Mechanism of Phosphine Borane Deprotection with Amines: The Effects of Phosphine, Solvent and Amine on Rate and Efficiency
Mechanism of Phosphine Borane Deprotection with Amines: The Effects of Phosphine, Solvent and Amine on Rate and Efficiency
复制标题
膦硼烷与胺的脱保护机制： 膦、溶剂和胺对速率和效率的影响
DOI：
10.1002/chem.201406585
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发表时间：
2015-03-27
影响因子：
4.3
通讯作者：
Taylor, Nicholas P.
中科院分区：
文献类型：
--
作者：
Lloyd-Jones, Guy C.;Taylor, Nicholas P.
本文测定了硼烷从简单的叔膦硼烷加合物到各种胺的转移动力学。所有获得的数据，包括二阶动力学，缺乏交叉，和三苯基膦硼烷与奎宁环和三乙胺的反应的活化负熵，是一致的直接（S（N）2-类）的转移过程，而不是解离（S（N）1-类）的过程。胺、膦和溶剂的特性都对转移的速率（k）和平衡（K）有很大影响，在某些情况下，其变化幅度可达许多数量级。P-N转移是更有效的环胺在非极性溶剂中，由于降低熵成本和基态不稳定。总的来说，数据允许从速率或合成便利性的角度对脱保护步骤进行明智的优化。在所有情况下，两种反应物都应以高初始浓度存在，以从该过程的双分子性中获得动力学益处。最终，胺的选择由膦硼烷络合物的特性决定。富含芳基的膦硼烷具有足够的反应性以允许使用二乙胺或吡咯烷作为挥发性低极性溶剂和反应物，而更多的富含烷基的膦受益于在非极性溶剂中在较高温度下使用更多反应性胺，例如1，4-二氮杂[2.2.2]双环辛烷（DABCO）。
The kinetics of borane transfer from simple tertiary phosphine borane adducts to a wide range of amines have been determined. All data obtained, including second order kinetics, lack of cross-over, and negative entropies of activation for reaction of triphenylphosphine borane with quinuclidine and triethylamine, are consistent with a direct (S(N)2-like) transfer process, rather than a dissociative (S(N)1-like) process. The identities of the amine, phosphine, and solvent all impact substantially on the rate (k) and equilibrium (K) of the transfer, which in some cases vary by many orders of magnitude. P-to-N transfer is more efficient with cyclic amines in apolar solvents due to reduced entropic costs and ground-state destabilisation. Taken as a whole, the data allow informed optimisation of the deprotection step from the stand-point of rate, or synthetic convenience. In all cases, both reactants should be present at high initial concentration to gain kinetic benefit from the bimolecularity of the process. Ultimately, the choice of amine is dictated by the identity of the phosphine borane complex. Aryl-rich phosphine boranes are sufficiently reactive to allow use of diethylamine or pyrrolidine as a volatile low polarity solvent and reactant, whereas more alkyl-rich phosphines benefit from the use of more reactive amines, such as 1,4-diaza[2.2.2]bicyclooctane (DABCO), in apolar solvents at higher temperatures.