Carbon-hydrogen bonds ester

Like carbene insertions into carbon-hydrogen bonds, metal nitrene insertions occur in both intermolecular and intramolecular reactions.For intermole-cular reactions, a manganese(III) meio-tetrakis(pentafluorophenyl)porphyrm complex gives high product yields and turnovers up to 2600 amidations could be effected directly with amides using PhI(OAc)2 (Eq. 51). The most exciting development in intramolecular C—H reactions thus far has been the oxidative cychzation of sulfamate esters (e.g., Eq. 52), as well as carbamates (to oxazolidin-2-ones), ° and one can expect further developments that are of synthetic... 

Another reaction in which the cleavage of a carbon-hydrogen bond is important is the bromination of ketones. In the bromination of ethyl acetoacetate and 2-carboethoxycyclopentanone, it was shown that multivalent cations are catalysts. In the latter reaction, cupric, nickelous, lanthanum, zinc, plumbous, manganous, cadmium, magnesium, and calcium ions were effective (45). One can interpret the effect of the metal ion in terms of its catalysis of the proton transfer from the ester to a base, whether the reaction is carried out in dilute hydrochloric acid solution (acid-catalyzed bromination) or in acetate buffer (base-catalyzed bromination). 

Benzyl esters, cyclization-hydrosilylation, 11, 386-387 Benzyl ethers, cyclization-hydrosilylation, 11, 386-387 Benzyl groups, C-H bond silylation, 10, 240 Benzylic alcohols, catalytic alkylation, 11, 146 Benzylic carbon-hydrogen bonds borylation, 9, 174... 

Further applications of nitrene cyclizations have also been reported in which the intermediate nitrene inserts into a 7r-deficient heterocycle or a nitrene on a -deficient nucleus inserts into a proximal carbon-hydrogen bond. In general, the preferred source of the nitrene is the photolysis or thermolysis of an azide. The versatility of this approach is shown in two syntheses of pyrrolo[3,2-J]thiazoles (selenazoles) (177), (178), (180) (Equations (52) and (53)). Thermolysis of the / -thiazolyl (selenazolyl)-a-azido-esters (176) or the / -(4-azido-5-thiazolyl)-x,/J-enones (179) affords these bicyclic heterocycles in excellent yields (79JHC1563, 92JCS(Pi)973>. The same strategy has also been... 

As a resirlt of the differences in polarity between the carbon fluorine and the carbon-hydrogen bond, fluorocarbon chemistry is wrought with more differences than similarities to hydrocarbon chemistry despite their similarities in van der Waal s radii (1.20 vs. 1.35 A). A great body of chemistry for the functionalized fluorocarbons has been developed in the areas of perfluoroalkenes, halofluoroalkanes, ethers, epoxides, peroxides, ketones, acids, and esters. 

Shilov chemistry, developed from 1970, employs [Pt(II)CLt] salts to oxidize alkanes RH to ROH or RCl with modest efficiency. Pt(IV) is an efficient (but economically impractical) primary oxidant that makes the process catalytic. This discovery strongly contributed to the continuing activity in CH activation. Periana developed a related and much more efficient system for methane oxidation to methanol using 2,2 -bipyrimidine ligands and sulfuric acid as solvent. In this case, the sulfuric acid is the primary oxidant and the methanol formed is protected by being converted in situ to MeOSOsH, an ester that strongly resists further oxidation. This area is more fully described under the entry Alkane Carbon-Hydrogen Bond Activation. 

Important advances in the field of C-H bond activation have involved the photochemical reactions of boryl complexes such as Cp W(CO)3(Bcat) (98, cat = 1,2-02-3,5-Me2C6H2). Transient species derived from these complexes efficiently activate the C-H bonds of alkanes and arenes (see Alkane Carbon-Hydrogen Bond Activation), and they can convert hydrocarbon solvents into alkylboronate esters (equation 27). Experimental and theoretical studies have shown that these reactions proceed via a boron-assisted, a-bond metathesis see a-Bond Metathesis) pathway involving back donation of electron density from the tungsten atom to a formally unoccupied p orbital centered on the boryl ligand. ... 

The fairly harsh conditions required to break the carbon-hydrogen bond in cyclohexane cause various side reactions, and the yield to the desired end products (based on cyclohexane converted) is only about 60 to 70%, even at low conversion. A higher yield could be obtained with added borate ester or boroxine (see Example 5.5 in Section 5.5), but this would require hydrolysis of the resulting cyclohexyl ester and is not practical in a process that calls for a dry product. 

Advantage can be taken of the enhanced polarity of a carbon-silicon bond over that of a carbon-hydrogen bond in the displacement of 2-tri-methylsilyl groups by reaction with a number of carbonyl reagents to give imidazoles substituted at C-2 by secondary alcohol, acyl, aroyl, ester, and amide functions. ... 

Finally it should be pointed out that carbanion formation is particularly easy when the carbon-hydrogen bond is weakened by two activating groups as it is in malonic ester, cyanoacctic ester, 0-ketoesters, 0-diketones, etc. In these compounds there is a doubled permanent polarization (XVIII), and the carbanion is a resonance hybrid of three forms ... 

The first step in this scheme is a Michael addition of the nucleophile to the j5-carbon of the alkynyliodonium salt to give the ylide 102. Loss of iodobenzene from 102 gives alkylidenecarbene 103, which rearranges to alkyne 104 in the absence of external traps. This mechanism is experimentally supported by the isolation of cyclic by-products 108 besides the major products, the alkynyl esters 107 in the reaction of alkynyliodonium salt 105 with nucleophiles (equation 67). These cyclic enol ethers are the result of the insertion of the intermediate carbene 106 into the tertiary-8-carbon-hydrogen bond. 

The ester linkages presumably form by a chain-transfer reaction with the carbon-hydrogen bonds of the acetate grouping both in the polymer and in the monomer. 

Catalytic Borylation of the Saturated C-H Bond. Hartwig and Chen et al. demonstrated that simple alkanes regiospecifi-cally react at terminal carbon-hydrogen bond with B2pin2, yielding pinacol 1-alkylboronic esters (eq 22). Photochemical borylation can be catalyzed by Cp Re(CO)3 (eq 23). Thermal borylation can be catalyzed by Rh and Ir complexes containing... 

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazoketones and esters. In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into a methine carbon-hydrogen bond is preferred to insertion at a... 

In addition to the carbon-carbon bond formation, the carbon-hydrogen bond-forming reaction under biphasic conditions was also controlled by the cinchonidine-derived PTC 20 and the synthetic utility of this reaction was demonstrated through the formal total synthesis of a natural product, (/ )- O-methyl-6-undecanolide 128 (Scheme 4.30). The enantioselective protonation of a chiral ammonium enolate via nonbiomimetic hydrolysis of the vinylic ester 129 was successfully developed to... 

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