Carbon tunnelling, carbene

Carbon Tunneling in Singlet Carbenes As described earlier in... 

Carbon tunneling in a second singlet chlorocarbene has also been proposed. It has proved impossible to observe noradamantylcarbene 73 spectroscopically, either by solution laser flash photolysis or with matrix isolation at low temperatures. It has been suggested that the carbene rearranges too rapidly, possibly via carbon tunneling, to adamantene (74). 

In contrast to the above examples, dihydroxycarbene 63 does not undergo a tunneling rearrangement. 63 was made by the HVFP of oxalic acid 62 (Reaction 5.12), and its structure was confirmed by the comparison of the experimental and computed (CCSD(T)//cc-pVTZ) vibrational frequencies. 63 persists for days at 11 K, as does its dideutrated analog 2-63. The carbene is stabilized by it-donation from each oxygen lone pair into the empty p-orbital on carbon. This donation manifests in the very short C-0 distance of 1.325 A in 63, compared with the C-0... 

Schreiner offers a correlation between the stabilities of the carbenes and their tunneling rates. Both the phenyl and cyclopropyl groups of 56 and 60 act as weak jt-donors into the adjacent empty p-orbital of the carbene carbon. This acts to stabilize the carbene, but a much greater stabilization is afforded when the atom adjacent to the carbene carbon has a lone pair. In both 63 and 64, the second adjacent oxygen atom donates significant Jt-densily to the carbene carbon, leading to a much more stable carbene with short C-O bonds. The adjacent nitrogen in 65 is an even better donor. 

C H bond (HOMO) with the empty orbital (LUMO) of the divalent carbon would necessarily reduce orbital overlap in 27ax and concommitantly suppress the formation of alkene 28. Of course, the conformation of 27 CyD must resemble that of 26 CyD, and also the classical energy barrier to a 1,2-hydride shift cannot be undermined by ultrafast quantum mechanical (QM) tunneling. The latter caveat, however, can probably be neglected at room temperature.122 Indeed, the H/D primary kinetic isotope effect (KIE) for 1,2-H(D) shifts in scaffolded tricyclo[5.3.0.04,8]decan-2-ylidene (38) is quite low (Scheme 7),112 indicating the absence of QM tunneling. Hence, the photolytic Hax/Heq migration preference of 1.2 that was determined for carbene 38 is a dependable benchmark. 

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