Catalytic tritiations bonds

There is a much wider variety of indirect replacement approaches. In most cases these approaches introduce the labels at structurally predefined positions generated by (formally) oxidative processes, which are then followed in a second step by reductive operations. Examples of such approaches preferentially designed for the introduction of tritium include halogenation followed by tritiodehalogenation, the introduction of carbon-carbon multiple bonds followed by catalytic tritiation, the oxidation of carbon-heteroatom bonds followed by reduction using tritide reagents, etc. These approaches, which do not alter the skeleton of the target in the process, are discussed in Chapter 4, Sections 1-3 and Chapter 10, Sections 10.1.1.2. ... 

Carbon-carbon multiple bonds are most commonly reduced by means of catalytic tritiation methods. Polarized multiple bonds such as those of carbonyl, carboxyl and nitrile groups are less often reduced by catalytic tritiation, since the use of tritide reagents is often more convenient and can provide greater selectivity (see Section 4.3). 

Since ivermectin (= 22,23-dihydroavermectin B ) is obtained by catalytic reduction of avermectin B, the same procedure using tritium gas convenientiy affords tritiated ivermectin (22,23- [JT]-22,23-dihydroavermectin B ). The preparation of a tritiated derivative containing a 22,23-double bond starts with the readily available 5-ketone, which is reduced with [JT]-sodium borohydride stereospecificaHy to a 5- [JT]-derivative (40). Carbon-14 labeled avermectins can be obtained by a biosynthetic process using sodium (l- C)propionate as labeled precursor (48). 

Tritiated GAs of very high specific radioactivity have been prepared by catalytic reduction. The 1,2 double bond of GAq can be selectively reduced, using a partially poisoned palladium catalyst, to give [1,2-3h2]GA] (7 -,(75,76), although some reduction of the 16,17 double bond and the lactone also occurs (76). Introduction of 3h at sites other than carbon atoms 1 and 2 has also been found (76). [3h]GAI has been prepared from GAy by a similar method JT7). [3h]GA was converted... 

When using, however, two-component catalysts alcohols also react with inactive metal-polymer (aluminium-polymer) bonds which are formed in the chain transfer reactions with a cocatalyst. It is expedient to use the alcohol method only for catalytic systems and polymerization conditions for which the number of inactive metal-polymer bonds is low. Such a case is the polymerization of 4-methyl-1-pentene on vanadium trichloride activated with various organoaluminium compounds. For this system the influence of catalyst composition and polymerization conditions on Cp and kp was determined by quenching the polymerization with tritiated alcohol. 

Tritiated oleate was prepared by catalytic reduction of one of the two double bonds of linoleic acid. To 5 mCi of NaBT4 (13.9 pmol) in 0.5 mL diethyl ether and 5 mg of 10% PdA3 was added 156 pL of 100 mg linoleic acid (55.7 pmol) (in 1 ml diethyl ether [7]). The free linoleic acid was added slowly (15 pL per min) formed sufficient tritium gas to stoichiometrically hydrogenate one of the two double bonds. The reaction mixture was taken to dryness under nitrogen, treated with 0.250 mL 10% methanolic HCl, heated to 70° for 1 hour to prepare the methyl ester. The product was separated by preparative TLC on silver impregnated silica gel as above. Three major radioactive peaks were observed, in addition to some material near the origin. The major radioactive peak corresponded with methyl oleate, the second peak with methyl vaccinate and the third peak was unidentified. 

As with tritiations of carbon-carbon multiple bonds, catalytic tritiodehalogenations are favored by polar and protic solvents, such as methanol, ethanol, and water. On the other hand, catalyst-mediated /H exchange between solvent and tritium gas may give rise to a... 

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