Following on the great study of hydroxycarbene1 (see my blog post), Schreiner now reports on the synthesis and characterization of dihydroxycarbene 1.2 It is prepared by high-vacuum flash pyrolysis of oxalic acid (Scheme 1).

Scheme 1

Dihydroxycarbene can exist in three different conformations characterized by the relationship about the C-O bond, either s-cis or s-trans. The three conformations are shown in Figure 1, and the s-trans,s-trans structure is the local energy minimum (computed at CCSD(T)/cc-pVTZ).

1tt (0.0)

1ct (0.1)

1cc (6.7)

Figure 1. CCSD(T)/cc-pVTZ optimized geometries and relative energies (kcal mol-1) of the conformers of 1.2

Identification of the 1 is made through comparison of the experimental and computed IR vibrational frequencies. As an example, the experimental and computed frequencies for the s-trans,s-trans conformer are listed in Table 1. The agreement is excellent.

Table 1. Computed and experimental vibrational frequencies (cm-1) and intensities (in parentheses) of the s-trans,s-trans conformation of 1.2

vibration

computed

experiment

1

3876.4 (23.5)

3633.2 / 3628.6 (w)

2

3871.4 (234.1)

3625.1 (s)

3

1443.1 (124.4)

1386.2 (m)

4

1370.5 (58.3)

1289.0 / 1287.4 (w)

5

1157.8 (470.6)

1110.3 / 1109.3 (vs)

6

1156.6 (1.4)

 

7

742.4 (178.8)

706.6 (s)

8

672.4 (0.0)

 

9

641.6 (11.2)

 

Unlike hydroxycarbene, dihydroxycarbene is stable. The amazing instability of hydroxycarbene is due to tunneling through a large barrier: nearly 30 kcal mol-1.1 The tunneling route for the decomposition of 1 is more difficult for two reasons. First, its C-O bond is quite strong; the C-O distance is quite short, 1.325 Å. This makes a long distance that must be traversed in the tunneling mode. (The strong bond is due to π-donation from the oxygen lone pair into the empty carbon p orbital; this is noted by the large rotational barrier about the C-O bonds of 17 kcal mol-1!) Second, the activation barrier for decomposition is very high, at least 34 kcal mol-1.

References

(1) Schreiner, P. R.; Reisenauer, H. P.; Pickard Iv, F. C.; Simmonett, A. C.; Allen, W. D.; Matyus, E.; Csaszar, A. G., "Capture of hydroxymethylene and its fast disappearance through tunnelling," Nature, 2008, 453, 906-909, DOI: 10.1038/nature07010.

(2) Schreiner, P. R.; Reisenauer, H. P., "Spectroscopic Identification of Dihydroxycarbene13," Angew. Chem. Int. Ed., 2008, 47, 7071-7074, DOI: 10.1002/anie.200802105

InChIs

1: InChI=1/CH2O2/c2-1-3/h2-3H