Department of Chemistry and ‡Department of Chemical Engineering, University of Florida , Gainesville, Florida 32611, United States.
J Am Chem Soc. 2015 Feb 11;137(5):1938-46. doi: 10.1021/ja511476n. Epub 2015 Jan 28.
Pairwise and random addition processes are ordinarily indistinguishable in hydrogenation reactions. The distinction becomes important only when the fate of spin correlation matters, such as in parahydrogen-induced polarization (PHIP). Supported metal catalysts were not expected to yield PHIP signals given the rapid diffusion of H atoms on the catalyst surface and in view of the sequential stepwise nature of the H atom addition in the Horiuti-Polanyi mechanism. Thus, it seems surprising that supported metal hydrogenation catalysts can yield detectable PHIP NMR signals. Even more remarkably, supported Pt and Ir nanoparticles are shown herein to catalyze pairwise replacement on propene and 3,3,3-trifluoropropene. By simply flowing a mixture of parahydrogen and alkene over the catalyst, the scalar symmetrization order of the former is incorporated into the latter without a change in molecular structure, producing intense PHIP NMR signals on the alkene. An important indicator of the mechanism of the pairwise replacement is its stereoselectivity, which is revealed with the aid of density matrix spectral simulations. PHIP by pairwise replacement has the potential to significantly diversify the substrates that can be hyperpolarized by PHIP for biomedical utilization.
在加氢反应中，通常无法区分成对加成和随机加成过程。只有在自旋相关性的命运很重要时，这种区别才变得重要，例如在 Para 氢诱导极化（PHIP）中。鉴于 H 原子在催化剂表面上的快速扩散以及 Horiuti-Polanyi 机理中 H 原子加成的顺序逐步性质，因此，负载金属催化剂不太可能产生 PHIP 信号。更令人惊讶的是，负载金属的氢化催化剂可以产生可检测到的 PHIP NMR 信号。更值得注意的是，本文表明负载的 Pt 和 Ir 纳米颗粒可以催化丙烯和 3,3,3-三氟丙烯的成对取代。只需将 Para 氢气和烯烃的混合物在催化剂上流过，前一种物质的标量对称化阶数就会被纳入后一种物质中，而分子结构不会发生变化，从而在烯烃上产生强烈的 PHIP NMR 信号。成对取代的一个重要机制指标是其立体选择性，借助密度矩阵光谱模拟可以揭示这一点。成对取代的 PHIP 有可能大大增加可通过 PHIP 进行生物医学利用的极化底物的多样性。
