Deuteriation selective

Small ey(Ib) that cobalt and niobium clusters react in a very selective fashion with deuterium was rapidly followed by similar studies on a variety of other metal systems. Three groups simultaneously reported similar dramatic behavior for iron c usters( 2b, 3b). Di hydrogen or di deuteri urn addition reactions have been reported for vanadium( 3e), i ron(lc,2a-d,3b), cobalt(lb),... 

FIGURE 46. Retinal chromophore in bR is attached via a protonated Schiff base to Lys-216 on helix G and is tilted toward the extracellular side. To determine its detailed structure, retinal was selectively deuteriated on the three methyl groups on the cyclohexene ring and incorporated into bR from H. Halobium. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

Methyl cation affinities of benzene and some substituted benzenes have been calculated. These follow a simple additivity rule and the value for benzene shows good agreement with the experimental estimate. Conclusive evidence is presented that these values are linearly related to the corresponding proton affinities. The competition between deuteriation and alkylation in the reaction of radiolytically formed perdeuterio ethyl cations with iV-methylpyrrole and with thiophene has been studied. Deuteriation, the Brpnsted acid pathway, predominates and intramolecular selectivities have been determined for each reaction. ... 

A problem with (—)-sparteine 362 is its lack of availability in both enantiomeric forms. Reversed selectivity in the generation of planar chirality has been achieved by second lithiations (see Schemes 163 and 171) and a remarkable modification of this strategy works with arenechromium tricarbonyls. By using excess BuLi (sometimes f-BuLi is required) in the presence of sparteine 362, a doubly lithiated species 450 may be formed from 448. The formation of the doubly lithiated species may be confirmed by double deuteriation with excess D2O. However, other electrophiles react selectively only once and give products of opposite absolute stereochemistry from those formed after monolithiation, if in rather low yield. Presumably, the first lithiation, which is directed by (—)-sparteine, produce an organolithium 448 whose complexation with (—)-sparteine remains favourable. The second lithiation must produce a less stable organolithium—one which cannot form a... 

Reaction of butyllithium with VBE tetrahydroisoquinolyl formamidine proceeds as shown in Scheme 56. The lack of a kinetic isotope effect in deuteriated substrates revealed that the slow step in the sequence is the complexation of the BuLi with the formamidine, followed by selective removal of the a proton as shown. A rationale for this selectivity is that the butyl group is oriented trans to the isopropyl of the 5-membered... 

The mechanism of this sequence is enlightening when contrasted with the mechanism of the formamidine auxiliary (Scheme 56). Scheme 59a illustrates the results of some deprotonation-alkylation experiments on deuteriated diastereomers. ° ° Two features of the product of these experiments were examined the diastereomer ratio and the percent deuterium incorporation. The deuterium incorporation in the product reveals that there is a preference for removal of the -proton. When deuterium is in the -position, this selectivity is opposed by the isotope effect, and the product has about half the original deuterium remaining. When deuterium is in the a-position, the selectivity for the -proton (imposed by the chiral auxiliary) and the isotope effect act in concert, and virtually all the... 

The reaction of stereomerically well-defined alkenylcopper species 91 obtained by a carbocupration followed by treatment with (KTFjiZn leads to a selective double methylene insertion providing the chelate-stabiUzed alkylzinc reagent 92, which leads after deuteriation with D2O to the unsaturated sulfoxide 93 in 80% yield. This method has been elegantly extended by Marek, Knochel and coworkers (Scheme 32). ... 

A study of the hyperfine splitting of the e.s.r. spectrum of the cation radical of oxidized zinc(n) bacteriochlorin, together with selective deuteriation, has allowed assignment of the electron density to the atoms of the system.279 There is high spin density on the protons of the saturated rings of the bacteriochlorin with or without the zinc. A similar situation may pertain in metal-free bacteriochlorophyll. 

Substituent effects in the more reactive oxazole system appear to be smaller (Table 7.3) which is to be expected by the reactivity-selectivity principle. The A rel value (0.95) [69JCS(B)270] for 2-deuteriation of 1-meth-yl-1,2,4-triazole (7.33) was also determined. Thus, assuming that the activation by the 5-nitrogen upon the 2-position is the same as in thiadiazole,... 

The effect of the high reactivity of indolizine also shows up in substituent effects that are very small, as they should be according to the reactivity-selectivity principle. The exchange-rate coefficients for deuteriation in DzO/dioxan at 50°C are given in Table 8.12 (71T4171), and from these data the methyl substituent effects may be calculated. Notable features are [87JCS(P2)591] summarized below. 

D20, 70 and 30% respectively of the axial [23] and equatorial [24] deuteriated ketone were formed. If it is assumed that protonation of the enolate oxygen atom is faster than that of C(c), this result corresponds to the stereoselectivity of axial and equatorial attachment of D+ to the enol (27). The higher selectivity between axial and equatorial deuteriated ketones observed for hydrolysis of the corresponding ethyl enol ether (90 10) and pyrrolidine enamine (>90 10) means that the stereoselectivity is probably smaller for the enol than for the related compounds. 

GS-MS and H NMR analysis of the selectively deuteriated intermediate product 2-amino-5,2 -dichloro-3,4, 6 -trideuteriobenzophenone, 46, have shown that in the first acid-catalysed key exchange reaction one deuterium has been incorporated into the triply substituted A ring and two deuterium atoms into the doubly substituted aromatic ring B Hydrogens a , b and c have been deuteriated in 99%, 98% and 92%, respectively. 

The H NMR spectra of methyl sulfonium salts of alkyl sulhdes, tetrahydrothiophene and benzothiophene (68) showed large shifts in the presence of [Eu(fod)4] A selectively deuteriated derivative of S-methyltetrahydrothiophenium iodide was used to confirm the assignment of the diastereotopic methylene hydrogen atoms. Studies with the S-methyltetrahydrothiophenium and Af-ethylquinohnium ions indicated that 1 1 complexes formed with [Eu(fod)4] and that the shifts were purely dipolar in origin . ... 

Competing dissociation and isomerization pathways in energy-selected ions of butene, pentene, 1,4-oxathiane, 1,4-dithiane, and dioxane A —> X Emission spectra of jet-cooled penta-1,3-diyne cations and deuteriated analogues. Vibrational frequencies and spin-orbit splittings A — X Emission spectra of jet-cooled CHjC C—C CCHJ and its [ Hj]- and analogues... 

Advantage may be taken of the selective hydroboration of alkynes as compared to alkenes by dicy-clohexylborane or disiamylborane (Section 3.10.4.3) to provide syntheses of dienes and their deuteriated derivatives e.g. equation 62). Dibromoborane-dimethyl sulUde may be used similarly (Section 3.10.4.4)." The approach has been used for the stereospecific synthesis of an allylic hydroxydiene (equation 63). ... 

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