Bu-i）（2）]，4和（TMP）Al（Bu-i）（2）中心点THF。与此相反，发现试剂2在LiTMP中心点THF和（TMP）Al（Bu-i）（2）中心点THF溶液中仅以两种分离的均相物种存在。还揭示了1和2在非极性己烷溶液中的组成。借助DFT计算，讨论集中在以下事实上，即1或2的THF溶液中存在的铝酸盐物质都不能使苯甲醚去质子化/金属化，相反，金属化过程似乎是LiTMP锂化，随后立即被烷基铝络合物捕获，在金属交换中将反应驱动到产物（芳基铝化）侧。
The lithium TMP-aluminate bases "LiTMP center dot Al(Bu-i)(3)" 1 and "LiTMP center dot Al(TMP)(Bu-i)(2)" 2, where TMP is 2,2,6,6-tetramethylpiperidide, have recently come under the spotlight as "aluminating" reagents in that they can perform aluminium-hydrogen exchange on a wide variety of aromatic substrates. Previous studies have intimated that 1 existed as a single species in THF solution formulated as [(THF)center dot Li(mu-TMP)(mu-Bu-i)Al(Bu-i)(2)]1 center dot THF, having a contacted ion pair structure as evidenced by an X-ray crystallographic study of isolated crystals. But here using anisole as a case substrate it is revealed that pre-crystallised 1 center dot THF cannot deprotonate anisole at all whether in hexane or THF solution contradicting earlier in situ applications of 1 which revealed near quantitative metallation of anisole. NMR spectroscopic studies of 1 made in situ in THF solution ascribe this reactivity distinction from 1 center dot THF to complex equilibria involving five major species in LiTMP center dot THF, Al(Bu-i)(3)center dot THF, [{Li(THF)(4)}(+){Al(TMP)(Bu-i)(3)}(-)] 1 center dot(THF)(4), [(THF)center dot Li(mu-TMP)(mu-OC4H7)-Al(Bu-i)(2)], 4, and (TMP)Al(Bu-i)(2)center dot THF. Reagent 2 in contrast is found to exist as only two separated homometallic species in LiTMP center dot THF and (TMP)Al(Bu-i)(2)center dot THF in THF solution. The constitutions of 1 and 2 in non-polar hexane solution are also revealed. With the aid of DFT calculations, discussion focuses on the fact that none of the aluminate species present in THF solutions of 1 or 2 can deprotonate/metallate anisole, instead the metallation processes appear to be LiTMP lithiations followed immediately by trapping by an alkylaluminium complex, in a metal exchange which drives the reaction to the product (arylaluminated) side.