Novel Alkali Metal-Induced H−D Atom Exchange Reaction of Diaryl Ketones with THF-d₈:  Unexpectedly High D Atom-Incorporation at Meta Position of Benzophenone

Abstract

This paper describes the hitherto unknown atom-exchange reaction between hydrogens of diaryl ketones and R-deuteriums of THF-d 8 oxygen, which proceeds under ordinary conditions during the preparation of absolute THF.Sodium and potassium benzophenone ketyls (anion radicals) are widely used in many laboratories as an indicator for the preparation of absolute THF because of their characteristic deep blue color which is highly sensitive to moisture and oxygen. 1 In connection with our recent study on the samarium(II) triflate-promoted pinacol-coupling reaction, 2 we attempted to prepare samarium(III) benzophenone ketyl by the transmetalation of sodium benzophenone ketyl with Sm(OTf) 3 in THF-d 8 , 3 in which we observed a novel H-D atom-exchange reaction. Further investigation revealed that the reaction is highly sodium-selective and proceeds through the formation of a benzophenone dianion. 4 Thus, when a deep blue solution of sodium benzophenone ketyl in THFd 8 was heated at 120°C with sodium metal in a sealed tube and then the resulting red-purple solution was treated with oxygen, partially deuterated benzophenone was obtained (Table 1). 5Careful analysis of the crude products by means of capillary GC-MS and FT-IR and 1 H and 13 C NMR revealed that a significant amount of the D atom was incorporated at the ortho, meta, and para positions in this order of magnitude. When the reaction time was short, the D atom was incorporated only at the ortho position (run 1). These results are in sharp contrast to the case of the samarium benzophenone dianion which was selectively protonated at the para position when aromatic alcohols were used as the proton source. 4a The present atom-exchange reaction took place at 95°C (runs 1 and 3) or even at the boiling point of THF. A prolonged reaction increased the D atom-incorporation affording up to a decadeuterated product as one of the isotopic isomers (run 6) (Figure 1).A relatively high D content at the meta position is remarkable because the electron density of the benzophenone dianion is expected to be high at the ortho and para positions. 4 The D atomincorporation appears to reach a maximum in ca. 48 h at 120°C (cf. runs 5 vs 6). When 1 equiv of sodium was used, no isotopic exchange reaction occurred (run 4), which indicates that the atomexchange reaction does not take place via the ketyl intermediate but proceeds through the formation of a dianion followed by the dedeuteration from THF-d 8 . 6,7 Figure 2 shows the 1 H NMR spectrum of the reaction mixture obtained after 48 h of refluxing at 66°C in a sealed NMR tube. The formation of the benzophenone dianion and its deuterated monoanion [Ph(D m )-CD(O -)-Ph(D n )] is apparent. 8 Based on the integral ratio of the signals, the ortho, meta, and para hydrogens of the intermediates can be unambiguously assigned as indicated.Interestingly, the use of potassium or lithium in place of sodium did not afford any deuterated benzophenone. In the former case, the starting benzophenone was recovered (run 7), although the reaction mixture tu...

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“…We are particularly interested in generating an isocyanate anion radical for spectroscopic investigation to explore the similarities, if any, with the structure and the electron-spin distribution found in ketyls (e.g., RNCO •– vs R 2 CO •– ), which are possibly the most important and most investigated class of anion radicals. − EPR spectroscopy has been instrumental in understanding the structure of ketyls as well as any perturbation in electron spin density as a consequence of ion association in solvents of varying polarity. ,,− As with MeNCO •– , aliphatic ketyls have the majority of the electron density residing in the carbonyl moiety resulting in a deviation from planarity for the sp 2 -hybridized carbon . Alternatively, aryl-substituted ketyls have the spin population throughout the extended conjugated π-system resulting in a structure where the ketyl moiety remains planar .…”

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