Dimethylallyl radicals

The values of pi, ps ps/pi are given in Table 3 where the average value of the spin densities for the dimethylallyl radical (Ic) and tri-methylallyl radical (Id) is taken as representative of the doubly substituted radical. 

The observation can be rationalized by generation of a biradical from the least hindered approach of the acrylonitrile to the allene with rotation about the allene to generate a nearly statistical ratio of the dimethylallyl radicals with closure occurring by least motion faster than rotation about the newly formed a bond (Scheme 6.40). 

Methylenecyclobutanes and Cyclobutenes.— The mechanism of allene dimerization has been further investigated. Dimerization of 1,1-dimethylallene at 150 C yielded the methylenecyclobutanes (86), (87), and (88). The product distribution at low total conversions was as indicated below. Under the same conditions, thermolysis of the tetrahydropyridazine (89) gave the same product distribution, affording strong evidence of a common intermediate, presumably the 2,2 -bis(l,l-dimethylallyl) radical, in the two reactions. 

Dienes As we saw in Sections 5.2-5.3, butadiene and allene react with a variety of hydrides by 1,2 insertion, but butadienes also react with HMn(CO)s to give an apparent 1,4 insertion. Since this 18e hydride has no vacant site and CO dissociation is slow, a different mechanism must be operating this is thought to be H atom transfer to give a 1,1-dimethylallyl radical that is subsequently trapped by the metal (Eq. 7.31). Only substrates that form especially stable radicals can react (e.g., 1,3-diene allyl radical), not simple alkenes. 

Terpenoid Synthesis from Isoprene.—Interest continues in new syntheses of iso-prene and its derivatives the dioxan (37) is obtained108 in good yield by the Prins reaction of methylallyl chloride with formaldehyde (cf. Vol. 5, p. 8) free-radical addition of isopropyl alcohol to vinyl acetate yields compound (38) which gives isoprene by acid-catalysed reaction over alumina.109 (Z)-2-Methylbut-2-en-l-ol and dimethylallyl alcohol are readily available from frans-crotyl alcohol.110... 

In 1972, Lewis and Winstein reported that the reaction of a,a-dimethylallyl phenyl sulfide (1) with thiophenol in the presence of tert-butyl hydroperoxide gave the isomeric compound 7,7-dimethylallyl phenyl sulfide (3) (Scheme 1) [1]. It was proposed that this reaction occurred by addition of thiophenoxy radical to the terminal end of the alkene to produce radical intermediate 2. This radical then underwent y9-scission with loss of the tertiary thiophenoxy group to form the rearranged alkene 3. 

The photodissociative pathway was confirmed by Meyer and Hammond, who foimd that essentially no o- or p-hydroxyacetophenone was formed in the photolysis of phenyl acetate in the gas phase. Instead, all products could be rationalized by recombination of phenoxy and methyl radicals (formed from decarbonylation of acyl radicals). Similarly, a photodissociative mechanism for the photo-Claisen reaction was supported by observation of products expected from the recombination of radicals produced by photodissociation of 3-methyl-l-phenoxybut-2-ene (113, Table 12.6). In addition to phenol, products of the reaction are the rearranged ether 114, the two y,y-dimethylallyl phenols 115 and 116, and the two rearranged allyl phenols... 

Finally, two important classes of compounds have to be mentioned since they have inspired many synthetic chemists. The pyrroloindole alkaloids result from the cyclization of tryptamine, as found in physostigmine (formed by a cationic mechanism after methylation in position 3 of the indole not shown) or in chimonanthine (presumably formed by a radical coupling mechanism Scheme 1.8). The ergot alkaloids are derived from the 3,3-dimethylallylation on position 4 of the indole in tryptophan whose further cyclization and oxidation processes afford the natural products (e.g., lysergic acid, Scheme 1.8, and ergotamine), which have had important medical applications [34]. 

---