Methyl-substituted Toluenes

3- or 1,4-Methyl-substituted toluenes can be ammoxidized to mono- or di-nitriles [e. g. 9,38]. The ammoxidation of o-xylene gives either phthalimide (e. g. over vanadium oxide on alumina [64] or vanadium oxide on zirconia [65]) or, under different reaction conditions, o-phthalodinitrile (e. g. over vanadium oxide on alumina [66]). m-Xylene is converted to isophthalodinitrile over multicomponent catalysts [67], and tellurium- [68] and niobium- [69] exchanged zeolite catalysts. /7-Xylene is ammoxidized to either /7-tolunitrile or terephthalonitrile, e. g. over multicomponent catalysts [12]. / -Tolunitrile and terephthalodinitrile are also formed with good selectivity from a mixture of m- and /7-xylenes by shape-selective ammoxidation with copper-containing ZSM-5 zeolites [33,34]. 

Toluenes with more than one additional methyl group are also converted to nitriles [70,71], although with higher substitution the number of products resulting from partial ammoxidation increases, e. g. the ammoxidation of mesitylene results in a mixture of l-cyano-3,5-dimethylbenzene, l,3-dicyano-5-methylbenzene and 

5-tricyanobenzene. Over vanadium-tin oxide [72] or over a VBPNa oxide catalyst (with addition of halocarbons to the feed) the yield of tricyanobenzene is markedly enhanced [73]. 

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In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

The acidity of benzylic protons of aromatics complexed to transition-metal groups was first disclosed by Trakanosky and Card with (indane)Cr(CO)3 [61]. Other cases are known with Cr(CO)3 [62], Mn(CO)3 [63], FeCp+ [64, 65], and Fe(arene)2+ [31, 66] but none reported the isolation of deprotonated methyl-substituted complexes. We found that deprotonation of the toluene complex gives an unstable red complex which could be characterized by 13C NMR ( Ch2 = 4.86 ppm vs TMS in CD5CD3) and alkylated by CH3I [58] Eq. (13) ... 

The catalyst jnecursors were tested in solution polymerization runs at 1.3 bar of elhylene pressure in toluene at tempraature (Tp) between 10 and 50 C and the results are summarized in Table 1. The active catalysts are generatKl in situ in toluene by the addition of MAO to the catalyst precursor in the prraence of ethylme. Methyl substituted catalyst (3a/MAO) showed the highrat activity while isopropyl homologue (3c/MAO) the lowest activity. The 3a/MAO catalyst showed higher activity than 3c/MAO by 2-fold at low Tp (say 10 C). However, as Tp... 

On the other hand, the oxidation of the alkyl substituent in alkyl aromatic compounds can be carried out by various methods efficiently. For example, CAN has been used to oxidize substituted toluene to aryl aldehydes. Selective oxidation at one methyl group can be achieved (Eq. 7.19).44 The reaction is usually carried out in aqueous acetic acid. 

Figure 7 displays the data of Ito4,13 and others29 in a 3 x 3 matrix of torsional potentials for o-fluorotoluene, m-fluorotoluene, and p-fluorotoluene in the three electronic states S0, S, and D0. The matrix reveals patterns that hold for other ortho, meta, and para substituents as well.9 In S0, ortho substitution creates a large barrier, while meta substitution does not. A sensible interpretation invokes steric repulsion between methyl and the ortho substituent. Hie menz-substituted cases have very small barriers like toluene itself, apparently for lack of steric effects. However, in ort/io-substituted toluenes, V3 decreases sharply on ji — n excitation from S0 to S, while in mefa-substituted toluenes, V3 increases substantially from S0 to S,. This suggests that steric interactions are not the complete story. Most intriguing of all,... 

In this section, we present a unified picture of the different electronic effects that combine to determine methyl rotor potentials in the S0, Sp and D0 electronic states of different substituted toluenes. Our approach is based on analysis of ab initio wavefunctions using the natural bond orbitals (NBOs)33 of Weinhold and cowork-ers. We will attempt to decompose the methyl torsional potential into two dominant contributions. The first is repulsive steric interactions, which are important only when an ortho substituent is present. The second is attractive donor-acceptor interactions between CH bond pairs and empty antibonding orbitals vicinal to the CH bonds. In the NBO basis, these attractive interactions dominate the barrier in ethane (1025 cm-1) and in 2-methylpropene (1010 cm-1) see Figure 3. By analogy, donor-acceptor attractions are important in toluenes whenever there is a substantial difference in bond order between the two ring CC bonds adjacent to the C-CH3 bond. Viewed the other way around, we can use the measured methyl rotor potential as a sensitive probe of local ring geometry. 

With accurate calculated barriers in hand, we return to the question of the underlying causes of methyl barriers in substituted toluenes. For simpler acyclic cases such as ethane and methanol, ab initio quantum mechanics yields the correct ground state conformer and remarkably accurate barrier heights as well.34-36 Analysis of the wavefunctions in terms of natural bond orbitals (NBOs)33 explains barriers to internal rotation in terms of attractive donor-acceptor (hyperconjuga-tive) interactions between doubly occupied aCH-bond orbitals or lone pairs and unoccupied vicinal antibonding orbitals. 

In general, symmetrical oxo-squaraines having the same end-groups are synthesized by reacting squaric acid with two equivalents of quatemized indolenine, 2-methyl-substituted benzothiazole, benzoselenazole, pyridine, quinoline [39, 45, 46] (Fig. 4) in a mixture of 1-butanol - toluene or 1-butanol - benzene with azeotropic removal of water in presence [39, 45] or absence [47] of quinoline as a catalyst. Other reported solvent systems include 1-butanol - pyridine [48], 1-propanol - chlorobenzene, or a mixture of acetic acid with pyridine and acetic anhydride [49]. Low CH-acidic, heterocyclic compounds such as quatemized aryl-azoles and benzoxazole do not react, and the corresponding oxo-squaraines cannot be obtained using this method [23, 50]. 

On going to methyl-substituted benzene derivatives the effects observed with benzene become more pronounced (Perkampus and Baumgarten, 1964a). Already with toluene it is found that the stability of the ternary complex has increased, since the latter decomposes into its components only above 250°K. From xylene onwards all complexes are stable, even at room temperature. This is already a manifestation... 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

The fluoro substituents proved to induce changes in the reactivities of the methyl- (X = H) and trifluoromethyl-substituted (X = F) pyrido[3, 2 4,5]furo[3,2- [l,3]oxazin-4(47r)-ones 105 with nucleophiles. When methyl-substituted compounds 105 (X = H) were reacted with piperidine in toluene, Wacetylamino carboxamides 106 were formed by nucleophilic attack at the carbonyl group of the l,3-oxazin-4-one ring (Scheme 16). However, the similar reactions of the trifluoromethyl-substituted analogs 105 (X = F) resulted in formation of amidino carboxylic acids 107 by attack at electron-poor position 2 <1995JFC(74)1>. 

Methyl-2-aminobenzophenone can be prepared similarly by substituting toluene for benzene. The yield of crude material, m.p. 85-88°, is 70%. On recrystallization from 95% ethanol, using 5 ml. per g., there is obtained, in two crops, a 70% recovery of 4/-methyl-2-aminobenzophenone, m.p. 92-93°. Because of the higher temperature required in the steam distillation (cf. Note 5), the sulfonamide is obtained in a form difficult to purify. As a result the crude aminoketone usually contains 1-2 g. of aluminum oxide. 

The course of the reaction of methyl-substituted benzene derivatives with cesium fluoroxysulfate in acetonitrile at 35-40 C strongly depends on the structure of the molecule. Toluene (16, R1 = R2 = H) and other alkyl-substituted benzenes 16 and 17 are mainly functionalized on the side chain, while 1,3.5-trimethylbenzene (18) gives mainly ring-substituted products 24 however. 1.2.4,5-tetramethylbenzene (19) and hexamethylbenzene again give mainly or exclusively side-chain products.25... 

Fig. 13. Infrared spectra of methyl-substituted benzenes on Ni/Si02 (A) toluene (B) o-xylene (C) /n-xylene (D) p-xylene. [(A), (C), and (D) from Ref. 358 (B) reprinted from Ref. 242, Kinet. Kami (Kinet. Catal. Transl.) 8, D. M. Shopov and A. N. Palazov, p. 862 (p. 732 Transl.). Copyright 1967 with kind permission of Elsevier Science-NL, Sara Burgerhart-straat 25, 1055 KV Amsterdam, The Netherlands.]...

The HO-toluene adduct corresponds to the methyl-substituted o-, m- and p-hydroxycyclohexadienyl radical isomers. The kinetics of formation and of unimolecular dissociation of HO-aromatic adducts have been studied extensively [30], The benzyl radical -CH2 is converted to the aldehydic product 0-CHO in the presence of NO via a series of reactions analogous to those involving simple alkyl radicals. Bandow et al. [144-146] have determined the yields of aromatic aldehydes to be < 12% of the overall reactions of toluene, xylenes, and trimethylbenzenes, and thus, the H-atom abstraction channel is relatively small but significant. In the case of 0-CHO, the... 

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