Insertion processes substitution

A related situation is found in the case of P-substituted cycloketones here, the electronic difference between the two a-carbons is almost insignificant, resulting in unselective migration upon chemical oxidation. BVMOs have a particularly different behavior, as they can influence the stereo- and/or regioselectivity of the biooxidation. In the latter case, the distribution of proximal and distal lactones is affected by directing the oxygen insertion process either into the bond close or remote to the position of the P-substituent. Consequently, a regioisomeric excess (re) can be defined for this biotransformation, similar to enantiomeric excess or diastereomeric excess values [143]. 

Shortly after this reaction was reported, Hegedus found46 that imines undergo a similar insertion process. The products (251) are structurally very similar to the alkyne adducts (247), and decomplexation with iodine, followed by treatment with trimethylamine TV-oxide, afforded a variety of substituted pyridones in good yield. 

In regard to the stereoselectivity of the insertion process, Murray and coworkers have shown that the CH oxidation of substituted cyclohexanes by dioxiranes is, like the already discussed epoxidation, highly stereo-controUed . A specific case is c -decalin, which gives only the cis alcohol, as exemplarily displayed in equation 27. A similar stereoselective retention of configuration was also obtained for frawi-decalin and cis- and frawi-dimethylcyclohexanes"°. In fact, complete retention of configuration was demonstrated in the CH oxidation of chiral alkanes ". For example, the optically active (f )-2-phenylbutane was converted by either DMD or TFD" to (5 )-2-phenylbutan-2-ol (equation 28) without any loss of the enantiomeric purity (ep) in the product. 

These Cl atoms are in fact free radicals and react very efficiently with hydrocarbons through substitution and insertion processes. 

Interrogation of the stereochemical course of the mechanism was obtained through submission of allylic ethers 208 and 210 (>95% ee) to reaction conditions (Scheme 7.58). The reaction of a 1 1 mixture of allylic ether 208 produced the allylic silane as a 1 1 mixture of diastereomers. Exposure of 210 to substoichio-metric amounts of copper triflate and cyclohexene silacyclopropane produced cfs-substituted allyl silanes 211 and 212 to reveal that C-Si bond formation occurs on the same face as the C-O bond that is cleaved. The loss of enantiopurity, however, indicates that the rate of allylic transposition is competitive with the insertion process. 

Tetracarbonylnickel is an inexpensive (although toxic) source of Ni° and its use for the in situ generation of allylnickel intermediates dates back at least 25 years. In fact, the nickel allylation/methoxycarbo-nylation of alkynes with allyl chloride and an excess of Ni(CO)4 in methanol has b n reviewed (Scheme 15). Accordbgly, dienoates (68) (35-80%) were obtained in a regio- and stereo-selective manner corresponding to a metallo-ene cis addition of an allylnickel compound at the less-substituted termirud C-1 of (67), followed by a CO insertion into the resulting vinylnickel species. The dienyl esters (68) were accompanied by variable amounts of cyclopentenones, indicating the possibility of consecutive CO insertion processes. 

The deuterium isotope effect on the rate of the insertion process was studied using perdeuteropropane, 1,1,1,3,3,3-propane-de and 2,2-pro-pane-d2- No change in the rate of insertion, upon isotopic substitution, was found i.e., kn ll)/ku, (D) — 1.0, within experimental error. This is very significant in view of the fact that the activation energy for sulfur insertion appears to be at least as large as that for methylene insertion, and in the latter case, an H/D isotope effect of ca. 1.5-2.0 has been reported. ... 

Electrophilic substitution of methane is to be considered as an insertion process into a C-H bond.This step, however, could be followed by a very facile bond-to-bond proton migration to the more symmetrical, energetically favored form EH2CH2, which subsequently gives the substituted methane by proton elimination [Eq. (6.46)]. 

Although 5"-hound sulhnato complexes are formally 1,1-insertion products, detailed mechanistic studies with CpFeR(CO)2 and related complexes have suggested that the SO2 insertion takes place via the species M+02SR , most likely a contact ion pair, as shown in Scheme 7.18. Generation of 0-sulfinato species in preference to the 5 -sulhnato complex has been confirmed by NMR and IR spectroscopy [108]. Moreover, the related kinetic results on the insertion into a series of CpFeR(CO)2 in liquid SO2 or in organic solvents are consistent with the electrophilic substitution mechanism [109]. The results of the detailed studies on the insertion processes can be summarized as follows ... 

Various addition reactions of alkenes and alkynes have been termed insertion processes. As the term insertion per se is a nonchemical term, it might be conveniently substituted with hydrometallation (H—M), carbometallation (C—M), heteroatom-metallation or heterometallation (X—M), and metallometallation (M—M), depending on the (7-bond that is added to tt compounds. These addition reactions involving Pd are represented by the general equations using alkenes as representative tt compounds as shown in Scheme 7. The alkenes in Scheme 7 may be replaced with alkynes and other 7T compounds. 

The insertion of cydohexene in Equation 9.45a occurs with an inverse kinetic order in PPhj. These data imply that a phosphine in the 18-electron starting complex is replaced by olefin and that the insertion process involves a hydride and a coordinated olefin. A study of the reaction between substituted styrenes and RhH2Q(PPh3)3 showed little variation in overall rate. However, this similarity in rate resulted from stronger binding of dectron-rich olefins, but slower rates for insertion of these olefins. - ... 

Skell and Owen observed the formation of methyl-substituted trisilanes from the reactions of thermally evaporated silicon atoms with methylsilanes. The most logical mechanism involves a di-insertion process Si atom reacts with- a substrate molecule by insertion into its Si-H bonds to form a methyl-substituted silylene, wich subsequently inserts into the Si-H bond of another substrate molecule to give the final product. 

The oxymercuration of 1-substituted (i,e, H, Me, and C02Me) tricyclo[4,l,0,0 ]-heptanes with mercuric acetate affords norcaranyi- and norpinyl-mercury compounds. A synthesis of 3,4-benzotricyclo[4,l,0,0 ]heptene (692) has been reported in which the usual cyclopropylcarbene C—H insertion process is employed. Isomerization of (692) with silver perchlorate gave benzocycloheptatriene which is also formed in the thermal isomerization of (692). Reaction of (692) with n-ally 1 palladium(ii) chloride dimer yielded 2-methylene-l T-naphthalene which rearranged readily to 2-methylnaphthalene at room temperature a carbenoid mechanism appears to be involved. 

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