Benzene as substrate

Note There is considerale variety in this category, including, for example, the use of hydroxyimino- or nitroso derivatives of benzene as substrates. 

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

Benzenes as substrates classified by atom number in the pyrimidine ring... 

The first carbon addition to allenes catalyzed by gold was described by Hashmi and was the addition of furans to allenones, a reaction that is discussed as a conjugate addition to carbonyls. Then the annulation of 2-allenyl indoles was described (equation 111)and an enantioselective version was later developed for terminal allenes. Then Nelson described the cyclization of an N-substituted pyrrol in the synthesis of (—)-Rhazinilam (equation 112). In that case, gold notably improved the yields and selectivities obtained with Pd(II) or Ag(l), and gave complete retention of the chiral information of the allene. The most recent report is on the hydroarylations of allenes with gold-covered substituted benzenes as substrates. ""... 

The reaction of carbenes with aromatic hydrocarbons is related to that with alkenes. Doering and Knox (1950, 1953) investigated this reaction using benzene as substrate even before their work with cyclohexene. They observed, however, ring expansion to give cycloheptatriene besides toluene (8-3, X = H). Norcaradiene as an intermediate was isolated only much later in the addition of dicyanomethylene... 

The same complexes were also tested in the catalytic borylation of arenes at 100 °C for 24 h using B2pin2 as borylating agent and 4% NaOtBu as additive to promote dehydrochlorination of the catalysts 47 and thus reduction from Ir(III) to Ir(I). In this borylation reaction, complex 47a also gave the best results with a conversion of 57% based on B2phi2 ( H NMR) and 44% yield ( H NMR). The electron-poor l,3-bis(trifluoromethyl)benzene as substrate led to the best conversion (94%) and yield (74%). The borylation occured selectively in the meta position (Figure 9.13). 

The effect of different substituents on the relative formylation rates of benzene derivatives was investigated. These results showed that methyl groups are activating while halogens are deactivating relative to benzene as substrate. The decrease in reactivity from fluorobenzene> chlorobenzene> bromobenzene is in accordance with formylation trends observed in other acidic systems. 

Using benzene as substrate neither the Brpnsted/Lewis acid site ratio nor the acidic strength increases the selectivity to DPM, the highest values being below 15%, irrespective of the catalyst nature or of the reaction conditions it is clear that, under these conditions, the etherification reaction is much faster than the benzylation due to the low reactivity of the substrate. 

An interesting approach to zr n.v-2,3-disubstituted cyeloalkanones is offered by auxiliary controlled intramolecular Michael additions. The diastereoselectivity depends on the chiral alcohol used193> l94. When the borneol derivative 7 was used as substrate, a single diastereomer of 8 resulted when the reaction was performed at 25 "C under thermodynamic control with a catalytic amount of sodium hydride in benzene. 

In a similar way as described for the hydroformylation, the rhodium-catalyzed silaformylation can also be used in a domino process. The elementary step is the formation of an alkenyl-rhodium species by insertion of an alkyne into a Rh-Si bond (silylrhodation), which provides the trigger for a carbocyclization, followed by an insertion of CO. Thus, when Matsuda and coworkers [216] treated a solution of the 1,6-enyne 6/2-87 in benzene with the dimethylphenylsilane under CO pressure (36 kg cm"2) in the presence of catalytic amounts of Rh4(CO)12, the cyclopentane derivative 6/2-88 was obtained in 85 % yield. The procedure is not restricted to the formation of carbocycles rather, heterocycles can also be synthesized using 1,6-enynes as 6/2-89 and 6/2-90 with a heteroatom in the tether (Scheme 6/2.19). Interestingly, 6/2-91 did not lead to the domino product neither could 1,7-enynes be used as substrates, while the Thorpe-Ingold effect (geminal substitution) seems important in achieving good yields. 

Other synthetic routes to phosphazenes, using phosphorus(v) compounds as substrates, are more limited. An example of the reaction of a nitrile with a phos-phorus(v) chloride is provided by Scheme 4.48 In benzene solution, or in the absence... 

The use of benzene as a solvent eliminates the chlorinated organic products in Table II. This point and the products in the table are consistent with radical abstraction (hydrogen atom transfer) from the substrate, cyclohexene, by the high valent (and likely oxometal) intermediate form of the TMSP complexes, followed by chlorine abstraction from the solvent by intermediate organic radicals. Separation and analysis of the two phases after the reaction reveals that the polyoxometalate is intact. 

The course of hydrocarbon photocatalyzed oxidations seems to depend significantly on the relative positions of the valence band edge of the active photocatalyst and the oxidation potential of the substrate. For example, in contrast to the clean oxidation of toluene described above, lower activity was observed in neat benzene, a substrate whose oxidation potential lies at or slightly below the valence band edge This observation implies the importance of radical cation formation (via photoinduced electron transfer across the irradiated interface) as a preliminary step to hydrocarbon radical formation. If beitzene-saturated aqueous semiconductor suspensions are... 

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