Benzene ambient-temperature activation

To isolate the neat product, the more volatile ether/tri-methylamine combination was used in the reaction, because a higher recovery of product was obtained in trials conducted with the racemic material. However, the neat, distilled, optically active product proved to be stereochemically labile at ambient temperature, and was considerably racemized compared to that obtained directly in the benzene/triethylamine solution. Moreover, the latter was relatively stable when further diluted in benzene solution at ambient temperature, showing only a 14% decrease in optical rotation after 70 hours at 26. For stereochemical studies,... 

Spontaneous racemization is very slow in nonpolar solvents at ambient temperature, but is greatly accelerated by protonic and Lewis acids. Racemization is first order in both 1-phenylethyl chloride and acid. Racemization catalyzed by SnCL, in CC14 at 25° C proceeds with a rate constant ka = 1.5 x 10-2 mol, -L-sec [48]. Because styrene and 1-phenylethyl chloride consumption is 90 times slower than racemization, the rate of racemization is not affected by adding styrene to the system. That is, the efficiency of ion capture by styrene is low, whereas the ion pair collapse must be very fast. Racemization of 1-phenylethyl chloride with SnCl4 is nearly 100 times faster in benzene than in CCL, ka = 1.3 mol-, L sec- at 25° C [49], with activation parameters AHt = 35 kJ-rnol- and A St = 120/mor K-. ... 

Although the related [PtH(CH2 Bu)( Bu2PCH2p Bu2)] reductively eliminates neo-pentane at ambient temperature in benzene solution, C—H bond activation does not occur. Instead, the dimeric complex [Pt2(/r- Bu2PCH2P Bu2)2] is formed . 

Azumaya et al. reported an interesting example of retention of the molecular chirality when the chiral crystal of l,2-bis(A-benzoyl-A-methylamino)benzene 65 was dissolved in a cold solution (Fig. 6) [37]. Furthermore, Tissot et al. reported a fine example of the formation of optically active complex 67 ( 100% ee in 93% yield) using axially chiral ligand 66 (Scheme 32) prepared by chiral crystallization [38]. Both achiral 65 and 66 exist as mixtures of many conformational isomers or diastereomers in solution at ambient temperature. If the molecular chirality is retained in homogeneous conditions like these examples, the frozen... 

The cyclopropyl-hydrido complex 415 exhibits appreciable thermal instability, such that it rearranges quantitatively in ambient temperature benzene solutions to the rhodacyclobutane complex Tp Rh(cr,cr -CH2CH2CH2)(CNCH2 Bu) (154, Scheme 34, Section II-C.l). Thermal isomerisation of 154 (65 °C, 2.5 h) affords a 7i-propene complex (153) that is able to activate benzene. Alternatively, in the presence of excess CNCH2 Bu, 154 thermally inserts two equivalents of the isonitrile to ultimately afford the rhodacyclohexane complex 427 (Scheme 34). 

Dilute solutions ( 10" moldm" ) of Wilkinson s complex in organic solvents such as ethanol/benzene actively catalyse the hydrogenation of alkynes and alkenes at ambient temperature and pressure (1 atm, H2). Traces of oxygen must be excluded. Hydrogenation rates are dependent on steric effects. The following general order has been established 1-alkenes > cis-2-alkenes > trans-2-alkenes > trans-3-alkenes. An exception is ethene itself, which forms a rather stable complex (Fig. 5.19) so that higher temperatures are required. Arenes, esters, ketones, carboxylic acids, amides and nitro compounds are unaffected, but aldehydes are slowly decarbonylated. 

Complex 13 undergoes electrophilic substitution with aromatic substrates. Thus, treatment with benzene in dichloromethane at ambient temperature results in the formation of the diphenyl complex 15 (Scheme V. Reaction of 13 with pyridine (5-6 equivs) in dichloromethane affords a new complex that is the result of pyridine a-CH activation. The NMR data clearly show two chemically equivalent coordinated pyridines and pyridine that has lost one of the a-hydrogens. Structure 16 is proposed from the preliminary data. The formation of 15 and 16 was quantitative by NMR monitoring, but these compounds are reactive and have not been isolated as pure solids. While main group Lewis acids are well known to undergo aromatic substitutions (e.g., mercurations, thallations, etc.) (33), relatively little is known about the ability of transition metal complexes to undergo electrophilic aromatic substitution (34). 

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