Intermediates oxidative addition

Table XII contains the enthalpy and free-energy differences of the critical intermediate species for the anthraphos Rh catalyst. Although no experimental results are available yet, our predicted energies show a much smaller dissociation energy for H2 loss, the first step in the dissociative mechanism (8 is more stable by 13 kcal/mol relative to 4). In contrast, the oxidative addition intermediate of the first step in the associative mechanism, the M(V) species 5, is 14 kcal/mol less stable...

While the aforementioned reaction works well for aminopyridines and alkoxypyridines, it is not operative for most electron-deficient pyridines as well as 2- and 4-bromopyridines. One of the possible reasons for its failure with 2-halopyridines is the formation of an unreactive dimer complex from the oxidative addition intermediate [130]. 

Applying Buchwald s Pd-catalyzed animation methodology, Thomas and coworkers prepared a range of bicyclic piperazine [108]. While Pd-catalyzed animation of 5-bromobenzofuran led to 5-benzofurylpiperizine 136 in 65% yield after deprotection, the corresponding reaction of 7-bromobenzofuran only gave 7-benzofurylpiperizine 137 in 20% yield. They speculated that either steric hindrance of the oxidative addition intermediate or the interaction between the oxygen lone pair and the metal center was responsible for the low yield. The debrominated benzofuran was the major by-product. 

Another theoretical study also showed that the third pathway (bl +b3+b4), 1,3 hydrogen shift, through a hydrido-alkynyl intermediate could compete with the 1,2 hydrogen shift pathway (bl+b2) when the metal center is electron-rich enough [29, 30]. Indeed several hydrido-alkynyl intermediates have been detected or even isolated during the q -l-alkyne-to-vinylidene rearrangement on electron-rich metal centers, such as Co(I), Rh(I) and Ir(I) [73-78]. The ab initio M P2 calculations by Wakatsuki, Koga and their coworkers on the transformation of the model complex RhCl(PH3)2(HC=CH) to the vinylidene form RhCl(PH3)2(C=CH2) indicated that the transformation proceeded via the oxidative addition intermediate RhCl(PH3)2(H) (C CH) [30]. 

Extensive studies by Amatore, Jutand, and co-workers have shed light on the structure and oxidative addition chemistry of a number of synthetically important palladium complexes [42], In particular, these workers have shown that the major species in a solution of Pd(dba)2 and BINAP is Pd(dba)BINAP and that oxidative addition of Phi to this complex generates (Bl-NAP)Pd(Ph)I [42d,43], In addition, it has been demonstrated that palladium halide complexes such as (PhjP jaryljPdCl do not dissociate the halide ligand in DMF solution [44], whereas the corresponding triflate complex is completely dissociated [44,45], As noted earlier, the nature of the oxidative addition intermediates defines two mechanistic pathways for the Heck reaction the neutral pathway for unsaturated halide substrates and the cationic pathway for unsaturated triflate substrates [2c-g,3,7-9]. Further, it is possible for halide substrates to be diverted to the cationic pathway by addition of Ag(I) orTh(I) salts [3], and it is possible to divert some triflate substrates to the neutral pathway by addition of halide additives [38]. Individual steps of these two pathways have recently received some scrutiny. 

Fig. 11. (A and B) Oxidative addition intermediates (pyrH)Pd(PPIi3)2Br and (pyr)Pd(PPh3)2r, (C D) Transmetallation intermediates l(pyrH)(R,R2C6H3)Pd(PPIi3)2 and (R,R2C6H3)Pd(PPIi3)2 (Ri = H or CH3 and R2 = H or CH3).

When cyclobutanone is treated under dihydrogen with a catalytic amount of a rhodium(I) complex containing a bidentate diphosphine ligand, the ring opened alcohol 89 is produced in goodyield [46,47]. The oxidative addition intermediate 87 is hydrogenated to give aldehyde 88, which is further reduced to the alcohol 89. 

The recognition that electron-rich late transition metals can activate C—H bonds via oxidative addition or by O-bond metathesis reactions poses a challenging mechanistic question. That is, for late transition metals with d-electron counts greater than zero, overall metathesis between M—R and R -H substrates to give M—R and R—H could proceed via concerted o-bond metathesis or through stepwise oxidative addition and reductive elimination sequences (e.g., see Scheme 11.29). If an oxidative addition intermediate is not observed, the experimental distinction between the two pathways is quite subtle and challenging. 

The double bond of certain sterically encumbered alkenylstannanes can undergo insertion into the oxidative addition intermediates leading to products of cine substitution (Scheme 1.7) [90], Mechanistic studies suggest the involvement of palladium carbenes 10 as intermediates, which are formed by evolution of intermediates 9 [90c,d]. 

Scheme 1.7 Insertion of the double bond of alkenylstannanes into the oxidative addition intermediates.

The key intermediates of the Suzuki-Miyaura reaction, species 13 and 14 corresponding to oxidative addition intermediates and species IS corresponding to transmetalation intermediates, were detected by ESI-MS in all the reactions for the first time in authentic reaction mixtures. Species 16 and 17, detected in most cases, were assigned to [ArPd(PPh3)2] and [Pd(PPh3)4] ", respectively, and were also observed in control experiments when the aryl halide was excluded from the reaction mixture. The authors postulate that intermediate 16 is a result of transmetalation with 17. This first study showed for the first time that the ESI-MS technique has great... 

In the case where Pd2(dba)3 was used as the catalyst, ESI-MS allowed oxidative addition intermediates 118 and diaryl Pd(II) transmetalated intermediates 119 to be... 

As depicted in Scheme 1.6 the oxidative addition intermediate adopts a trico-ordinated T-shaped geometry in contrast to the four coordinated square planar complexes formed, when less hindered phosphines are used (Scheme 1.7) [37]. 

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