Uranium reaction with benzophenone

However, the soqrce of the uranium hydride in the reaction of benzophenone with U wasn t clear. When the reaction was worked up with D90, no deuterium incorporation in TPA or DPM was seen, nor was any seen when using toluene-dg as a solvent. All of the naphthalene could be recovered, which led us to conclude that TMEDA (presumably coordinated to uranium) was reacting with the low valent uranium generating a uranium hydride which became incorporated into the TPA or DPM. 

This reaction with TMEDA is temperature dependent. When the reaction of U with benzophenone is carried out at 70 °C, TPE is produced exclusively, with no TPA or DPM seen. As the reaction temperature is increased above 70 °C, the amounts of TPA and DPM produced increase. It appears that th hydrogenated products TPA and DPM arise when benzophenone reacts with U containing uranium hydrides. These hydrides may be formed from either substrates containing acidic hydrogens, or by thermal reaction of the low valent uranium species with coordinated TMEDA. 

Active uranium reacts with diphenylmethanol (benzhydrol) to give exclusively TPA. A 60% yield of TPA was observed after 4h in refluxing xylenes (2 1 benzhydrol/uranium). No reaction was seen at room temperature. No TPE was ever observed in this reaction. On the basis of these observations, it appears that the TPA formed in the coupling reaction of benzophenone arises from the initial reduction of benzophenone to a benzhydrol alkoxide followed by coupling of this alkoxide rather than by further reduction of the TPE [83, 86]. When 4,4 -dimethylbenzophenone is reacted with 3, the intermediacy of 4,4 -di-methylbenzhydrol is observed [87]. 

Figure 13.1 Surface metallopinacols on active titanium and uranium. Table 13.2 Temperature effect on reaction of benzophenone with U. ...

Further evidence for surface activation of active uranium by alcohols was seen in the reaction of active uranium with benzophenone in the presence of methanol. When the methanol was added along with the benzophenone to freshly prepared active uranium, TPE production could be observed even at room temperature (3 h, 13%). After 70 h at 50°C, 38% TPE was seen. In the absence of methanol, only negligible amounts of TPE can be formed even at 50°C. 

In addition to aromatic solvents, we have seen reactivity of active uranium prepared in 1-decene and TMEDA. The yields of TPE and TPA resulting from preparation and reaction of active uranium with benzophenone in 1-decene are only slightly lower than those in aromatic solvents, presumably due to solubility considerations. There was little difference in the proportion of TPA to TPE. It appears that decene does not serve as either a hydride source (allylic hydrogens) or a hydride sink (via hydrogenation of the double bond). The preparation and reaction of active uranium with benzophenone in TMEDA gave yields of TPE comparable to other aromatic solvents at that temperature ( 20%). Negligible amounts of TPA were seen, however. The presence of a large excess of a basic solvent could serve to reduce the amount of metal hydrides present. 

For most of the reactions in this study, a stoichiometry of two ketones per uranium has been used (Table 13.1). This is in accord with the postulated stoichiometry of active uranium (U ) producing UO2. When only a twofold excess of benzophenone is used, however, the combined yields of coupled products is 94%. This is in marked contrast to the titanium systems where each titanium is only capable of reacting with 1 equiv of ketone [82]. In the titanium systems, optimal yields of alkene are obtained with excess titanium, whereas we find that excess ketone produces the highest yields of alkene. This difference in M O stoichiometry is suggestive of a different type of metallopinacol intermediate (Figure 13.1). Further evidence which suggests the presence of this type of intermediate is the fact that 3 reduces the a-diketone benzyl to diphenyla-cetylene, whereas active forms of titanium have been shown by two authors not to effect this transformation [40,88]. 

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