Parahydrogen induced polarization

Mobile single-sided NMR

The theory and practice of hyperpolarization in magnetic resonance using parahydrogen

The formal combination of a carbene and an organometallic fragment yields carbene complexes. They are commonly classified as Fischerand Schrock-type complexes. While Schrock-type compounds (first described in the early 1970s) play an important role in olefin metathesis (acknowledged by the Nobel Prize 20051), Fischer carbene complexes, which were first reported in 19642a and 1965,2b have been developed into powerful reagents for organic synthesis.3 This review concentrates on group 6 metal carbene complexes with a special focus on chromium complexes since, because of their balance of reactivity and stability combined with easy accessibility (see below), they are the most prominent members of the Fischer carbenes and have found the broadest application. While most of the recent chemistry of Fischer carbene complexes has been periodically summarized during the last 5-10 years,4 this review, apart from the basic characteristics of Fischer carbene complexes in the introductory part, will focus on the developments since the mid1990s. The literature is covered up to April 2009.

Fischer Carbene Complexes in Organic Synthesis: Metal-Assisted and Metal-Templated Reactions

Recent developments in NMR hyperpolarization have enabled a wide array of new in vivo molecular imaging modalities, ranging from functional imaging of the lungs to metabolic imaging of cancer. This Concept article explores selected advances in methods for the preparation and use of hyperpolarized contrast agents, many of which are already at or near the phase of their clinical validation in patients.

NMR Hyperpolarization Techniques for Biomedicine

Hydrogenation de composes acetyleniques ou ethyleniques en presence de complexes de rhodium carbonyle ou de palladium

Para hydrogen induced polarization in hydrogenation reactions

S = s/2 impurity determined from data in the range 1.7-50 K. In CD3CN solution at room temperature22 an upper bound for the magnetic moment of 1.5 pug agrees well with that in the solid state of 1.59 pB. It is clear from Table I that the antiferromagnetic coupling is independent of the Fe-O separation and the Fe-O-Fe angle-the latter has been noted previously.I6 Stronger coupling is observed for multiply bridged species (e.g., the symmetrical [ ( Bpz3) Fe( CH,C00)2( 0) Fe( Bpz,)] complex7a and unsymmetrical methemer~thrin~~ ), unsymmetrical species (this work and metHrk), and, possibly, heme species with linear FeOFe groups.* The magnetic properties of the isosceles (p3-oxo)triiron(III) cluster are different from those of the more symmetrical clusters.I2 Weaker coupling is observed for ~xyHr.~~ Earlier theoretical workz3 offers few clues for consequences of asymmetry. We intend examining in detail the proton NMR of this complex, which is the first (p-oxo)diiron(III) complex with an imidazolederived ligand, for comparison and amplification of the corresponding data for he~nerythrin.~~ We are attempting to isolate the one-electron reduced, mixed-valence compound that we observe formed in a quasi-reversible manner under cyclic voltammetry20 and to substitute other ligands at the tetrahedral site. Acknowledgment. The partial support of the Research Corporation and of the NIH-BRSG is gratefully acknowledged.

Haptotropic rearrangements in naphthalene-chromium tricarbonyl complexes

electrochemical methods, to furnish 38-52% yields of a blue-green intermediate 6. The optical spectrum [A, 303 (e 15 300), 380 (45 300), 646 (inf; SOOO), 704 nm (9300)] was almost identical with that of the previous intermediate; the greatly simplified proton NMR spectrum showed three methine peaks (6.26, 5.35, 5.00 ppm), two NHs (13.84, 13.22 ppm), nine methyl resonances (2.03-1.77, 1.40 ppm), and an AB quartet [2.98, 2.52 ppm (each d, JAe = 15.3 Hz)] (Figure 3). Insert A in Figure 3 shows the methine protons of the intermediate from the unsymmetrical a,c-biladiene 3 and demonstrates the presence of unequal amounts of two isomeric structures depending upon which of the two terminal methyls in 3 forms the macrocyclic bridging carbon. Irradiation of the methyl singlet in 6 at 1.40 ppm gave a nuclear Overhauser enhancement at the upfield doublet (2.52 ppm) and also at a methyl resonance (1.77 ppm). On the basis of this evidence, we propose structure 6 for the intermediate, with proton NMR assignments as annotated. High resolution FAB mass spectrometryI6 confirmed the expected molecular weight. The mechanism shown in Scheme I is proposed for the decamethyl-apbiladiene 5 electrocyclization; following deprotonationI7 the conjugated tetrapyrrole suffers two-electron oxidation and macrocyclization to give the intermediate 6. Nucleophilic attack,18 presumably by the electrolyte, causes formation of the phlorin 7 which undergoes spontaneous oxidation19 to give porphyrin. Thin-layer spectroelectrochemistry (not shown) indicates that the order of the nucleophilic attack/oxidation steps may be reversed in the electrochemical conversion of 6 into porphyrin.

Rein U. Kirss

Papers

Para hydrogen induced polarization in hydrogenation reactions

Haptotropic rearrangements in naphthalene-chromium tricarbonyl complexes

Para hydrogen induced polarization in hydrogenation reactions catalyzed by ruthenium phosphine complexes

Para-hydrogen-induced polarization in rhodium complex-catalyzed hydrogenation reactions

Haptotropic Rearrangements in Naphthalene-Chromium Tricarbonyl Complexes