Cyclopropylcarbinyl radical opening

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. 

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]. 

This approach has been used, for example, to find whether the intramolecular photocycloaddition reaction of the triplet excited cyclopropyl-substituted 4-(buteny-loxy)acetophenone 220 proceeds via the 1,4-biradical 221 (Scheme 6.87).827 This presumption was confirmed by identifying the three rearrangement cyclization products 222 224. Because the rate constant of the cyclopropylcarbinyl radical opening to the allylcarbinyl radical is known to be 7 x 107 s 1,828 it was suggested that the rate constant for the formation of the (not observed) or// o-photocycloaddition adduct (225) must be less than 3 x 106s This technique comparing the rate constants of two parallel processes, of which one is known is often referred to as a kinetic (or radical) clock 29... 

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. ... 

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. 

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... 

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. 

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. 

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... 

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 dominant contributor to the reactivity of vinylcyclopropanes in any radical reaction is the form (4a), the cyclopropylcarbinyl radical system. The opening of a cyclopropylcarbinyl radical to a butenyl radical is among the fastest radical processes known, with a rate constant of 1.3 x 10 sec". - The various stereoelectronic effects of this rearrangement have been reviewed. The structure of (4a), deduced from its ESR spectrum - and in agreement with calculations (STO-36 basis set), is in the bisected conformation shown, predicted to be 1.4 kcal mol more stable than its perpendicularly oriented counterpart. Above -KX) T only the butenyl radical (4b) can be detected. Substituent efiects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cy-clopropylcaibinyl cation and anion have structures similar to (4a), bisect conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeai Reviews of cyclopropylcarbinyl anions and carbenes are also available. - ... 

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

Table 1. Calculated Barriers (AHt) and Reaction Enthalpies (AH) (kJ mol" ) for the Ring Opening of the Cyclopropylcarbinyl Radical ...

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