Monoalkylated aromatic compounds

The allcylation of a number of aromatic compounds through the use of a chloroa-luminate(III) ionic liquid on a solid support has been investigated by Holderich and co-workers [87, 88]. Here the allcylation of aromatic compounds such as benzene, toluene, naphthalene, and phenol with dodecene was performed using the ionic liquid [BMIM]C1/A1C13 supported on silica, alumina, and zirconia. With benzene, monoalkylated dodecylbenzenes were obtained (Scheme 5.1-56). 

Monoalkylation products, 3-aryl-1,1 -dichloro-1 -silabutanes, were obtained from the alkylation of aromatic compounds with I in the presence of aluminum chloride catalyst in good isolated yields (60-80%) along with small amounts of higher alkylation products. Dialkylation products were obtained in yields ranging from 2 to 8% when a 5-fold excess of the aromatic compounds with respect to 1 was used. The amount of dialkylated products can be further reduced by using a greater excess of the aromatic compounds. 

Monoalkylbenzene or other aromatic compounds react more rapidly than benzene itself in alkylation with hydrogen fluoride and the dialkyl-benzene react less rapidly in general to form tri and higher alkylated products. The polyalkylated products require more strenuous conditions. To form the monoalkyl product the alkylating agent should be added slowly to a large excess of the aromatic compound. 

Second, Friedel-Crafts monoalkylations of aromatic compounds such as benzene or naphthalene, in which the introduction of a second alkyl group is not prevented by steric hindrance, can only be carried out with a trick The reaction is performed with a large... 

Second, Friedel-Crafts monoalkylations of aromatic compounds such as benzene or naphthalene, in which the introduction of a second alkyl group is not prevented by steric hindrance, can be carried out (only) with a trick The reaction is performed with a large excess of the aromatic compound. There is almost no overalkylation at all. Simply for statistical reasons virtually the entire attack is directed at the excess starting material, not the primary product. For example, the following substitution succeeds in this way ... 

The third case shows the immobilisation of Lewis-acidic ionic liquids. The resulting catalysts, named Novel Lewis-Acidic Catalysts (NLACs), are highly active in the Friedel-Crafts alkylation of aromatic compounds with dodecene. Conversions and selectivities to the desired monoalkylated products were excellent. No leaching of the catalytically active component could be observed. The isomer distribution of the monoalkyated products is very similar to that obtained over pure aluminum(III)chloride. The main drawback of the NLACs is that thy are very sensitive towards water, which leads to irreversible deactivation. A second problem is the deactivation after long reaction times. The most likely cause is olefin oligomerisation. 

There is some spectroscopic evidence that aromatic compounds complex carbenium ions [42]. For example, the complexation equilibrium constant between trityl ions and hexamethylbenzene is K = 68 mol-1 L at 0° C [43]. Complexation should be stronger with more electrophilic carbenium ions such as those derived from styrene and a-methylstyrene. On the other hand, the monoalkyl-substituted phenyl rings attached to the polymer chain are weaker nucleophiles than hexamethylbenzene. A complexation constant K = 4 mol 1 L was reported for trityl cation and styrene [43]. Similar complexes have been proposed to explain the red color observed in inifer systems based on l,4-bis(I-chIoro-l-methyl-ethyl)benzene and BCI3 in CH2C12 at low temperature [44],... 

Aromatic Aldehydes. The preparation of aromatic aldehydes from benzene or monoalkyl and polyalkyl aromatic compounds by means of carbon monoxide and hydrogen chloride has been reviewed by Crounse (19). 

The Catellani s alkylation-alkenylation sequence using norbomene offers a useful synthetic method for 2,6-dialkylated 1-substituted benzenes. Lautens applied the reaction to the synthesis of fused aromatic compounds using ort/jo-substituted iodobenzenes and bromoalkenes. Reaction of o-iodotoluene (11) with ethyl 6-bromo-2-hexenoate (13) afforded the benzocarbocycle 14 via monoalkylation and intramolecular Heck reaction. It is important to use tri-2-furylphosphine (1-3) as a ligand [4]. Similarly the 2,5-disubstituted 4-benzoxepine 17 was obtained in 72% yield by the reaetion of 1-iodonaphthalene (15) with the unsaturated bromo ester 16 [5]. 

Measurement of the equilibrium constants for such processes allows one to estimate the relative electron affinities (in solution) of different aromatic compounds. Measurements of this kind for the equilibria between benzene and various monoalkyl and dialkyl benzenes, i.e.. 

In Grignard reactions, SiCU, HSiCl3 or organosilicon compounds are used as raw materials. For example, in the case of HSiCl3, the reaction is as shown in eq. (8.8). On the other hand, in using monoalkyl silicon compounds, the reaction with aromatic compounds is shown in eq. (8.9) [9]. 

The ruthenium-, rhodium-, and palladium-catalyzed C-C bond formations involving C-H activation have been reviewed from the reaction types and mechanistic point of view.135-138 The activation of aromatic carbonyl compounds by transition metal catalyst undergoes ortho-alkylation through the carbometallation of unsaturated partner. This method offers an elegant way to activate C-H bond as a nucleophilic partner. The rhodium catalyst 112 has been used for the alkylation of benzophenone by vinyltrimethylsilane, affording the monoalkylated product 110 in 88% yield (Scheme 34). The formation of the dialkylated product is also observed in some cases. The ruthenium catalyst 113 has shown efficiency for such alkylation reactions, and n-methylacetophenone is transformed to the ortho-disubstituted acetophenone 111 in 97% yield without over-alkylation at the methyl substituent. 

As with phosphoric acid, three H atoms in the molecule can be replaced by aliphatic or aromatic radicals to become carbamate insecticides. However, the second H atom on the nitrogen is not replaced in making insecticides, because the monoalkyl structure is more toxic than the N-disubstituted compound. A typical structure is represented by carbaryl. 

Synthesis of Phosphoric Acids and Their Derivatives. - A series of monoalkyl and dialkyl phosphorus acid chiral esters have been synthesised for use as carriers for the transport of aromatic amino acids through supported liquid membranes. The compounds acted as effective carriers but enantio-separation was at best moderate. A range of phosphono- and phosphoro-fluoridates have been prepared by treatment of the corresponding thio- or seleno- phosphorus acids with aqueous silver fluoride at room temperature (Scheme 1). In some cases oxidation rather than fluorination occurred. Stereospecifically deuterium-labelled allylic isoprenoid diphosphates, e.g. (1), have been synthesised from the corresponding deuterium-labelled aldehyde by asymmetric reduction, phosphorylation and Sn2 displacement with pyrophosphate (Scheme 2). ... 

In Chapter 6 we discussed the remarkable mixing effect of Friedel-Crafts reactions. Let us briefly review it and then discuss its application to various aromatic and heteroaromatic compounds (Scheme 8.7). Friedel-Crafts reaction of 1,3,5-trimethoxybenzene with a highly reactive electrochemically generated N-acyliminium ion pool using a conventional macrobatch reactor results in the formation of an essentially 1 1 mixture of the monoalkylation product and the dialkylation product. The reaction is very fast and is complete within Is even at —78°C. 

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