Cyclopropylmethyl radicals ring-opening

A novel synthesis of a C-3-branched glycal derivative, by application of a cyclopropylmethyl radical ring-opening reaction, is illustrated in Scheme 4. ... 

As discussed previously, radical ring-opening reactions of three-membered systems via cyclopropylmethyl and oxiranylmethyl radicals represent a fruitful method in organic synthesis [111]. De Kimpe and coworkers have now shown that azirid-ines can also be used, featuring a radical one-step synthesis of pyrrolizidines 3-280... 

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... 

Below — 140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above — 140°C, however, the spectmm of a second species was seen, and above — 100°C, this was the only spectmm observed. This second spectmm could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to — 140°C. The conclusion is that the ring opening of the cyclopropyl radical is a very facile process and that the lifetime of the cyclopropyl radical above — 100°C is very short. Even though the equilibrium favors the 3-butenyl radical, the reversible ring closure can be detected by isotopic labeling experiments, which reveal the occurrence of deuterium migration ... 

The reaction of [H2C(SiMe3)2C]2Si 180 with cyclopropylmethyl chloride proceeds via ring opening and formation of product 192 containing a spirocyclic Si atom, whose formation can be attributed to a radical-reaction pathway... 

MISCELLANEOUS REACTIONS OF DIHYDROPYRIDINES Additional tests for net hydride transfers initiated by single-electron transfer include the use of substrates in which such pathways would necessarily involve readily ring-opened cyclopropylmethyl or readily cyclized 5-hexenyl radicals. Products from these radical reactions are not formed in NAD+/ NADH dependent enzymic reductions or oxidations (Maclnnes et al., 1982, 1983 Laurie et al., 1986 Chung and Park, 1982). Such tests have also been applied in non-enzymic reductions. Thus cyclopropane rings in cyclopropyl 2-pyridyl ketones, or imines of formylcyclopropane (van Niel and Pandit, 1983, 1985 Meijer et al., 1984) survive Mg+2 catalysed reduction by BNAH or Hantzsch esters but are opened by treatment with tributylin hydride. 

Rate constants for ring-opening of cyclopropylmethyl radicals... 

Ester, nitrile, or ketone functions are tolerated in the method. Aryl chlorides can be applied with only slightly decreased yields. For 3-bromopropionate the bipyridine ligand was mandatory for the success of the reaction. The involvement of radicals is supported by ring opening occurring when cyclopropylmethyl bromide was used as the substrate (cf. Part 1, Fig. 8). 

Cyclopropropylmethyl radicals.1 Generation of a radical center adjacent to a cyclopropane can result in ring opening of the cyclopropylmethyl system. This reaction can be used to convert allylic cyclohexenols to alkylcyclohexenes, as shown for conversion of 1 into 3 via the phenyl selenide 2. When the cyclopropyl ring is... 

Radical clock rearrangements can be used to provide evidence for radical intermediates these include the ring opening of cyclopropylmethyl radical and the ring closing of the hexenyl radical (equation 21). 

The hydrostannation of carbonyl compounds to give alkoxystannanes (equation 14-5) can follow a homolytic or heterolytic mechanism depending on the structure of the reactants and on the reaction conditions (Section 20.1.3). This existence of alternative mechanisms is elegantly demonstrated in the reduction of cyclopropyl methyl ketone by tributyltin hydride. In methanol, 1-cyclopropylethanol is formed in a polar process, but, with irradiation with UV light, the main product is pentan-2-one, which is formed by ring opening of the cyclopropylmethyl radical by P-scission (Scheme 14-1)14... 

A number of intermediates related to the cyclopropylmethyl radical undergo rapid ring opening. 1-Cyclopropylvinyl radicals 35 rearrange to 2-(allenyl)ethyl radicals 36, while radical anions 37, formed by one-electron reduction of cyclopropyl ketones, ring open to give enolate radicals... 

Photoexcitation of 3-(5-cyclopropylpent-4-enyl)cyclohex-2-enones, e.g. 25, yielded diradicals which took part in hex-5-enyl type radical cyclizations prior to coupling. The cyclopropylmethyl-containing diradicals, formed in this cyclization process, either coupled directly to afford the major products, or first rearranged by jS-scission. The minor products were obtained by intramolecular coupling of the ring-opened diradicals. 

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