Aryl-alkyl bond formation

Another example of transient formation of a palladacycle is the Pd-mediated ortho-alkylation and ipso-vinylation of aryl iodides depicted in Scheme 8.23. In this multicomponent reaction the ability of norbomene to undergo reversible arylation and palladacycle formation is exploited. This reaction also illustrates that aryl halides undergo oxidative addition to Pd faster than do alkyl halides, and that aryl-alkyl bond-formation by reductive elimination also proceeds faster than alkyl-alkyl bond-formation. The large excess of alkyl iodide used in these reactions prevents the formation of biaryls. Benzocyclobutenes can also be formed in this reaction, in particular when the alkyl group on the aryl iodide is sterically demanding or when a secondary alkyl iodide is used [161]. 

Aryl-Aryl and Aryl-Alkyl Bond Formation... 

Aromatic C(sp )—H C—H Activation Aryl-Alkyl Bond Formations.126... 

Aromatic C—H Alkylations, C(sph—Aryl-Alkyl Bond Formations.185... 

AROMATIC C—H ALKYLATIONS, C(sp )—H ARYL-ALKYL BOND FORMATIONS... 

The following paragraphs on aryl/aryl(het-eroaryl)- and aryl/alkyl-bond formation centre on organometallic mediated C-C coupling reactions. For want of space, certain methods have had to be omitted and only those most commonly used have been included. Indeed, in certain cases combina-... 

The aryl group participation process is mechanistically a Friedel-Crafts alkylation. The r value of the Friedel-Crafts alkylation by alkyl carbocations was found to be significantly lower than that of protonation or of halogenation of aromatic substrates (Yukawa et al., 1966). Olah interpreted the low r value in terms of an earlier transition state, i.e. less advanced aryl C bond formation with r = 0.6 at the transition state prior to formation of the... 

Photochemistry offers a convenient access to the formation of aryl—alkyl bonds under mild reaction conditions. This can be accomplished either via fragmentation of an aromatic derivative to produce a trappable aryl radical or aryl cation as intermediate, or via the activation of an aliphatic component that then reacts with the arene derivative. 

Examples of the intermolecular C-P bond formation by means of radical phosphonation and phosphination have been achieved by reaction of aryl halides with trialkyl phosphites and chlorodiphenylphosphine, respectively, in the presence of (TMSlsSiH under standard radical conditions. The phosphonation reaction (Reaction 71) worked well either under UV irradiation at room temperature or in refluxing toluene. The radical phosphina-tion (Reaction 72) required pyridine in boiling benzene for 20 h. Phosphinated products were handled as phosphine sulfides. Scheme 15 shows the reaction mechanism for the phosphination procedure that involves in situ formation of tetraphenylbiphosphine. This approach has also been extended to the phosphination of alkyl halides and sequential radical cyclization/phosphination reaction. ... 

Asymmetric conjugate addition of dialkyl or diaryl zincs for the formation of all carbon quaternary chiral centres was demonstrated by the combination of the chiral 123 and Cu(OTf)2-C H (2.5 mol% each component). Yields of 94-98% and ee of up to 93% were observed in some cases. Interestingly, the reactions with dialkyl zincs proceed in the opposite enantioselective sense to the ones with diaryl zincs, which has been rationalised by coordination of the opposite enantiofaces of the prochiral enone in the alkyl- and aryl-cuprate intermediates, which precedes the C-C bond formation, and determines the configuration of the product. The copper enolate intermediates can also be trapped by TMS triflate or triflic anhydride giving directly the versatile chiral enolsilanes or enoltriflates that can be used in further transformations (Scheme 2.30) [110],... 

Sulfate monoesters can react by dissociative paths, and this is the favored path. Whether such reactions are concerted or involve a very short-lived sulfur trioxide intermediate has been the subject of debate. ° Benkovic and Benkovic reported evidence suggesting that the nucleophile is present (though there is little bond formation) in the transition state for the reaction of amines with p-nitrophenyl sulfate. Alkyl esters of sulfuric or sulfonic acids normally react with C-0 cleavage only when this is disfavored, as in aryl esters, does one see S-0 cleavage. Sulfate diester... 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

Crowe proposed that benzylidene 6 would be stabilised, relative to alkylidene 8, by conjugation of the a-aryl substituent with the electron-rich metal-carbon bond. Formation of metallacyclobutane 10, rather than 9, should then be favoured by the smaller size and greater nucleophilicity of an incoming alkyl-substituted alkene. Electron-deficient alkyl-substituents would stabilise the competing alkylidene 8, leading to increased production of the self-metathesis product. The high trans selectivity observed was attributed to the greater stability of a fra s- ,p-disubstituted metallacyclobutane intermediate. 

While C-C bond formation via metallation and alkyl or aryl halides is well known (COMC (1982) 5.4.2.7.5), the C-C bond formation with TBAF and aldehyde, enones, or enals is new (Figure 9).318 Carboranyl groups can actually be used as protecting groups for aldehydes and ketones due to the facile cleavage of the cage C-C(OH) bond with... 

The redox reaction shown in Scheme 7.60 results in the formation of an amide a-radical and tetrathiafulvalene cation-radical. These initially formed a-radical and cation-radical combine to give salts of the S-arylated tetrathiafulvalene (a minor product) and C-alkylated tetrathiafulvalene (the main product). The latter demonstrates an unprecedented carbon-carbon bond formation with the cation-radical of tetrathiafulvalene the structure depicted was confirmed by single crystal x-ray analysis (Begley et al. 1994). 

The formation of the heterocycle 1 from the xylylene-bis-phosphonium salt 2 and PCI3 proceeds via a detectable intermediate 3 in a cascade of condensation reactions that is terminated by spontaneous heterolysis of the last remaining P-Cl bond in a cyclic bis-ylide-substituted chlorophosphine formed (Scheme 1) [15]. The reaction scheme is applicable to an arsenic analogue of 1 [15] and to bis-phosphonio-benzophospholides with different triaryl-, aryl-alkyl- and aryl-vinyl-phosphonio groups [16, 18, 19], but failed for trialkylphosphonio-substituted cations here, insufficient acidity prohibited obviously quantitative deprotonation of the phosphonium salts, and only mixtures of products with unreacted starting materials were obtained [19]. The cations were isolated as chloride or bromide salts, but conversion of the anions by complexation with Lewis-acids or metathesis was easily feasible [16, 18, 19] and even salts with organometallic anions ([Co(CO)4] , [CpM(CO)3] (M=Mo, W) were accessible [20]. 

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