Methylallyl radical

This is related to reaction (X) for propene, but for isobutene this process is unlikely because it involves formation of a 2-methylallyl ion and destruction of a tertiary ion in the gas phase this reaction would be highly endothermic [113] because the ionisation potential of the 2-methylallyl radical [114] is appreciably greater than that of the tertiary butyl radical [115], and the difference in the homolytic C—H bond dissociation energies is in the same... 

Free radical addition of HBr to buta-1,2-diene (lb) affords dibromides exo-6b, (E)-6b and (Z)-6b, which consistently originate from Br addition to the central allene carbon atom [37]. The fact that the internal olefins (E)-6b and (Z)-6b dominate among the reaction products points to a thermodynamic control of the termination step (see below). The geometry of the major product (Z)-(6b) has been correlated with that of the preferred structure of intermediate 7b. The latter, in turn, has been deduced from an investigation of the configurational stability of the (Z)-methylallyl radical (Z)-8, which isomerizes with a rate constant of kiso=102s 1 (-130 °C) to the less strained E-stereoisomer (fc)-8 (Scheme 11.4) [38]. 

Scheme 11.4 Products of HBr addition to buta-1,2-diene (lb) in the gas phase (top), preferred site of hydrogen atom trapping of 2-bromobutenyl radical 7b (center) and isomerization of methylallyl radical (Z)-8 (bottom) [13, 37, 38].

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. 

In passing it is worth noting that the concept of resonance in a conjugated radical can be demonstrated in a direct, yet simple, way by this technique. The 1 -methylallyl radical can be prepared by the reaction of sodium with either of the two isomeric chlorobutenes (3a and 3b) to give the corresponding two canonical structures (3c and 3d) of the radical. 

Using the same approach and interpretation, values of — jq-ii.io o.44 jjj3 molecule s and Eub = 161.2 6.4 kJ mol were obtained [45] from studies of isobutene oxidation, as predicted by the similar thermochemistry and inert nature of methylallyl radicals due to electron delocalization. The agreement is good, and moreover the Arrhenius parameters are entirety consistent with Aif= 10 " cm molecule s and Elf = 163 kJ mot , which were obtained from studies of HCHO oxidation under conditions where the chain length was reduced virtually to zero. In the initial stages of reaction, the mechanism in KCl-coated vessels, where HO2 and H2O2 are efficiently destroyed at the vessel surface, is very simple. 

Further consideration of the behaviour of the smaller alkenyl radicals at temperatures near to 1000 K is merited. As indicated earlier, methylallyl radicals undergo C—C homolysis to some extent at 750 K. 

Fig. 6. Reaction rate coefficients for the combination of f-butyl radicals in -hexa-decane solvent V, n-dodecane solvent , M-decane solvent X, n-octane solvent n-heptane solvent and of allyl radicals in propane ( ) and methylallyl radicals in isobutane (O) plotted against the Smoluchowski—Stokes—Einstein rate coefficient, eqn. (30). The broken line is of unit slope. The solid line is a comparison of the steady-state Collins and Kimball rate coefficient [eqn. (26)] with the activation ...

Assignments based on internal consistency with the 1-methylallyl radicals. 

An Important feature of butene pyrolysis Is the existence of free radicals with resonant stabilization energy. The allyl and the 2-butenyl (methylallyl) radicals are stabilized by the fact that they have delocalized electrons. Added stability of these free radicals Is about 10 to 13 kcal/mole (16), which significantly increases the rate of their production and decreases the rate of their consumption. 

Electron impact ionization of both disubstituted cyclopropanes 145 and 146 lead to the elimination of the Br radical. From the CA spectra of the resulting [M—Br]+ ions and those of C4H+ ions generated from other suitable precursors it must be concluded that both cyclopropane derivatives give only the 1-methylallyl cation 147. There is no experimental evidence for the generation of the isomeric 2-methylallyl cation 148. Moreover, it was shown that 147 and 148 do not interconvert under the experimental conditions used (36). 

Of special interest are also the reactions of allyl-substituted element-centered radicals. For instance, AIIBu2Gc and dibutyl(2-methylallyl)germyl radicals undergo disproportionation reactions leading to derivatives of tetra- and divalent germanium56. 

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... 

Trimethyl-6-(l-methylallyl)oxycarbonylmethylpyrazine (115) underwent a Carrol type1716 rearrangement with loss of C02 to give 2,3,5-trimethyl-6-(pent-3-enyl)pyrazine (116) [Ph20, 2,6-di-tert-butyl-4-methylphenol (radical inhibitor), 200°C, 18 h 46%].1384... 

Below 800 K, H abstraction from isobutene results almost uniquely in methylallyl (MA) radicals which undergo homolysis to a small degree to give allene and CH3 radicals, but otherwise are as unreactive as allyl radicals. With their resulting high radical concentration, significant yields (up to 30%) of 2,5-dimethylhexa-l,5-diene (DMHDE) are observed in the initial products of isobutene oxidation. 

Summarising the essentials, two sub-mechanisms appear to operate in the oxidation of the four alkenes just discussed. With butene-1 and butene-2, the mechanism is sharply dependent on HO2/H2O2 chemistry, whereas with the unique cases of and isobutene, [HO2] is relatively low, and the kinetics of allyl and methylallyl reactions through radical-radical processes are of key importance. The recombination products HDE and DMHDE are responsible through homolysis and reaction with O2 for the striking autocatalysis observed (Fig. 1.17). 

As a final comment here, it is pertinent to point out that the problem over lower alkene formation arises solely with CaHg and isobutene which are the only alkenes to give stable electron delocalized radicals, allyl and methylallyl, which do not have a facile reaction with O2 and are consumed (at least in the range 600-1000 K) mainly through radical-radical processes. Together with the mutual reactions of the two species, their reactions with HO2 will be the major processes involved. It is conceivable that C2H4 is formed in one of the reaction channels from the CH2CHCH2 + HO2 reaction. 

Attempts to use other hydroxymethyl monomers such as allyl alcohol, 2-methylallyl alcohol, and 2-chloroallyl alcohol for isomerizational copolymerizations with methyl acrylate gave mixed results due to the poor copolymerization rate constants of these olefins and the ability of acrylic radicals to abstract hydrogen atoms from allyl alcohols. 

Another complexity with allylic substrates is the potentitil tor formation of allylic isomers. Reviews by De Wolfe and Young (93). Figiderc and Franck (94). and Magid (951 are relevant to this question. Scheme 2.2 illustrates the consequences of Sx I and SET in the prototype substituted allylic system, erotyl/o -methylallyl. The new bond I onnation. at either end of the allylic system, could be subject to a memory effect due to restricted reorientation during the lifetime of a radical or ion pair. Alternatively, competition between Ss2 and S, 2 could lead to isomer mixtures (Scheme 2.3). In early work (96, rciictions of crotyl and o -methylallyl chlorides with PhMeBr led to mixtures (9 II I l 75 25) which... 

Chiral oxazolidinones are the most popular auxiharies in the field of asymmetric synthesis. Chen and Sibi [76] employed this auxiliary for the domino asymmetric C-C/C-C single bond formation under convenient radical conditions (Scheme 5.51). During the reaction, the radical intermediates 240 was coordinated onto the Lewis acid Yb(OTf)3 at —78°C to form the desired s-cis conformation. Upon the treatment of this intermediate 240 by 2-methylallyl stannane, the product 241 was generated with a diastereoselectivity of >20 1. Similar radical conditions using a bromooxazolidinone imide source also resulted in allylated products with excellent diastereoselectivity (>50 1) and yield (>90%) even at room temperature. 

Benzylideneacetone)Fe(CO)3 may be reduced stepwise electrochemically to yield a radical anion and dianion, respectively. Reaction of the former with crotyl bromide yields ( -l-methylallyl)2Fe(CO)2. (Benzylideneacetone)Fe-(CO)aPPh3 has been prepared, and acts as a useful source of the Fe(CO)2PPh3 moiety in the preparation of (diene)Fe(CO)2PPh3 complexes. A kinetic study of the reaction between chpt and several (a/S-unsaturated ketone)Fe(CO>3 complexes to yield (chpt)Fe(CO)s reveals a stepwise displacement containing both associative and dissociative pathways. ... 

---