Expanding Water/Base Tolerant Frustrated Lewis Pair Chemistry to Alkylamines Enables Broad Scope Reductive Aminations.
Expanding Water/Base Tolerant Frustrated Lewis Pair Chemistry to Alkylamines Enables Broad Scope Reductive Aminations.
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DOI：
10.1002/chem.201605466
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发表时间：
2017-02-10
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影响因子：
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通讯作者：
Ingleson MJ
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文献类型：
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作者：
Fasano V;Ingleson MJ
与B(C6F5)3相比，低路易斯酸度的硼烷对水/强Brnsted碱的组合表现出更大的耐受性，这使得在H2O/烷基胺存在的情况下，通过受挫刘易斯对(FLP)机制进行的Si−氢键活化。具体地说，与H2O-B(C6F5)3相比，BPh3具有更好的耐水性，因为与H2O-B(C6F5)3相比，Brnsted酸性加合物H2O-BPh3不会与脂肪胺发生不可逆的去质子化反应。因此，BPh3是以硅烷为还原剂与烷胺进行醛和酮的还原胺化反应的催化剂。使用B(C6F5)3作为催化剂无法获得的一系列胺，可以通过BPh3催化的还原胺化反应获得，方法简单，不需要提纯BPh3或试剂/溶剂。BPh3与B(C6F5)3具有互补的还原胺化范围，前者不是芳胺还原胺化反应的有效催化剂，后者不是烷胺还原胺化反应的有效催化剂。这种差异是由于水-硼烷加合物的pKa值不同以及BPh3物种对原脱冠反应的更大敏感性所致。通过对使用B(C6F5)3和BPh3作为还原胺化催化剂的失活过程的了解，确定了第三种三芳基硼烷B(3，5-Cl2C6H3)3，它具有更宽的底物范围，能够催化芳基胺和烷基胺与羰基的还原胺化反应。
Lower Lewis acidity boranes demonstrate greater tolerance to combinations of water/strong Brønsted bases than B(C6F5)3, this enables Si−H bond activation by a frustrated Lewis pair (FLP) mechanism to proceed in the presence of H2O/alkylamines. Specifically, BPh3 has improved water tolerance in the presence of alkylamines as the Brønsted acidic adduct H2O–BPh3 does not undergo irreversible deprotonation with aliphatic amines in contrast to H2O–B(C6F5)3. Therefore BPh3 is a catalyst for the reductive amination of aldehydes and ketones with alkylamines using silanes as reductants. A range of amines inaccessible using B(C6F5)3 as catalyst, were accessible by reductive amination catalysed by BPh3 via an operationally simple methodology requiring no purification of BPh3 or reagents/solvent. BPh3 has a complementary reductive amination scope to B(C6F5)3 with the former not an effective catalyst for the reductive amination of arylamines, while the latter is not an effective catalyst for the reductive amination of alkylamines. This disparity is due to the different pK a values of the water–borane adducts and the greater susceptibility of BPh3 species towards protodeboronation. An understanding of the deactivation processes occurring using B(C6F5)3 and BPh3 as reductive amination catalysts led to the identification of a third triarylborane, B(3,5‐Cl2C6H3)3, that has a broader substrate scope being able to catalyse the reductive amination of both aryl and alkyl amines with carbonyls.