PhiP, addition

During the homogeneous hydrogenation of a variety of substrates - and in particular of those containing aryl groups - satellites appear in addition to the expected PHIP-NMR spectra of the usual para-hydrogenated products, typically shifted upfield relative to those of the authentic parent compounds [37]. 

The formation and decay of these product-catalyst-7i-complexes are expected to occur according to the sequence of reactions as outlined in Scheme 12.4. The kinetic constants associated with the occurrence of kHYD and the decay of k0FF> respectively, can both be determined by PHIP-NMR using a process termed dynamic PASADENA (DYPAS) spectroscopy, as has been outlined previously [37]. For this purpose the addition of parahydrogen to the reaction is synchronized with the pulse sequences of the NMR spectrometer, whereby the time for acquiring the NMR spectra is delayed by variable amounts. The results thereof are listed in Table 12.3. A variety of kinetic constants can be determined, and the method is reasonably accurate the margins of error are also indicated in Table 12.3 [37]. 

In addition to the observed polarization transfer, attachment of the hydrogenated product to the catalyst - most likely in the form of a re-complex between the aromatic portion of a product and the cationic catalyst - has also been observed in the 13C-PHIP-NMR spectra. The associated larger shift range of the affected 13C will make it possible to characterize the nature of this attachment as well as the associated binding energies of the hydrogenation product to the catalysts metal center more precisely and effectively. 

In addition, it is important to note that the phase behavior of PHIP NMR signals differs substantially from that in standard NMR experiments. For normal NMR spectroscopy, a 90° pulse renders the maximum possible signal intensity, while a 180° pulse gives no intensity at all. This is not true for PHIP spectra In PHIP NMR, the maximum signal intensity is achieved by a 45° pulse a 90° pulse is here the zero-crossing (like 180° in standard NMR, yielding minimum intensity). 

For all the reasons mentioned above, it is more desirable to obtain PASADENA spectra when applying the PHIP method, at least initially. Nevertheless, in certain cases ALTADENA spectra can be very helpful, too, especially for a quick screening, because these experiments can be performed without any specialized NMR probes. Rather, in these cases, standard NMR equipment will suffice totally. Furthermore, in other cases, the (normally unwanted) isotropic mixing can be used advantageously to obtain information about other additional aspects of the system. A typical such case results when it is desirable to transfer the original proton polar-... 

It is also possible to screen catalysts according to their selectivities (regio- and stereoselectivity), their stabilities, and rates, since kinetic investigations can be carried out as well [23]. In addition, the application of especially designed NMR pulse sequences further extends the possibilities of the PHIP method [19]. For all of these applications, the main advantage of PHIP is the remarkable signal enhancement which is combined with a spectroscopic in situ method of observation. 

Platinum complexes (continued) with aryls, thallium adducts, 3, 399 with bis(alkynyl), NLO properties, 12, 125 with bisalkynyl copper complexes, 2, 182-186 with bis(3,5-dichloro-2,4,6-trifluorophenyl), 8, 483 and C-F bond activation, 1, 743 in C-H bond alkenylations, 10, 225 in C-H bond electrophilic activation studies, 1, 707 with chromium, 5, 312 with copper, 2, 168 cyclometallated, for OLEDs, 12, 145 in diyne carbometallations, 10, 351-352 in ene-yne metathesis, 11, 273 in enyne skeletal reorganization, 11, 289 heteronuclear Pt isocyanides, 8, 431 inside metallodendrimers, 12, 400 kinetic studies, 1, 531 on metallodendrimer surfaces, 12, 391 mononuclear Pt(II) isocyanides, 8, 428 mononuclear Pt(0) isocyanides, 8, 424 overview, 8, 405-444 d -cP oxidative addition, PHIP, 1, 436 polynuclear Pt isocyanides, 8, 431 polynuclear Pt(0) isocyanides, 8, 425 Pt(I) isocyanides, 8, 425 Pt(IV) isocyanides, 8, 430... 

When 20 amino acids were individually heated with glucose and creatinine in an aqueous model system, the presence of IQx, MelQx, and DiMelQx was established in most of the samples, PhIP, Trp-P-1, and Trp-P-2 being present in some in addition. The last two compounds have not received much attention hitherto, as... 

The mechanism of H2 addition to Cp 2Ta(=CH2)H has been studied using parahydrogen-induced polarization (PHIP), C labeling, and comparison to the related complex Cp 2Ta(=CH2)Me. The reaction of />ara-enriched H2 with labeled Cp 2Ta(= CH2)H leads to polarized resonances for both Cp 2TaH3 and Cp 2Ta(CH3)H2, where the polarized resonances correspond to the two hydride hgands. The results thus support a pathway of H2 addition to a Ta(lll) intermediate Cp 2TaCH3, rather than addition directly across the Ta=C bond. ... 

All methodology was developed in order to determine HAA concentrations in cooked foods or to assess human exposure. In addition, the formation of HAA upon cooking of meat was studied [94]. It appears that HAA are especially abundant in well-done grilled chicken meat. Widely different HAA concentrations can be found in different preparations of different meat types, e.g., concentration ranges between 0.045 and 45,500 pg/kg [92]. Typical amounts in cooked meat, as indicated by Skog [94], are 35 pg/kg for PhIP, 10 pg/kg for MelQx and IFP, and 20 pg/kg for AaC. With respect to human exposure, Knize et al. [95] reported that after eating a well-done chicken meal 8-fold variation in the total amount of PhIP metabolites were found in the urine of volunteers, and 20-fold variation in the relative amounts of the various metabohtes. 

The first example (see Pig. 21.9) shows a H-PHIP spectrum ofthe hydration of perdeuterated styrene obtained under ALTADENA conditions, i.e. with hydration outside the NMR magnet taken from Ref [94]. The spectrum of the hydration product ethyl-benzene exhibits the typical strong spin polarized signals at 1 ppm and 3 ppm. Between 5 ppm and 6 ppm there are additional spin polarized signals, which stem from a side reaction. In Ref [94] it was shown that the ratio of hydration versus geminal exchange is controllable by addition of CO. 

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