Indirect characterisation of hydrogenation intermediates using PASADENA NMR spectroscopy — evolution of zero‐quantum coherence in AB spin systems

Abstract

An indirect method is presented for the detection and characterisation of intermediates that occur during homogeneously catalysed hydrogenations. In combination with parahydrogen as a reactand, this method takes advantage of NMR spectroscopy and zero‐quantum coherence that becomes modified under the Hamiltonian of an intermediate. The method allows the investigation of highly reactive intermediates that are inaccessible for conventional analytical methods. A theoretical framework is presented based on a density matrix description. It is shown that PASADENA (Parahydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment) NMR spectra of strongly coupled spin systems (i.e., AB systems) contain structural information of intermediates, even if they cannot be detected directly.

A simple and low‐cost device for generating hyperpolarized contrast agents using parahydrogen

“…The process described above is referred to in the literature as ‘parahydrogen‐induced polarization’ or, alternatively, ‘PASADENA’ (‘parahydrogen and synthesis allows dramatically enhanced nuclear alignment’). The underlying effect was first demonstrated by Bowers and Weitekamp in 1986, and has been used as a tool to elucidate reaction intermediates, particularly involving transition‐metal catalysts , for general NMR signal enhancement and for investigational medical imaging . Other devices , operating on the same general principles, have been described, and one such device, geared primarily to studies of hydrogenation chemistry, is offered commercially (the ‘Parahyperpol’, Stelar, s.r.l, Pavia, Italy).…”

“…The process described above is referred to in the literature as 'parahydrogen-induced polarization' or, alternatively, 'PASADENA' ('parahydrogen and synthesis allows dramatically enhanced nuclear alignment'). The underlying effect was first demonstrated by Bowers and Weitekamp (5) in 1986, and has been used as a tool to elucidate reaction intermediates, particularly involving transition-metal catalysts (19)(20)(21), for general NMR signal enhancement and for investigational medical imaging (7,22,23).…”

Probing signal amplification by reversible exchange using an NMR flow system

“…However, polarization transfer to heteronuclei in these types of systems with AB, AX, AA′X, ABX, AMXY and AA′XX′Y nuclear spin topologies has been studied. 20 , 21 The effects of variable magnetic fields on PHIP in multispin systems have also been investigated. 22 Increasingly, in more recent years, applications of PHIP in the MRI field have been investigated and pursued.…”

“…This requirement generally limits the range of organic compounds that can be studied by this approach. However, polarization transfer to heteronuclei in these types of systems with AB, AX, AA′X, ABX, AMXY and AA′XX′Y nuclear spin topologies has been studied . The effects of variable magnetic fields on PHIP in multispin systems have also been investigated .…”

High field parahydrogen induced polarization of succinate and phospholactate

“…Please note that analytical equations of the time-averaged density matrix were derived for AB and AA'X spin systems by Hu ¨bler et al and Natterer et al 54,55 In the latter paper, however, a potential 13 C isotope effect was neglected.…”