Five-membered palladacycle

In the case of 2-bromo-l,l-diphenylethene (Scheme 9),90 the resulting five-membered palladacycle adds a further equivalent of the starting material because a highly strained and antiaromatic benzocyclobutadiene would be formed. Finally, the ring closure via the reductive elimination gives the product. A similar reaction is observed in the case of 11 -(bromomethylene)dibenzoheptatriene.91... 

The Pd(0) complexes [Pd(PPh3)4] and [Pd P(OPh)3 4] also react with hexafluoroacetone to give compounds containing a three-membered ring. With [Pd P(OMe)3 4] or [Pd P(OMe)2Ph 4], however, coupling of two ketones occurs to generate a five-membered palladacycle - ... 

Due to the inability of the norbomylpalladium(II) species 4 to undergo a syn (3-hydride elimination, a variety of alternative reactions can take place. Catellani has shown a number of reactions are possible including alkyne addition [19] and carbonylation [20], In addition, and perhaps the most intriguing reaction of complex 4 is the formation of the five membered palladacycle 5 in the presence... 

For substrates that have a methyl group in the benzylic position, C-H activation of this methyl group can be chemoselectively achieved. In this event, P-hydride elimination is impossible, and the formation of benzocyclobutene products is observed instead (Table 7) [66], However, even if there is a choice between methyl C-H bonds and methylene C-H bonds (the latter allowing for p-hydride elimination), steric factors favor methyl C-H activation, leading to the formation of benzocy-clobutenes. That being said, this reaction may not mechanistically involve a complete migration of palladium. The product may simply be formed by reductive elimination from a five-membered palladacycle intermediate. 

It is important to mention that no reductive elimination of the five-membered palladacycle 129 to form four-membered ether occurs. Instead, oxidative addition of 127 to 129 takes place to form 130. 

Furthermore, Dyker reported the Pd-catalyzed reaction of 2-t-butyliodobenzene (147) gave the benzocyclobutane 148 and the trimer 149 [38]. Key steps are facile formation of the palladacycles 150, 153, and 156 by C—H activation of the methyl group. Oxidative addition of 147 to the five-membered palladacycle 150 to generate 151 and reductive elimination gives 152. In this case, the palladacycle 153 undergoes very unusual reductive elimination to give the strained benzocyclobutane 148 in 75 % yield. Further reaction of 153 via 154 and 155 yields again the... 

Phenylpyridine and P,P-disubstimtedbenzylphosphines can also participate in cy-clometallation reactions leading to five-membered palladacycles, as shown in Table 11. Oxygen and sulfur analogs have also been used. 

Benzylic C—H activation can also occur readily to give five-membered palladacycles containing Pd—CspS bonds (Scheme 36). Five-membered palladacycles containing Pd—Csp3 bonds may also be prepared via transmetallation (Scheme 36). 

Complex 66 is supposed to be present as a mixture of two diastereomers (epimers). Rapid interconversion between them relative to reductive elimination enables the dynamic kinetic resolution to take place. The striking performance of 2-iodo-A,A-diisopropylbenzamides was not mechanistically investigated but coordination of the amide carbonyl group to the Pd centre, forming a five-membered palladacycle, was suggested. 

The formation of 5.5-membered palladacycles becomes preferred again when a biphenyl or a phenyl vinyl fragment (that is two -bonds instead of two r/> -bonds) are involved in the cycle. At variance with palladium(O), which prefers to coordinate the arene as rf, the higher preference of palladium(ll) for -coordination gives rise also to five-membered palladacycles. 

C—H activation of sp bonds is not limited to arylation, olefination, or oxidative methods. Recently, Baudoin has leveraged Pd(0)-mediated C—H functionalization toward the construction of benzocyclobutenes through oxidative addition to aryl halides followed by intra-molecular carbopaUadation leading to a five-membered palladacycle which rednctively eliminates to afford the listed benzocyclobutene ... 

Model stoichiometric reactions of [PdCH3(CO)(Pr DAB)] [B 3,5-(CF3)2C6H3 4] (Pr DAB = l,4-diisopropyl-l,4-diaza-l,3-butadiene) with alkynes and carbon monoxide have been investigated by NMR spectroscopy and DFT studies to identify the putative intermediates involved in the cydocarbonylation of alkynes [47]. Addition of but-2-yne (R = CH3) or 1-phenylpropyne (R= Ph) results in regioseledive insertion into the Pd-acyl bond to afford a five-membered palladacycle 54 that undergoes rapid cydocarbonylation at low temperature to afford a palladium-coordinated, q -allylic lactone 55. The a,f -unsaturated y-lactone could be liberated either by proton abstraction with a stoichiometric amount of Na[BEt3H] or by nucleophilic addition... 

The same strategy was employed for the synthesis of pyrrolidines (Scheme 2.12) from substrates containing 5-methyl C-H bonds. The author believed that compared with the ring contraction from a five-membered palladacycle to a four-membered azetidine product, the formation of a five-membered pyrrolidine product from a six-membered palladacycle intermediate would be much more favorable. However, substrates lacking substituent at the y-positions provide only low yields of the pyrrolidine products, which suggest the possible importance of the Thorpe-Ingold effect to the efficiency of the transformation. 

A further experiment was especially significant (Scheme 15.6). Treatment of the substrate with the standard conditions only gave the six-membered ring lactam. In contrast, the first cyclometalation of substrate gave the favored five-membered palladacycle which was applied to the carbonylation with CO in the absence of BQ, affording the five-membered lactam in 86% yield as well as a minor amount (6%) of the isomer. The steric hindrance around the amino group is pivotal for the success of this transformation because the full substitution of the carbon in a-position of the amino esters plays an important role in their cyclopalladation step. 

A copper-catalysed CDC of benzothiazoles with thiazoles and polyfluoroarene leads to the synthesis of the 2,2 -linkage of thiazoles and 2-polyfluoroarylthiazoles. Silver carbonate has been used as a terminal oxidant. Palladium-catalysed dual C-H functionalization of benzophenones to form fluorenones by oxidative dehydrogenative cyclization has been developed using silver oxide as the oxidant. The reaction is postulated to involve a five-membered palladacycle intermediate. Os(VIII)-catalysed oxidation of 6-aminopenicillanic acid and chlorpheniramine by diperiodatoar-gentate(III) involves an initial formation of an Os(VIII)-reductant complex followed... 

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