Carbenes concerted reactions

A related issue is whether the carbene, when it is involved, is in equilibrium with a ring-closed isomer, an oxirene.233 This aspect of the reaction has been probed using isotopic labeling. If a symmetrical oxirene is formed, the label should be distributed to both the carbonyl and a-carbon. A concerted reaction or a carbene intermediate that did not equilibrate with the oxirene should have label only in the carbonyl carbon. The extent to which the oxirene is formed depends on the stmcture of the diazo compound. For diazoacetaldehyde, photolysis leads to only 8% migration of label, which would correspond to formation of 16% of the product through the oxirene.234... 

Preliminary calculations of reaction paths have proved encouraging. Thus singlet carbene is predicted to insert into CH bonds, and to add to double bonds, by concerted processes involving no activation the critical geometries are as indicated in 32 and 33. The latter is of course that predicted by Skell56) and supported experimentally by ClossS7) it is also in accord with predictions based on considerations of orbital symmetry or Evans principle 31). The total lack of discrimination shown by carbene in reactions of this type also indicates that the activation energies must be zero or close to zero. 

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

Singlet carbenes add stereospecifically to olefins in a concerted reaction mechanism. 

The mechanism of this nonspecific reaction must be different. In fact, a concerted reaction is impossible for triplet carbenes because of the spins of the electrons involved. After the carbene adds to the alkene in a radical reaction, the diradical (triplet) intermediate must wait until one of the spins inverts so that the second C-C bond can be formed with paired electrons. This intermediate also lives long enough for C-C bond rotation and loss of stereochemistry. 

An efficient route to dicyclopropylcarbene (173) has also been reported . In contrast to other singlet carbenes the reaction of 173 with chloroform proceeded by C-H insertion, and this result was rationalized in terms of a non-synchronous concerted process in which the transition state had significant character of the dicyclopropylmethyl cation (equation 39) . 

The second category includes reactions in which the simultaneous formation of two carbon-carbon bonds completes the framework of the bicyclobutane moiety. For the sake of simplicity, carbene insertion reactions as well as some photochemical transformations are considered in this context as being concerted reactions . 

If, as seems possible, four-valence electron cations prefer Lewis acid - Lewis base reactions forming just one bond at a time, opportunity will be provided to answer a very basic question about six-valence electron species like carbenes, silylenes, nitrenes, phosphinidenes, etc. how can their tendency toward concerted formation of two bonds be explained The presence of HOMOs and LUMOs of accessible energies may be insufficient. So the last question I will leave with you is how can we modify the structures of electron-deficient species that are reluctant to participate in concerted reactions forming more than one bond in order to encourage them to do so ... 

The lack of stereospecificity in some non-concerted reactions is discussed in Chapter 38 in relation to carbenes. 

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