An alkylidyne analogue of tebbe's reagent: Trapping reactions of a titanium neopentylidyne by incomplete and complete 1,2-additions
An alkylidyne analogue of tebbe's reagent: Trapping reactions of a titanium neopentylidyne by incomplete and complete 1,2-additions
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DOI：
10.1002/anie.200703079
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Mindiola, Daniel J.
中科院分区：
文献类型：
--
作者：
Bailey, Brad C.;Tout, Alison R.;Mindiola, Daniel J.
含羰基官能团的亚甲基化已经推动了用于有机化学的有机钛试剂的研究。[1，2] Tebbe试剂，[Cp 2 Ti {CH 2AlCl-（CH 3）2}]（Cp= C5 H5），[3]通过将两当量的Al（CH 3）3加入到[Cp 2 TiCl 2]中制备的刘易斯酸稳定的亚甲基络合物[Eq. (1)]是最早报道的以催化方式进行烯烃复分解的早期过渡金属体系之一。[4]因此，这并不奇怪，这种复合物可以经常被用作维蒂希样试剂羰基亚甲基化反应，因为它是比原型磷酸-维蒂希反应物更具反应性，并特别适用于空间位阻羰基存在于醛，酯，内酯，和酰胺。[5]有人认为，Tebbe试剂中的AlMe 2Cl部分使亚烷基α碳的亲核性质变得更好，从而使该试剂的碱性远低于Wittig体系。因此，涉及Tebbe络合物和手性底物的亚甲基基团转移反应通常不会导致产物的差向异构化。[5]虽然Tebbe试剂在300年前就被首次制备出来，但这种种子物种的次烷基类似物的例子仍然很难找到，大概是因为Ti C多键α-碳原子上的初始负电荷。本文中，我们报道了由[（PNP）Ti= CHtBu（CH 2 tBu）]（1）[6]产生的[（PNP）Ti CtBu]（A; PNP=[2-{P（CHMe 2）2}-4-甲基苯基] 2NH 3）中的次烷基部分可以方便地用Al（CH 3）3稳定，以提供刘易斯酸稳定的第4族次烷基[（PNP）Ti {C-（tBu）Al（CH 3）3}]（2）的第一个实例。我们的研究表明，Al（CH 3）3可能在制备化合物2的过程中加速α-氢的提取。我们还表明，配合物2是非常稳定的，但在吡啶的存在下，作为一个亚烷基类似物的Tebbe的试剂，从而干净地开环的N-杂环的强CCN-N键。在我们的研究过程中，我们发现配合物1可以切断刘易斯酸B（OCH 3）3的B1 O键，得到一种不寻常的钛配合物，其含有被亲电基序B（OCH 3）2取代的亚烷基配体。对两性离子物种[（PNP）Ti {CtBuAl（CH 3）3}]以及BH 2 O键断裂产物的固态结构进行了讨论，发现不能通过添加刘易斯碱如PR 3来捕获A中的次烷基部分（R= CH 3，Ph）、THF、HMPA（六甲基磷酰胺）、OP（CH 3）3和吡啶[7，8]促使我们研究Ti C键中的亲核性质是否可以被刘易斯酸（如Al（CH 3）3）螯合。Tebbe及其同事报道了涉及[Cp 2 Ti（CH 3）2]和Al（CH 3）3的类似反应，得到[Cp 2 Ti {CH 2Al（CH 3）3}]，其通过光谱表征。[3]然而，这种两性霉素从未以纯的形式产生，并且被残留的二甲基前体污染。[3]我们在低温下用纯的或化学计量的Al（CH 3）3处理化合物1 [6]，这导致在CH 3 tBu挤出的同时立即形成两性离子络合物[（PNP）Ti {C（tBu）Al（CH 3）3}]（2）。[9]配合物2已通过1H、13 C、31 P和27 Al NMR光谱、元素分析以及单晶X射线衍射研究进行了表征。[9]而31 P NMR谱清楚地表明2在溶液中具有C1对称性（两个双峰，JPP= 36 Hz），1H NMR谱显示2中Al-（CH 3）3部分的三个不等价的甲基共振，与两个末端Al（CH 3）2甲基一致。
Methylenation of carbonyl-containing functionalities has driven the research of organotitanium reagents for use in organic chemistry.[1, 2] Tebbe s reagent,[Cp2Ti {CH2AlCl-(CH3) 2}](Cp= C5H5),[3] a Lewis acid stabilized methylidene complex prepared from addition of two equivalents of Al (CH3) 3 to [Cp2TiCl2][Eq.(1)], was one of the first reported early-transition-metal systems to perform olefin metathesis in a catalytic manner.[4] Hence, it is not surprising that this complex can be often utilized as a Wittig-like reagent for carbonyl methylenation reactions, since it is more reactive than prototypical phospha-Wittig reactants, and works particularly well for sterically encumbered carbonyl groups present in aldehydes, esters, lactones, and amides.[5] It has been argued that the AlMe2Cl moiety in Tebbe s reagent alleviates the nucleophilic nature of the alkylidene α carbon, thus rendering this reagent far less basic than a Wittig system. Consequently, methylidene-group transfer reactions involving Tebbe s complex and a chiral substrate often do not result in epimerization of the product.[5] Although Tebbe s reagent was first prepared almost 30years ago, examples of an alkylidyne analogue for this seminal species have remained elusive, presumably because of the incipient negative charge confined at the Ti C multiply bonded α-carbon atom. Herein we report that an alkylidyne moiety in [(PNP) Ti CtBu](A; PNP=[2-{P (CHMe2) 2}-4-methylphenyl] 2NÀ), generated from [(PNP) Ti= CHtBu (CH2tBu)](1),[6] can be conveniently stabilized with Al (CH3) 3 to afford the first example of a Lewis acid stabilized Group 4 alkylidyne [(PNP) Ti {C-(tBu) Al (CH3) 3}](2). Our studies suggest that Al (CH3) 3 likely accelerates α-hydrogen abstraction in the process of making compound 2. We also demonstrate that complex 2 is remarkably stable, but in the presence of pyridine behaves as an alkylidyne analogue of Tebbe s reagent, thus cleanly ring opening the strong CÀN bond of the N-heterocycle. During the course of our studies we discovered that complex 1 can cleave the BÀO bond of the Lewis acid B (OCH3) 3 to afford an unusual titanium complex containing an alkylidene ligand substituted with an electrophilic motif ÀB (OCH3) 2. The solid state structures for the zwitterionic species [(PNP) Ti {CtBuAl (CH3) 3}] as well as the product resulting from BÀO bond cleavage across the Ti C moiety in A are also presented and discussed.Our inability to trap the alkylidyne moiety in A by addition of Lewis bases, such as PR3 (R= CH3, Ph), THF, HMPA (hexamethyl phosphoramide), OP (CH3) 3, and pyridines,[7, 8] prompted us to investigate whether the nucleophilic nature in the Ti C linkage could be sequestered by a Lewis acid, such as Al (CH3) 3. Tebbe and co-workers reported an analogous reaction involving [Cp2Ti (CH3) 2] and Al (CH3) 3 to afford [Cp2Ti {CH2Al (CH3) 3}], which was characterized spectroscopically.[3] However, this zwitterion was never generated in pure form and was contaminated by residual dimethyl precursor.[3] We treated compound 1 [6] with neat or stoichiometric Al (CH3) 3, at low temperatures, which resulted in immediate formation of the zwitterionic complex [(PNP) Ti {C (tBu) Al (CH3) 3}](2) concurrent with CH3tBu extrusion.[9] Complex 2 has been characterized by 1H, 13C, 31P, and 27Al NMR spectroscopy, elemental analysis, as well as by single crystal X-ray diffraction studies.[9] Whereas the 31P NMR spectrum clearly suggests 2 to have C1 symmetry in solution (two doublets, JPP= 36 Hz), the 1H NMR spectrum displays three inequivalent methyl resonances for the Al-(CH3) 3 moiety in 2, consistent with the two terminal Al (CH3) 2 methyl groups being …