Insertion involving radicals

Molecular oxygen, as distinct from reactions involving radicals or singlet oxygen, is directly inserted into free fatty acids by lipoxygenase (LOX) enzymes. Lipoxygenases, both regio- and stereospecific enzymes, react on the 1,4-pentadienyl moieties such as those of linoleic and a-linolenic acids. 

Photocross I inking of poly(vinyl-p-azidocinnamate) (XIII) is reported to involve competitive photodimerization of cinnamoyl groups to cyclobutenes along with the dominant nitrene reactions of insertion and radical abstraction. The reaction can be triplet-sensitized (19). 

They have compared the reaction energy profiles of five different reaction paths, including oxidative insertion, nucleophilic substitution, and single-electron transfer mechanisms involving radical species, both in the gas phase as well as using an electrostatic continuum model to include the effect of a solvent in an approximate fashion (Structures 3-5). 

Insertion of O2 into element-carbon bonds is not well understood. Most examples of this reaction appear to involve radical intermediates that form metal alkylperoxy compounds [e.g., reactions (a)-(g)]. 

Figure 46. Net insertion of CO, aldehydes, isocyanides, and olefins into Rh—H bonds of [Rh (TPP)(H)] and [Rh (OEP)(H)] complexes involving radical pathways.

Side-chain brominated amino acid derivatives have been exploited in stereo-controlled syntheses of dehydro, cyclopropyl and hydroxy amino acids [39-41], The biochemical hydroxylation of amino acids often involves radical oxidation, where the regioselectivity is analogous to that of bromination. Consequently bromination followed by hydrolysis provides a convenient method for synthesis of the natural products [41], Alternatively, dimethyldioxirane has been used for the direct synthesis of y-hydroxyleucine derivatives through oxygen atom insertion into the yC-H bond [42],... 

In contrast, the insertion of a triplet carbene is a two-step process involving radical formation and combination. 

Carbenes are highly reactive species, and if an olefin or other addition partner is not available, carbenes will indiscriminately insert into C-H bonds. Singlet carbenes insert into C-H bonds in a concerted reaction that always proceeds with retention of configuration at the C-H bond undergoing insertion. Triplets undergo insertions via a stepwise process involving radical intermediates. Reactions of this type are covered in more detail in Chapter 11, where we discuss substitutions at saturated centers. 

Although considerations such as these have led to a less active field of UV/visible spectroscopic study compared with infrared study, this wavelength region offers some unique advantages, particularly for studies oriented toward kinetics, dynamics, thermodynamics, and mechanisms of gas-phase ion chemistry. As a particular example, we can mention the ability to initiate a chemical process with the insertion of a very accurately known (single-photon) increment of several eV of internal energy at a precisely determined time (for a view of some of these possibilities, see, for instance, [132]). Control of photon polarization is possible [133]. Moreover, excited electronic reaction and dissociation pathways, often involving radical ion chemistry, can be accessed in this way, which is not possible with vibrational activation. 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

The main aim of this review is to survey the reactions by which the Co—C bond is made, broken, or modified,.and which may be used for preparative purposes or be involved in catalytic reactions. Sufficient evidence is now available to show that there exists a general pattern of reactions by which the Co—C bond can be made or broken and in which the transition state may correspond to Co(III) and a carbanion (R ), Co(II) and a radical (R-), Co(I) and a carbonium ion (R ), or a cobalt hydride (Co—H) and an olefin. Reactions are also known in which the organo ligand (R) may be reversibly or irreversibly modified (to R ) without cleavage of the Co—C bond, or in which insertion occurs into the Co—C bond (to give Co—X—R). These reactions can be shown schematically as follows ... 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

Studies have been carried out on the methylated complex [H3C-Niin(17)(H20)]2+, which is obtained from the reaction of methyl radicals (generated by pulse radiolysis) with [Ni(17)]2+. The volumes of activation are consistent with the coherent formation of Ni—C and Ni—OH2 bonds, as expected for the generation of a Ni111 complex from a square planar Ni11 precursor.152 The kinetics of reactions of [H3C-Niin(17)(H20)] + involving homolysis, 02 insertion and methyl transfer to Crn(aq) have been determined, and intermediates have been considered relevant as models for biological systems.153 Comparing different alkyl radicals, rate constants for the... 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

Alkylzirconocene derivatives can be converted to alcohols with H202/NaOH, /BuOOH, or m-chloroperbenzoic acid (m-CPBA) [98], These reactions appear to involve migratory insertion processes (Pattern 14) similar to those observed with organoboranes (Scheme 1.20). On the other hand, oxidation with 02 may be a radical process. 

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