Hydrogenation Hydrogen elimination

Another reaction in the last step is the syn elimination ofhydrogen with Pd as H—Pd—X, which takes place with alkyl Pd complexes, and the Pd hydride and an alkene are formed. The insertion of an alkene into Pd hydride and the elimination of, (3-hydrogen are reversible steps. The elimination of, 3-hydrogen generates the alkene, and both the hydrogen and the alkene coordinate to Pd, increasing the coordination number of Pd by one. Therefore, the / -elimination requires coordinative unsaturation on Pd complexes. The, 3-hydrogen eliminated should be syn to Pd. 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

Conversion of 4 to 6 consumes no hydrogen and appears to be a consequence of double-bond migration. In this case, however, the reaction proceeded in two stages, hydrogen addition (5) followed by hydrogen elimination and migration (2S). 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

Aliphatic aldehydes and ketones can be converted to gem-dichlorides by treatment with PCI5. The reaction fails for perhalo ketones. " If the aldehyde or ketone has an a hydrogen, elimination of HCl may follow and a vinylic chloride is a frequent side product ... 

Me3SiMn(CO)j has also been obtained from Mn2(CO)io, and HMn(CO)5 was a postulated intermediate 31). Thus, method (a) comprises a formally oxidative M—M cleavage and a subsequent hydrogen elimination between Si—H and H—M moieties. 

An obvious limitation to the hydrogen-elimination method, especially for early transition metal elements, is the availability of transition metal hydrides (this applies also to the HCl elimination) or binuclear complexes. 

A number of hydrogen elimination reactions [Eq. (51)] from SiM precursors were discussed in Section another example is that from... 

The UMP nucleotide contains uracil, ribose, and one phosphate group. The structure of uracil and the hydrogen eliminated during the condensation appear in Figure 13-22. Here are the component parts, drawn in position to eliminate water molecules and link ... 

Wherever possible, we have sought a direct comparison of the reactivities of structurally related Crni and q-II alkyls with ethylene. For example, after having established the catalytic activity of complexes of the type [( Cri (L)2R] (see above), we showed that the isostructural neutral compounds Cp Crn(L)2R did not polymerize ethylene instead facile P-hydrogen elimination was observed. [3) This difference in reactivity was not due to the charge of the complexes. Thus, we have subsequently shown that neutral Cr J alkyls are also active polymerization catalysts. For example, Cp Cr I(THF)Bz2 and even anionic Li[Cp Cr H(Bz)3] (Bz = benzyl) polymerized ethylene at ambient temperature and pressure, while the structurally related CpCrD(bipy)Bz proved inert.[5]... 

Based on our observation in these two systems, it would appear that Cp Cr -alkyls, if rendered electrophilic and/or sufficiently coordinatively unsaturated, will both bind and insert a-olefins. However, the more heavily substituted alkyl ligands thus formed (i.e. CrBl-CH2-CH(R)-P vs. Crni-CH2-CH2-P resulting from ethylene insertion) seem to be very susceptible to facile 3-hydrogen elimination. Rapid chain transfer and very low molecular weights are the results of this tendency. Whether the latter is an innate property of all chromium alkyls or reflects the particular chemical nature of the Cp Cr-moiety remains to be established. To this end, investigation of chromium alkyls with a variety of other ancillary ligands are needed. 

The alkyls Tp Cr-R are the best test case yet of the catalytic activity of CrU alkyls (see Section 1). However, they did not react with ethylene, even at elevated temperature. On the contrary, Tp - Cr-Et eventually decomposed by an apparent P-hydrogen elimination yielding Tp - Cr-H and ethylene. Thus our notion that divalent chromium alkyls are not the chain propagating species in polymerization catalysis receives further support... 

An alternative hydrogen elimination process has been proposed by Severens et al. [432, 468], They argue that hydrogen can be eliminated by a cross-linking step [469] immediately after a physisorbed SiHj radical has chemisorbed on a... 

As reported in Scheme 1 the process involves a series of steps. The alkylpalladium species 1 forms through oxidative addition of the aromatic iodide to palladium(O) followed by noibomene insertion (4-7). The ready generation of complex 2 (8-11) from 1 is due to the unfavourable stereochemistry preventing P-hydrogen elimination from 1 (12). Complex 2 further reacts with alkyl halides RX to form palladium(IV) complex 3 (13-15). Migration of the R group to the... 

An additional prerequisite in this reaction, however, is inhibition of a premature P-hydrogen elimination. Reaction of 6/4-56 and 6/4-57 led to 6/4-58 with 41 % yield. Again, one can assume that first a Ni-complex 6/4-59 is formed, which gives the bicyclic 6/4-60 followed by formation of the triquinane skeleton 6/4-58 via 6/4-61 with a P-hydride elimination being the last step (Scheme 6/4.15). 

Complexes ligated by alkylphosphines had been used rarely as catalysts in cross-coupling chemistry, but several studies suggested that they could catalyze the amination of aryl halides with higher selectivity and activity than catalysts of arylphosphines. Steric hindrance promotes reductive elimination at the expense of /3-hydrogen elimination.54 Therefore, reactions of primary amines and, in... 

---