3-Aryl eliminations

Zhao P, Incarvito CD, Hartwig JF (2007) Directly observed transmetalation from boron to rhodium. /3-Aryl elimination from Rh(I) arylboronates and diarylbori-nates. J Am Chem Soc 129 1876... 

Based on mechanistic investigations conducted by Hartwig [56], P-aryl elimination from trisubstituted aryl methanols 63 derived from acridinone 65 was exploited to generate an aryl rhodium species which then undergoes 1,4-addition to a range of acyclic a,P-unsaturated carbonyl compounds 62. 

Migratory insertions are one step of many different types of catalytic processes, several of which are conducted on large industrial scales and are presented in later chapters of this text. For example, the mechanism of carbonylation processes, such as hydroformylation, includes ttie insertion of CO into a metal-carbon bond. Likewise, catalytic hydrogenation occurs by insertion of an olefin into a metal-hydride bond, and olefin polymerizations and couplings of olefins with haloarenes occur by insertions of olefins into metal-carbon bonds. The reverse of these reactions, p-hydride, p-alkyl, and p-aryl eliminations, are principal pathways for the decomposition of metal-alkyl complexes. 

P-Eliminations (Equation 10.1) are the most common type of elimination reaction from transition metal complexes, and p-hydrogen eliminations from metal-alkyl complexes are the most common type of p-elimination reactions. p-Hydrogen elimination from aUcoxo and amido complexes has also been observed in a few cases. p-Alkyl elimination, p-aryl elimination, p-aUcoxide elimination, and p-chloride elimination have also been observed and have been studied carefully because of their importance as side reactions in catalytic chemistry. Although P-hydrogen elimination from metal-alkyl complexes occurs almost exclusively by migratory de-insertion pathways, p-hydrogen ehmmation from alkoxides has been shown to occur by several different pathways. 

P-Hydrogen eliminations and p-aryl eliminations from alkoxo and amido complexes are also known. Such eliminations have been shown to occur by migratory de-insertion pathways, as well as alternative p-hydride abstraction mechanisms. P-Hydrogen eliminations from metal-silyl complexes are rare because the silicon-carbon double bond in the product is weak. For similar reasons, p-hydrogen eliminations from metal-thiolate complexes are rare. 

Zhao P, Incarvito CD, Hartwig JE (2006) Direct observation of p-aryl eliminations from Rh (I) alkoxides. J Am Chem Soc 128 3124... 

Alternatively, should there be an equilibrium between 30 and epimer O, and between O and M (via p-aryl elimination), it is conceivable that 30 and 31 would also be in equilibrium. In order to examine this, 30 was subjected to the reaction conditions with [RhCl(C2H4)2]2 in toluene without detection of 31 by H NMR spectroscopy. Although this proves that 30 and 31 do not equilibrate, it does not infer about the relative energy levels of M and N. 

A palladacycle reacted with alkyl halides to afford a sterically bulky arylpalla-dium complex with liberation of norbornene (Scheme 1.34) [42]. P-Aryl elimination from the norbornylpalladium intermediate would be responsible for the formation of these products. 

Three oxidative reactions of benzene with Pd(OAc)2 via reactive rr-aryl-Pd complexes are known. The insertion of alkenes and elimination afford arylalk-enes. The oxidative functionalization of alkenes with aromatics is treated in Section 2.8. Two other reactions, oxidative homocoupling[324,325] and the acetoxylation[326], are treated in this section. The palladation of aromatic compounds is possible only with Pd(OAc)2. No reaction takes place with PdCl2. 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

In the reaction of Q,/3-unsaturated ketones and esters, sometimes simple Michael-type addition (insertion and hydrogenolysis, or hydroarylation, and hydroalkenylation) of alkenes is observed[53,54]. For example, a simple addition product 56 to methyl vinyl ketone was obtained by the reaction of the heteroaromatic iodide 55[S5]. The corresponding bromide affords the usual insertion-elimination product. Saturated ketones are obtained cleanly by hydroarylation of o,/3l-unsaturated ketones with aryl halides in the presence of sodium formate, which hydrogenolyses the R—Pd—I intermediate to R— Pd—H[56]. Intramolecular hydroarylation is a useful reaction. The diiodide 57 reacts smoothly with sodium formate to give a model compound for the afla-toxin 58. (see Section 1.1.6)[57]. Use of triethylammonium formate and BU4NCI gives better results. 

An Q-arylalkanoate is prepared by the reaction of aryl halide or triflate with the ketene silyl acetal 74 as an alkene component. However, the reaction is explained by transmetallation of Ph - Pd—Br with 74 to generate the Pd eno-late 75, which gives the a-arylalkanoate by reductive elimination[76]. 

When allylic alcohols are used as an alkene component in the reaction with aryl halides, elimination of /3-hydrogen takes place from the oxygen-bearing carbon, and aldehydes or ketones are obtained, rather than y-arylated allylic alcohoIs[87,88]. The reaction of allyl alcohol with bromobenzene affords dihydrocinnamaldehyde. The reaction of methallyl alcohol (96) with aryl halides is a good synthetic method for dihydro-2-methylcinnamaldehyde (97). 

Aryl or alkenyl halides attack the central carbon of the allene system in the 2,3-butadien-l-ol 120 to form the 7r-allyl intermediate 121, which undergoes elimination reaction to afford the o,/3-unsaturated ketone 122 or aldehyde. The reaction proceeds smoothly in DMSO using dppe as a ligandflOl]. 

The unconjugated alkenyl oxirane 133 reacts with aryl halides to afford the arylated allylic alcohol 134. The reaction is explained by the migration of the Pd via the elimination and readdition of H—Pd—1[107]. 

In the reaction of aryl and alkenyl halides with 1,3-pentadiene (248), amine and alcohol capture the 7r-allylpalladium intermediate to form 249. In the reactions of o-iodoaniline (250) and o-iodobenzyl alcohol (253) with 1,3-dienes, the amine and benzyl alcohol capture the Tr-allylpalladium intermediates 251 and 254 to give 252 and 255[173-175]. The reaction of o-iodoaniline (250) with 1,4-pen tadiene (256) affords the cyclized product 260 via arylpalladiuni formation, addition to the diene 256 to form 257. palladium migration (elimination of Pd—H and readdition to give 258) to form the Tr-allylpalladium 259, and intramolecular displacement of Tr-allylpalladium with the amine to form 260[176], o-Iodophenol reacts similarly. 

The terminal diyne 320 is prepared by coupling of the zinc acetylide 318 with /rfln.s-l-iodo-2-chloroethylenc (319), followed by elimination of HCI with sodium amide[231]. Similarly, terminal di- and triynes are prepared by using cw-l,2-dichloroethylene[232]. The 1-alkenyl or l-aryl-2-(perefluoroalkyl) acetylene 321 is prepared by the reaction of a zinc acetylide with halides[233]. 

The carbonylation of aryl iodides in the presence of alkyl iodides and Zn Cu couple affords aryl alkyl ketones via the formation of alkylzinc species from alkyl iodides followed by transmetallation and reductive elimination[380]. The Pd-catalyzed carbonylation of the diaryliodonium salts 516 under mild conditions in the presence of Zn affords ketones 517 via phenylzinc. The a-diketone 518 is formed as a byproduct[381],... 

Organotin compounds such as aryl-, alkenyl-, and alkynylstannanes are useful for the ketone synthesis by transmetallation of acylpalladium 529 and reductive elimination of 530 as shown[389-393]. Acetophenone (531) is obtained by the carbonylation of iodobenzene with Me4Sn. Diaryl ketones... 

The aryl- and heteroarylfluorosilanes 541 can be used for the preparation of the unsymmetrical ketones 542[400], Carbonylation of aryl triflate with the siloxycyclopropane 543 affords the 7-keto ester 545. In this reaction, transme-tallation of the siloxycyclopropane 543 with acylpalladium and ring opening generate Pd homoenolate as an intermediate 544 without undergoing elimination of/3-hydrogen[401],... 

The 2-substituted 3-acylindoles 579 are prepared by carbonylative cycliza-tion of the 2-alkynyltrifluoroacetanilides 576 with aryl halides or alkenyl tri-flates. The reaction can be understood by the aminopalladation of the alkyne with the acylpalladium intermediate as shown by 577 to generate 578, followed by reductive elimination to give 579[425]. 

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