C=O addition

In order to disfavor aryl C-H addition, and hopefully thereby allow C-O addition to proceed, we investigated anisole derivatives in which the ortho-C-R bonds were either replaced with orf/io-methyl groups or sterically blocked by meta-methyl groups. Unfortunately, reactions with either 2,6-dimethylanisole or 3,5-dimethylanisole resulted in complex mixtures that showed no indication of the desired C-O activation products. 

DFT calculations were employed to gain insight into the mechanism of the 1,2-dehydroaryloxylation. One possible pathway could involve initial direct C-O oxidative addition followed by j8-hydride elimination however, the earlier observations that direct C-O oxidative addition does not occur for methyl aryl ethers along with the observation that 4-ethoxy-2,3,5,6-tetrafluorotoluene reacts faster than 4-methoxy-2,3,5,6-tetrafluorotoluene (i.e., the substrate with the /3-C-H bond reacts faster than the substrate with the a-C-H bond) would argue against such a mechanism. Accordingly, the barriers to direct C-O addition for 4-ethoxy-2,3,5,6-tetrafluorotoluene and ethoxybenzene were calculated to be prohibitively high, 35.0 and 40.8 kcal/mol, respectively. In contrast, the barriers for addition of the fi-C-H bond followed by /3-aryloxy elimination and loss of ethylene (Fig. 4.4) were found to be considerably lower (Fig. 4.5). 

DFT calculations were employed to study the mechanism of methyl ester C-O addition. A low barrier [8.4 kcal/mol relative to free (PCP)Ir and methyl acetate] was calculated for C-H addition to give (PCP)Ir(H)(/c -CH20Ac), the species that was observed to form at room temperature. This intermediate has two potential coordination isomers one where... 

As in the case of the C-O addition reactions, a KIE was measured to probe the reaction pathway. (PCP)Ir was reacted with a fivefold excess of 3,5-bis(trifluoromethyl)benzylfluoride and its deuterated (CD2F) analog at 60 °C (Fig. 4.13). The KIE ( ch2f/ cd2f) was determined to be 2.7, indicating that the C-F addition proceeds via C-H addition. Analogously to the C-O additions, we propose that the reaction proceeds via the initial C-H activation to yield the five-coordinate... 

As discussed earlier, alkyl oxygenates with /3-C-H bonds undergo 1,2-H-O elimination as opposed to C-O addition. The analogous behavior is observed with alkyl fluorides. Thus, the reaction of (PCP)Ir(NBE) with fluoroethane results in rapid formation of an equimolar mixture of (PCP)Ir(H)(F) and (PCP)Ir(ethylene) (Scheme 4.22). Reaction of (PCP)Ir(NBE) with 2-fluoropropane, followed by heating to 80 °C, resulted in quantitative conversion to (PCP)Ir(H)(F) (Scheme 4.23). 

Davies and Warren have investigated the nitration of naphthalene, ace-naphthene and eight dimethylnaphthalenes in acetic anhydride at o °C. Rates relative to naphthalene were determined by the competition method, and the nitro-isomers formed were separated by chromatographic and identified by spectrophotometric means. The results, which are summarised in the table, were discussed in terms of various steric effects, and the applicability of the additivity rule was examined. For the latter purpose use was made of the data of Alcorn and Wells (table 10.2) relating to the nitration of monomethyl-naphthalenes at 25 °C. The additivity rule was found to have only limited utility, and it was suggested that the discrepancies might be due in part to the... 

The Pd-catalyzed elimination of the mesylate 909 at an anomeric center, although it is a saturated pseudo-halide, under mild conditions is explained by the facile oxidative addition to the mesylate C—O bond, followed by elimination of /3-hydrogen to give the enol ether 910[767],... 

Fig. 3. The tetrahedra in the schematics represent four oxygens clustered around a siUcon. Schematic representation of (a) an ideal crystalline stmcture (Si—O—Si bond angles = 180°) (b) a simple glass (Si—O—Si bond angles = 144° according to Fig. 2) and (c) the addition of a modifier, in this case one molecule of Na20, causes the breaking of one Si—O—Si bond to form two Si—ONa linkages.

Nucleophilic Substitution Reactions. Many of the transformations reali2ed through Michael additions to quiaones can also be achieved usiag nucleophilic substitution chemistry. In some iastances the stereoselectivity can be markedly improved ia this fashion (100), eg, ia the reaction of ben2enethiol with esters (R = CH C O) and ethers (R = 3) 1,4-naphthoquiaones. 2-Bromo-5-acetyloxy-l,4-naphthoquiQone [77189-69-6J, R = Br, yields 75% of 2-thiophenyl-5-acetyloxy-l,4-naphthoquinone [71700-93-1], R = SC H. 3-Bromo-5-methoxy-1,4-naphthoquinone [69833-10-9], R = Br, yields 82% of 3-thiophenyl-5-methoxy-l,4-naphthoquinone [112740-62-2] R = SC H. ... 

Determination of surface functional groups, e.g., —OH, —C - C—, and >C = O, and identificadon of adsorbed molecules comes principally from comparison with vibrational spectra (infixed and Raman) of known molecules and compounds. Quick qualitative analysis is possible, e.g., stretching modes involving H appear for v(C—H) at 3000 cm and for v(0—H) at 3400 cm L In addition, the vibrational energy indicates the chemical state of the atoms involved, e.g., v(C=C) " 1500 cmT and v(C=0) " 1800 cm"L Further details concerning the structure of adsorbates... 

The acetal polymer moleeules have a shorter backbone (—C—O)—bond and they pack more closely together than those of polyethylene. The resultant polymer is thus harder and has a higher melting point (175°C for the homopolymer). The position of the glass transition is a subjeet of debate since at least two transitions in addition to the melting point are discernible. The true glass transition is usually associated with the temperature at which movement of segments of about 50-150 baekbone atoms becomes relatively easy, in the... 

In addition to diamond and amorphous films, nanostructural forms of carbon may also be formed from the vapour phase. Here, stabilisation is achieved by the formation of closed shell structures that obviate the need for surface heteroatoms to stabilise danghng bonds, as is the case for bulk crystals of diamond and graphite. The now-classical example of closed-shell stabilisation of carbon nanostructures is the formation of C o molecules and other Fullerenes by electric arc evaporation of graphite [38] (Section 2.4). 

Aldiough diese structures have a positive charge on a more electronegative atom, diey benefit from an additional bond which satisfies file octet requirement of the tricoordinate carbon. These carbocations are well represented by file doubly bonded resonance structures. One indication of file participation of adjacent oxygen substituents is file existence of a barrier to rotation about the C—O bonds in this type of carbocation. 

Other examples of the successful displacement of tosylates are the preparation of 31 -, 16a-,16j - and27- labeled steroids. This displacement reaction fails, however, with certain C-18 and C-19 alcohol derivatives which give mainly O—S instead of C—O bond cleavage. Unsatisfactory results were also obtained with sterically hindered tosylate esters at C-11, C-12 and C-20, which give considerable amounts of olefinic products in addition to O—S bond cleavage. ... 

Examples of linear addition reactions that form C-O, C-S, C-N, C-P, C-C, and C-Si bonds are reviewed. Only a few of the growing number of linear additions that form a carbon-transition metal bond are included... 

C. Other Methods of Enamine Formation O. Additional Examples of Enamine Formation... 

What controls selectivity Draw the products that would result from O addition and from ortho C addition (as well as "para C addition shown above) of phenoxide anion and phenyl diazonium ion. Compare the energies of these products (para C product, ortho C product and O product). Which is most stable Is this the observed product Can thermodynamics explain the outcome ... 

The most widely accepted mechanism of reaction is shown in the catalytic cycle (Scheme 1.4.3). The overall reaction can be broken down into three elementary steps the oxidation step (Step A), the first C-O bond forming step (Step B), and the second C-O bond forming step (Step C). Step A is the rate-determining step kinetic studies show that the reaction is first order in both catalyst and oxidant, and zero order in olefin. The rate of reaction is directly affected by choice of oxidant, catalyst loadings, and the presence of additives such as A -oxides. Under certain conditions, A -oxides have been shown to increase the rate of reaction by acting as phase transfer catalysts. ... 

The o-aminophenylpropiolic acid 4 (20 g) in water (60 mL) and aqueous ammonia (9 mL, d = 0.88) was added with shaking during 15 minutes to a mixture prepared from ferrous sulfate (220 g), water (440 mL), and aqueous ammonia (110 mL, d = 0.88). After 45 minutes, with occasional shaking but no external cooling, the suspension was filtered. The residue was washed with water, and the combined filtrates were treated with ammonium acetate (60 g) and made weakly acidic with acetic acid. The solution was then cooled to 0°C by addition of crushed ice, and then made acidic to Congo-red with concentrated hydrochloric acid (70-80 mL). Additional hydrochloric acid (20 mL, 2 N) was immediately added, and the turbid solution which resulted was diazotized with 20% aqueous sodium nitrite, after which the mixture was kept at 70°C. The cinnoline acid 6 was separated over 45 minutes as a dark brown, granular solid (12.5 g), m.p. 260-265°C. ... 

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