Radical cyclopropylcarbinyl

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. ... 

When free radicals are added to 1,5- or 1,6-dienes, the initially formed radical (9) can add intramolecularly to the other bond, leading to a cyclic product (10). When the radical is generated from an precursor that gives vinyl radical 11, however, cyclization leads to 12, which is in equilibrium with cyclopropylcarbinyl radical 13 via a 5-exo-trig reaction. A 6-endo-trig reaction leads to 14, but unless there are perturbing substituent effects, however, cyclopropanation should be the major process. 

Fragmentation of cyclopropylcarbinyl radicals has been incorporated into several synthetic schemes.357 For example, 2-dienyl-l,l-(dimethoxycarbonyl)-cyclopropanes undergo ring expansion to cyclopentenes. 

The general idea of this concept was first outlined by Nugent and Rajan-Babu [17-20] as shown in Scheme 3, and constitutes an analogue of the well-established opening of a cyclopropylcarbinyl radical [21,22]. Titanocenes have emerged as the most powerful reagents in these transformations. However, it is clearly attractive to find other metal complexes in order to develop novel reactivity patterns. 

Scheme 3 Formal analogy between epoxide opening by metal complexes and cyclopropylcarbinyl radical opening...

Photoexcitation of the ketone gives a 1,2-diradical, which undergoes Norrish type I cleavage of the C1-C2 bond to give a 1,5-diradical. The cyclopropylcarbinyl radical opens up to give a 1,3-diradical, which finally loses CO to give the observed diene. Some of these steps may be concerted. 

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).]...

Formation of trapping product 140 in over 54% yield provides strong support for preferential formation of the first bond at the w-carbon, as could be predicted by the rule of five . Higher trapping yield (65%) was obtained in the irradiation of 137 (R = t-Bu) at 50 °C. Formation of the Z-trapped product 141 and the /Z ratio of the trapped products 140/141 (52 13 respectively) could be attributed to an equilibrium between the cyclopropylcarbinyl radical 138 and its corresponding homoallyl radical. However, further studies are required to verify this assumption (Scheme 30). 

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

Reaction of 711 with terminal alkynes occurs regioselectively living 3-thiahexa-l,5-dienyl radicals 715 which also undergo selective 1,5-ring closure to allylcarbinyl radicals 716. Depending on the substitution pattern, radicals 716 rearrange to cyclopropylcarbinyl radicals 717. From 716 and 717,2,3-dihydrothiophene-3-acetate 718 and 3-thiabicyclo[3.1.0]hexane-6-carboxylates 719 are formed, respectively. 

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. 

By incorporating a cyclopropylcarbinyl radical clock, Cheng and Wagner have confirmed the intermediacy of a 1,4-biradical 222, which cyclized very slowly [274] (Scheme 64). 

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. 

The Barton-McCombie reaction of bicyclo[3.1.0]-, [4.1.0]-, and [5.1.0]-imidazoylthio-carbonates (264) induces the formation of cyclopropylcarbinyl radicals, and subsequent (3-cleavage, to generate one-carbon ring-expanded 6-, 7-, and 8-membered compounds (265) in good yields [271]. The driving force of these reactions is the reduction of ring strain (eq. 3.106). 

As a combined reaction of (3-cleavage of a cyclopropylcarbinyl radical and intermolecular addition, treatment of vinylcyclopropane (266) and activated alkyne in the presence of PhSSPh and AIBN forms a cyclopentene skeleton (267), through the initial addition of a thiyl radical to the vinyl group, P-cleavage of the cyclopropylcarbinyl radical, addition of the carbon-centered radical to the alkyne, ring closure of a vinyl radical via 5-exo-trig manner, and finally subsequent P-elimination of the thiyl radical, as shown in eq. 3.107 [272-276]. Here, PhSSPh acts as a catalyst, since the thiyl radical is regenerated. Aliphatic disulfides such as... 

---