Understanding a Hydroformylation Catalyst that Produces Branched Aldehydes from Alkyl Alkenes

This paper reports experimental and computational studies on the mechanism of a rhodium-catalyzed hydroformylation that is selective for branched aldehyde products from unbiased alkene substrates. This highly unusual selectivity relies on a phospholane–phosphite ligand prosaically called BOBPHOS. Kinetic studies using in situ high pressure IR (HPIR) and the reaction progress kinetic analysis methodology suggested two steps in the catalytic cycle were involved as turnover determining. Negative order in CO and positive orders in alkene and H2 were found and the effect of hydrogen and carbon monoxide partial pressures on selectivity were measured. Labeling studies found rhodium hydride addition to the alkene to be largely irreversible. Detailed spectroscopic HPIR and NMR characterization of activated rhodium-hydrido dicarbonyl species were carried out. In the absence of H2, reaction of the rhodium-hydrido dicarbonyl with allylbenzene allowed further detailed spectroscopic characterization of four- and five-coordinate rhodium-acyl species. Under single-turnover conditions, the ratios of branched to linear acyl species were preserved in the final ratios of aldehyde products. Theoretical investigations uncovered unexpected stabilizing CH−π interactions between the ligand and substrate which influenced the high branched selectivity by causing potentially low energy pathways to become unproductive. Energy span and degree of TOF control analysis strongly support experimental observations and mechanistic rationale. A three-dimensional quadrant model was built to represent the structural origins of regio- and enantioselectivity.本论文报道了铑催化氢甲酰化反应（hydroformylation）机理的实验与计算研究，该反应可从无偏烯烃底物选择性生成支链醛产物。这一极不寻常的选择性依赖于一种被通俗称为BOBPHOS的膦杂环烷-亚磷酸酯配体（phospholane–phosphite ligand）。采用原位高压红外（high pressure IR, HPIR）动力学研究与反应进程动力学分析（reaction progress kinetic analysis）方法表明，催化循环中有两步为周转决速步。研究发现反应对一氧化碳呈现负级数，对烯烃与氢气呈现正级数，并测定了氢气与一氧化碳分压对选择性的影响。同位素标记研究表明，氢化铑（rhodium hydride）对烯烃的加成反应基本不可逆。对活化态的二羰基氢化铑物种开展了详细的原位高压红外与核磁共振（nuclear magnetic resonance, NMR）表征。在无氢气存在的条件下，二羰基氢化铑与烯丙基苯（allylbenzene）的反应可进一步实现四配位与五配位酰基铑物种（four- and five-coordinate rhodium-acyl species）的详细光谱表征。在单周转条件（single-turnover conditions）下，支链酰基物种与直链酰基物种的比例与最终醛产物的比例保持一致。理论研究揭示了配体与底物之间存在未被报道过的稳定化CH-π相互作用（CH−π interactions），该相互作用通过使原本低能的反应路径变为非生产性路径，从而影响了高支链选择性。能垒跨度（energy span）与周转频率（turnover frequency, TOF）控制分析强烈支持实验观测结果与机理解释。研究构建了三维象限模型（three-dimensional quadrant model），用以阐释区域选择性与对映选择性（regio- and enantioselectivity）的结构起源。