Reaction Initiated by C-H Bond Formation

Enantioselective 1,3-dipolar cycloaddition of imino esters to electron-deficient alkenes is one of the most powerful and atom-economical C-C bond-forming reaction that facilitates the synthesis of a range of structurally and stereochemically rich pyrrolidines [35]. Wang developed the asymmetric 1,3-dipolar cycloaddition of naphthalene-1,4-dione (116) with imino esters 117 catalyzed by the Cu(I) complex of ferrocenyl ligand 115, followed by silica-gel-promoted aromatization [39]. 

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In aromatic combustion flames, cyclopentadienyl radicals (c-CgHj ) can be precursors for PAH formation. " At high temperatures, benzene is oxidized by reaction with an oxygen molecule to yield phenylperoxy (C6H5O2 ) radical, via the initial formation of the phenyl radical (by C-H bond cleavage) and then the rapid addition of O2 (reaction 6.16). After expulsion of CO from phenylperoxy radical, a resonance-stabilized cyclopentadienyl radical (c-CgHg ) is formed (reaction 6.16). 

Complexes 98 [L = PPh3, P(Ph-p-F)3, P(Ph-p-Me)3] react with methyl-lithium to give, after methanolysis, the orthometallated complexes 99 (Scheme 5). Complex 98 (L = PPh3) also leads to 99 by reaction with phenyllithium or Red-Al 54). The formation of 99 suggests that the initial reduction of 98 leads to a 16-electron ruthenium (0) intermediate followed by C—H bond activation as for the transformations of 90 and 91. Treatment of complex 98 (L = P-i-Pr3) with methyllithium produces the cyclo-metallated diastereoisomers 100. Complexes 101 and 102 are obtained by treatment of 98 (L = PPh2-f-Bu) with methyllithium at -78°C and at +70°C, respectively. Complex 101 isomerizes to 102 by a first-order process (k 0.2 hour-1 in C6D6 at 50°C) when L is PPh2-i-Pr 98 leads to 103 which isomerizes to the orthometallated complex 104 54). 

The formation of tri- and especially tetrasilanes which are already branched (tertiary Si-units) as the first reaction products (described elsewhere [4]) suggests the appearance of intermediate silylene species which could enter in insertion reactions of Si-Si as well as Si-Cl bonds. The tri- and tetrasilanes undergo thermal crosslinking reactions at reaction temperatures of 165-250 °C. In addition dehydrochlorination reactions initiated by acid H-abstraction of methyl groups cause the formation of carbosilane (methylene) units in the polymer framework. Table 1 shows the gross compositions of poly(methylchlorosilanes) which are determined by the reaction temperature. 

Brook et al. 5X1 observed such reactions during the formation of siienes by photolysis. Using radiation with A > 360 nm, they photolyzed acylsi-lanes such as 127, which bears a mesityl group attached to the carbonyl carbon. On prolonged photolysis of the initially formed silene 128, the C—H bond of the ortho methyl group of the mesityl group added to the silicon-carbon double bond to form the benzocyclobutane 129. Alternatively a 1,5-H shift would lead to the species 130, which would also yield the benzocyclobutane on electrocyclic rearrangement. 

The Harmata group s initial report concerned a one-pot, one-operation procedure <99AG(E)2419> for the synthesis of enantiomerically pure 2,1-benzothiazines via the Buchwald-Hartwig reaction reported by Bolm <98TL5731 OOJOC169> for sulfoximine N-arylation. For example, treatment of ortho-bromobenzaldehyde 78 with enantiomerically pure N-H sulfoximine 77a in the presence of a palladium catalyst and base afforded the benzothiazine 79 in 78% yield (Scheme 22). Both C-N bond formation and condensation occurred during the reaction, a phenomenon that appears general for aldehydes like 78. 

Free radical formation in oxidized organic compounds occurs through a few reactions of oxygen bimolecular and trimolecular reactions with the weakest C—H bond and double bond (see Chapter 4). The study of free radical generation in polymers (PE, PP) proved that free radicals are produced by the reaction with dioxigen. The rate of initiation was found to be proportional to the partial pressure of oxygen [6,97]. This rate in a polymer solution is proportional to the product [PH] x [02]. The values of the apparent rate constants (/ti0) of free radical formation by the reaction of dioxygen (v 0 = k 0[PH][O2]) are collected in Table 13.8. 

The fact that only trans-1,2- and cis- 1,4-glycols are obtained implies that they cannot actually be formed by the simplified mechanism in Scheme 9. The carbenium ions 99-101 should give a mixture of cis and trans glycols. However, the reaction can be neither entirely concerted, as shown for a 1,5-hydride shift in Equation 6.46, nor involve initial formation of a carbonium ion, as shown in Equation 6.47 The kHlkD isotope effects are too small for C—H bond breaking... 

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