Tunneling Carbenes

Considerable experimental and theoretical evidence indicates that ring expansions of singlet cyclopropylhalocarbenes (65) have minimal contributions from tunneling. Carbene 65 has been shown to be indefinitely stable in low temperature matrices. ... 

The chemistry of carbenes is inextricably linked with issues of spin state. This is equally true for tunneling as for classical reactions. The factors that determine the... 

As expected, introducing a CD3 group in 51 impeded the reaction considerably. The deuterated carbene was found to be stable up to 59 K in Xe. It was not possible to find conditions where the trideutero carbene decayed. Hence, k ilk or Ah/Aq ratios, which might help support a tunneling mechanism, could not be determined. 

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). 

Tunneling in a Cyciopropene to Triplet Carbene Rearrangement In previous sections, tunneling in the reactions of a number of carbenes to give covalently saturated products has been described. We conclude this section with a unique example where tunneling appears to dominate the converse of this process, cleavage of a cyciopropene to a triplet carbene. 

The second of our principal concerns is the contribution of quantum mechanical tunneling (QMT) to singlet carbene 1,2-H shifts and related reactions. There is strong evidence that QMT is important in the low temperature matrix reactions of (e.g.) t-buty 1 chlorocarbene (18)58 and benzylchlorocarbene (10a).59... 

The photolyses of diazirines 9a and 9b were similarly studied in Ar matrices at 10-34.5 K 59 Eq. 10. Benzylchlorocarbene (10a) and its ct,a-d2 analogue (10b) were observed by UV or IR (10b) spectroscopy, and their decay to styrenes 11 and 12 could be monitored. Tunneling in these 1,2-H(D) shifts was indicated by (a) much higher rates of carbene decay at 10 K than could be anticipated from extrapolation of the 298 K LFP kinetic data, (b) a kinetic isotope effect (KIE) for the 1,2-H(D) shifts estimated at 2000, and (c) little temperature dependence of the rate at low temperature.59 Accepting that QMT is important in the very low temperature H shifts of carbenes 10 and 18, the obvious question becomes is QMT important at higher or even ambient temperatures ... 

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