Obligacion Jennifer V, Zhong Hongyu, Chirik Paul J
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
Isr J Chem. 2017 Nov;57(10-11):1032-1036. doi: 10.1002/ijch.201700072. Epub 2017 Sep 25.
The activation of readily prepared, air-stable cobalt (II) bis(carboxylate) pre-catalysts for the functionalization of C()-H bonds has been systematically studied. With the pyridine bis(phosphine) chelate, PNP, treatment of with either BPin or HBPin generated cobalt boryl products. With the former, reduction to (PNP)CoBPin was observed while with the latter, oxidation to the cobalt(III) dihydride boryl, -(PNP)Co(H)BPin occurred. The catalytically inactive cobalt complex, Co[PinB(OCBu)]2, accompanied formation of the cobalt-boryl products in both cases. These results demonstrate that the pre-catalyst activation from cobalt(II) bis(carboxylates), although effective and utilizes an air-stable precursor, is less efficient than activation of cobalt(I) alkyl or cobalt(III) dihydride boryl complexes, which are quantitatively converted to the catalytically relevant cobalt(I) boryl. Related cobalt(III) dihydride silyl and cobalt(I) silyl complexes were also synthesized from treatment of -(PNP)Co(H)BPin and (PNP)CoPh with HSi(OEt), respectively. No catalytic silylation of arenes was observed with either complex likely due to the kinetic preference for reversible C-H reductive elimination rather than product- forming C-Si bond formation from cobalt(III). Syntheses of the cobalt(II) bis(carboxylate) and cobalt(I) alkyl of PONOP, a pincer where the methylene spacers have been replaced by oxygen atoms, were unsuccessful due to deleterious P-O bond cleavage of the pincer. Despite their structural similarity, the rich catalytic chemistry of PNP was not translated to PONOP due to the inability to access stable cobalt precursors as a result of ligand decomposition via P-O bond cleavage.
已对易于制备、空气稳定的双（羧酸盐）钴（II）预催化剂用于C（）-H键官能化的活化进行了系统研究。使用吡啶双（膦）螯合物PNP，用BPin或HBPin处理会生成钴硼基产物。使用前者时，观察到还原为（PNP）CoBPin，而使用后者时，氧化为钴（III）二氢硼基化物-（PNP）Co（H）BPin。在这两种情况下，催化惰性的钴配合物Co[PinB（OCBu）]2伴随着钴硼基产物的形成。这些结果表明，双（羧酸盐）钴（II）的预催化剂活化虽然有效且使用空气稳定的前体，但效率低于钴（I）烷基或钴（III）二氢硼基配合物的活化，后者可定量转化为催化相关的钴（I）硼基。还分别通过用HSi（OEt）处理-（PNP）Co（H）BPin和（PNP）CoPh合成了相关的钴（III）二氢硅基和钴（I）硅基配合物。两种配合物均未观察到芳烃的催化硅烷化反应，这可能是由于动力学上倾向于可逆的C-H还原消除，而不是由钴（III）形成产物的C-Si键。由于钳形物中有害的P-O键断裂，用氧原子取代亚甲基间隔基的钳形物PONOP的双（羧酸盐）钴（II）和钴（I）烷基合成未成功。尽管它们结构相似，但由于通过P-O键断裂导致配体分解而无法获得稳定的钴前体，PNP丰富的催化化学性质并未转化为PONOP。
