C- Alkylation reactions

The general feature of alkylation reactions at a carbon atom is that they can be achieved under sonication using solid bases even in apolar solvents. The advantage is that side reactions are generally minimised. Deprotonation occurs readily on a benzylic position in the presence of aqueous sodium hydroxide, as shown with indene (Eq. 3.21) [117]. A quantitative yield of the alkylated product can be obtained using sonication in the presence of a PTC. It was suggested that alkylation of cyclopentadiene or indene by secondary or tertiary alkyl halides in the presence of potassium hydroxide and Ali-quat occurred via a SET process [118]. 

Alkylation at carbon positions activated by electron-withdrawing groups is illustrated by the spiroannelation of cyclopentanone and cyclohexanone by sonication of the parent compound with potassium t-butoxide and 1,4-dibromobutane (Eq. 3.22). The so-nochemical improvement is especially large with cyclopentanone which is normally particularly prone to self-condensation [119]. 

A rather difficult double Wittig reaction (Eq. 3.24) has been effected with enhanced efficiency under sonication [123], The process constitutes a novel type of annelation of an aromatic ring when applied to o-quinones. 

It is liquid-liquid reactions involving phase transfer catalysts which generally benefit from the use of ultrasound. Sonication produces homogenisation - i. e. very fine emulsions - which greatly increase the reactive interfacial area and allows faster reaction at lower temperatures. Davidson has reported an example of this with the ultrasonically enhanced saponification of wool waxes by aqueous sodium hydroxide using tetra n-heptyl ammonium bromide as a PTC [124]. 

In another example increased yields of products are obtained under ultrasonic irradiation in the PTC alkylation of the isoquinoline derivatives using 50% aqueous NaOH as base [125]. Efficient mixing is not easy to achieve for this system under normal reaction conditions due to the viscosity of the aqueous base. In the specific case of alkylation with benzyl chloride ultrasound plus [Et3NCH2Ph] Cl achieved 60% yield in 20 min compared with only 50% in 2 h with stirring (Eq. 3.25). 

---

Pyridones were studied for N- and C-alkylation reactions by de la Hoz et al. [100] as already mentioned for 1,2,4 triazoles, the selectivity of the alkylation is highly dependent on the activation technique (microwave or conventional heating). 

The first clue to the existence of the SrnI mechanism came from product studies both in aliphatic and aromatic cases. It was noticed that in the reaction of benzyl and substituted benzyl chlorides with the 2-nitropropane anion, oxygen alkylation, yielding the oxime and then the aldehyde, occurs exclusively in the case of benzyl chloride and 3-nitrobenzyl chloride, whereas, with 4-nitrobenzyl chloride, the yield of aldehyde is only 6% and the carbon-alkylated (104) product is obtained in 92% yield (Kornblum, 1975). This was interpreted as the result of a competition between 8, 2 (O-alkylation) and S l (C-alkylation) reactions. In the aromatic case, it was observed that the reaction of 5- and 6-halopseudocumenes with KNHj in liquid ammonia (Kim and Bunnett, 1970) forms the 5- and 6-pseudocumi-dines in a ratio which is the same whether the starting compound is the 5- or 6-isomer in the case of the chloro- and bromo-derivatives, as expected from an aryne mechanism (Scheme 9), whereas much more non-rearranged... 

In addition to C-alkylation reactions, hydroaminations are of great interest in organic synthesis [85, 86]. Recently, Shibasaki and coworkers developed a Bi(OTf)3-catalyzed intermolecular hydroamination using styrenes [87] and... 

C-alkylation reactions 0-alkylation reactions N-alkylation reactions... 

Umbelliferone is the pivotal metabolite in the pathways to substituted coumarins and furano- or pyranocoumarins. Its 7-0-methylation to herniarin (Fig. 4.3) or 0-prenylation and C-alkylation reactions are often observed in coumarin-producing higher plants where the prenylation at C-6 or C-8 mark the entry to the routes branching linear from angular furano- and pyranocoumarins (Fig. 4.5). In Ruta graveolens and Ammi rmjus, the prenylation reactions are catalysed by particulate enzymes (Hamerski et at, 1990), and... 

Dong JH, Dong WK, Dae YC (2010) Facile ring-closure cyclization of atenes by nucleophilic C-alkylation reaction in ionic Uqitid. Tetrahedron Lett 51 54-56... 

Graft and block copolymers of propylene and styrene have been developed to compatabilize PP/PS blends. Del Giudice et al. (167) and Xu and Lin (168) have synthesized PP-b-PS. Kim et al. (169) and Li et al. (170) first polymerized propylene together with some functional monomers, then polymerized styrene from these monomers units to form polystyrene branches. Diaz et al. (171,172) grafted PP chains onto PS chains based on F-C alkylation reaction when mixing PP/PS blends in the presence of AICI3 catalyst and styrene. All these copolymers help form very... 

In 1877, Charles Friedel and James M. Crafts discovered new methods for the preparation of alkylbenzenes, known as Friedel-Crafts alkylation reactions. The mechanism includes an electrophilic aromatic substitution whereby a carbocation is generated as the electrophile in the presence of a Lewis acid catalyst. The general scheme of F-C alkylation reaction is (16) as follows ... 

---