Deciphering the Mechanism of the Nickel-Catalyzed Hydroalkoxylation Reaction: A Combined Experimental and Computational Study

The
[Ni­(0)­(cod)2]/P∩P-catalyzed hydroalkoxylation
of butadiene to form butenyl ethers is studied mechanistically, where
P∩P = 1,4-bis­(diphenylphosphino)­butane (dppb) and
1,2-bis­(diphenylphosphinomethyl)­benzene (dppmb). Experimental studies
suggest the intermediacy of [(P∩P)­Ni­(0)­(butadiene)]
and [(P∩P)­Ni­(II)­(allyl)] intermediates and rule
out the involvement of Ni–H species. The related species [(dppb)­Ni(0)­(1,4-diphenylbutadiene)], 1, and [(P∩P)­Ni­(II)­(crotyl)­(Cl)] complexes 2 (P∩P = dppmb) and 3 (P∩P = dppb) have been synthesized and characterized on
the basis of VT NMR spectroscopy and X-ray crystallographic studies.
Compounds 2 and 3 are shown to be catalytically
competent for the hydroalkoxylation reaction. Computational studies
on [(dppmb)­Ni(0)­(butadiene)] indicate a facile protonation that forms
a cationic allylic intermediate [(dppmb)­Ni­(II)­(η-C4H7)]­OMe. C–O bond formation then occurs via external
attack by the solvent-stabilized methoxide nucleophile. Hydroalkoxylation
proceeds with modest computed barriers of ca. 18 kcal/mol, and the
butenyl ether product formation is only marginally exergonic. Overall,
the results are consistent with initial kinetic control leading to
the major branched isomer followed by a reversible isomerization process
operating under thermodynamic control.

本研究对[双(1,5-环辛二烯)合镍(0)]（Ni(0)(cod)₂）/双膦配体（P∩P）催化丁二烯氢烷氧基化制备烯丁基醚的反应机制开展了系统性探究，其中P∩P指代1,4-双(二苯基膦基)丁烷(dppb)与1,2-双(二苯基膦甲基)苯(dppmb)。实验研究证实，[(P∩P)Ni(0)(丁二烯)]与[(P∩P)Ni(II)(烯丙基)]两类中间体参与了该催化循环，并排除了Ni-H物种参与反应的可能性。研究者已成功合成相关配合物[(dppb)Ni(0)(1,4-二苯基丁二烯)]（化合物1）以及[(P∩P)Ni(II)(巴豆基)(Cl)]（当P∩P为dppmb时为化合物2，为dppb时为化合物3），并通过变温核磁共振(VT NMR)光谱学与X射线晶体学手段完成了结构表征。实验结果证实，化合物2与3具备催化该氢烷氧基化反应的能力。针对[(dppmb)Ni(0)(丁二烯)]的计算化学研究表明，该中间体可经历易于发生的质子化过程，生成阳离子型烯丙基中间体[(dppmb)Ni(II)(η-C₄H₇)]·OMe⁻；随后，经溶剂稳定的甲氧基亲核试剂的外部进攻完成C-O键的构建。该氢烷氧基化反应的计算能垒适中，约为18千卡/摩尔(kcal/mol)，且烯丁基醚产物的生成仅为微弱放能过程。综合来看，实验与计算结果均符合如下反应路径：反应初始阶段由动力学控制生成主要的支链异构体，随后在热力学控制下发生可逆异构化过程。