Cyclopropylcarbinyl radical ring opening

A significantly more important tool to decipher the reaction mechanism is probe reactions (Fig. 8). Most commonly used are cyclopropylcarbinyl radical ring opening reactions and radical 5-exo cyclizations to intercept coupling reactions with metal centers. Cyclopropylmethyl bromide 20 is reduced by a metal complex and generates cyclopropylcarbinyl radical 20A. Unimolecular ring opening to... 

Tian, F., Bartberger, M. D., Dolbier, W. R., Jr. Density Functional Theory Calculations of the Effect of Fluorine Substitution on the Kinetics of Cyclopropylcarbinyl Radical Ring Openings. J. Org. Chem. 1999, 64, 540-546. 

However, more rapid events, such as 5-exo cyclization and cyclopropylcarbinyl radical ring opening, can lead to radical formylation (Scheme 4-7). Thus, 1,6-dienes, when exposed to tin hydride/CO conditions, give fair yields of slannylfor-mylation products via cyclization (Scheme 4-7) [20J. On the other hand, treatment of vinylcyclopropanes with CO under similar reaction conditions leads to stannylformylation products via a cyclopropylcarbinyl radical opening to a homoallylic radical (Scheme 4-7) [21]. 

There are just a few examples of such radical ring-openings in the literature (See Sect. 6.6.4), and all of them proceed with exclusive distal bond cleavage and with no observed trapping of the precursor cyclopropylcarbinyl radical. 

This is less common than rearrangement of carbocations, but it does occur (though not when R = alkyl or hydrogen see Chapter 18). Perhaps the best-known rearrangement is that of cyclopropylcarbinyl radicals to a butenyl radical. The rate constant for this rapid ring opening has been measured in... 

Cyclopropylcarbinyl radicals (5) are alkyl radicals but they undergo rapid ring opening to give butenyl radicals." The rate constant for this process has been measured by picosecond radical kinetic techniques to be in the range of 10 M s for the parent to lO Af s for substituted derivatives. This process has been observed in bicyclo[4.1,0]heptan-4-ones. ... 

Rate constants for ring openings of substituted cyclopropylcarbinyl radicals show the same types of kinetic effects as seen in radical additions to substituted alkenes with an obvious relationship to the thermodynamics of the reaction (Fig. 4.16). Minor steric effects influence the kinetics, however, and... 

Figure 4.16. First-order rate constants for ring openings of substituted cyclopropylcarbinyl radicals at ambient temperature. [Data from (71, 113, 115-121).]...

TABLE 4. Effect of substituents on the rate of ring opening of cyclopropylcarbinyl radicals and related radical anions... 

Smith, D. M. Nicolaides, A. Golding, B. T. Radom, L. Ring opening of the cyclopropylcarbinyl radical and its N- and O-substituted analogues a theoretical examination of very fast unimolecular reactions./. Am. Chem. Soc. 1998, 120,10223-10233. 

In the hydroxylahon of cyclopropyl probes, the cyclopropylcarbinyl radical clock can either rearrange to ring-opened 3-buten-l-yl radical before being trapped or rebound with the hydroxyl carrier to yield the alcohol product direchy (Scheme... 

The ring opening of a range of cyclopropylcarbinyl radicals has been investigated. 

---