Palladium transition state

In the lower section of Table 5, the energy characteristics of the palladium transition states are collected. One can notice that the Pd activation barriers are substantially lower than the corresponding barriers for hydration of the Pt(II) complexes. The average difference between the corresponding barriers is about 10 kcal/mol. The smallest difference of 6.2 kcal/mol occurs in the second chloride replacement of the trans-Pt(NH3)2Cl(OH) structure. 

The successful implementation of this strategy is shown in Scheme 4. In the central double cyclization step, the combined action of palladium(n) acetate (10 mol %), triphenylphosphine (20 mol %), and silver carbonate (2 equiv.) on trienyl iodide 16 in refluxing THF results in the formation of tricycle 20 (ca. 83 % yield). Compound 20 is the only product formed in this spectacular transformation. It is noteworthy that the stereochemical course of the initial insertion (see 17—>18) is guided by an equatorially disposed /-butyldimethylsilyl ether at C-6 in a transition state having a preferred eclipsed orientation of the C-Pd a bond and the exocyclic double bond (see 17). Insertion of the trisubstituted cycloheptene double bond into the C-Pd bond in 18 then gives a new organopal-... 

Figure 6.38. Potential energy diagram for the hydrogenation of ethylene to the ethyl (C2H5) intermediate on a palladium(m) surface. The zero of energy has been set at that of an adsorbed H atom, (a) Situation at low coverage ethylene adsorbed in the relatively stable di-cr bonded mode, in which the two carbon atoms bind to two metal atoms. In the three-centered transition state, hydrogen and carbon bind to the same metal atom, which leads to a considerable increase in the energy...

They demonstrated that electron-deficient R groups and electron-rich R substituents at S accelerated the reductive elimination. They proposed 123 (Lj = DPPE, R = Ph, R = Ar) as a transition state, where R acts as an electrophile and thiolate as a nucleophile. The Hammet plot for the reductive elimination showed that the resonance effect of the substituent in R determines the inductive effect of the R group, and the effect in SR showed an acceptable linear relationship with the standard o-values. The relative rate for sulfide elimination as a function of the hybrid valence configuration of the carbon center bonded to palladium followed the trend sp > sp spl... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Insertion of ethylene into the Ni-C bond in 3a leads to the alkyl complex 4a via the transition state TS[3a-4a] with a barrier [13a] of 17.5 kcal/mol relative to 3a It is worth to note that in TS[3a-4a both ethylene and the a-carbon of the growing (propyl) chain are situated in the N-Ni-N plane. For the corresponding palladium complex the insertion barrier [13c] is somewhat higher at 19.9 kcal/mol. 

For entries 3-5 the increase in molecular weight observed can be assigned to the increase in the rate of insertion and the rate of termination remains practically the same. An increase of the rate of polymerisation with the steric bulk of the ligand is usually ascribed to the destabilisation of the alkene adduct while the energy of the transition state remains the same. As a chain transfer reaction presumably P-hydride elimination takes place or traces of water might be chain transfer agents. Chain transfer does occur, because a Schulz-Flory molecular weight distribution is found (PDI 2, see Table 12.2). Shorter chains are obtained with a polar ortho substituent (OMe, entry 2) and in methanol as the solvent, albeit that most palladium is inactive in the latter case. 

Furukawa and co-workers (368,369) succeeded in applying the softer dicationic Pd-BINAP 260 as a catalyst for the 1,3-dipolar cycloaddition between 225 and 241a (Scheme 12.82). In most cases, mixtures of endo-243 and exo-243 were obtained, however, enantioselectives of up to 93% ee were observed for reactions of some derivatives of 225. A transition state structure has been proposed to account for the high selectivities obtained for some of the substrates (368). In the structure shown in Scheme 12.82, the two phosphorous atoms of the Tol-BINAP ligand and the two carbonyl oxygens of the crotonoyl oxazolidinone are arranged in a square-planar fashion around the palladium center. This leaves the ii-face of the alkene available for the cycloaddition reaction, while the re-face is shielded by one of the Tol-BINAP tolyl groups. 

Benzamido allylic acetates 242 and 243 undergo palladium-catalyzed cychza-tion to oxazolines. Excellent yields and very high diastereoselectivity is observed for the conversion of several acyclic primary and secondary benzamido aUyhc acetates to tran -5-vinyl substituted oxazolines 244. The diastereoselectivity of the reaction is determined by the the steric interactions between the R group and the hydrogen of the 7i-allylpalladium complex in the transition state, trans-Oxazolines are obtained since transition state A is favored over transition state B (Scheme 8.66). 

Oxazoline formation from 5-vinyloxazolidinones promoted by palladium (0) is also known. Oxidative insertion of palladium with loss of CO2 results in a pair of equihbrating 71-allyl palladium complexes. The stereochemistry of the vinyl group is therefore not important. Ring closure from the thermodynamically more stable transition state accounts for the trans-isomer as the major product. Depending on the exact substitution, diastereoselectivities ranging from 2.5 1 to 16 1 can be obtained (Scheme 8.68). 

These palladium- or nickel-catalyzed reactions are radical reactions leading to an organometallic product. By using a precursor such as 37 as a 1 1 mixture of diastereoisomers, the palladium-catalyzed cyclization provides in a stereoconvergent way the cyclopentylmethylzinc derivative 38 which, after allylation, produces the unsaturated ester 39 in 71% yield". The intermediate radical cyclizes via a transition state A where all the substituents are in an equatorial position. Interestingly, the analogous reaction using Ni(acac)2 as a catalyst allows the preparation of heterocyclic compounds such as 40. The... 

Kinetic analysis of the palladium catalyzed acylation reaction of 1 (R = i-Pr) and 23 indicates that the rate does not depend on the bulk of the trialkylsilyl substituent. Since the rate limiting step of this reaction is the interaction of a coordinatively unsaturated acylpalladium chloride with the cyclopropane (Cf. Eq. 59), the observed independence can reasonably be taken as an evidence that the Si—O bond remains intact in the transition state [56], Semiquantitative data on the cleavage of I (R = i-Pr) and 23 with ZnCl2 in ether, Eq. (13), led to the same conclusion [27]. 

Substituted derivatives with bulky groups, however, are reduced at higher rates and with opposite selectivity on palladium [Eq. (11.18)]. This was explained by the partial release of peri strain in the transition state during hydrogenation. 

The square planar palladium complexes which give values of m of ca. 0.4 (Table 1) are known to react via a mainly associative mechanism so that the values of m are taken to indicate that Pd—Cl bond cleavage and leaving Cl solvation were both important in determining the reactivity trend for these complexes, i.e. there is a greater degree of M—Cl bond breaking in the transition state of palladium compared with cobalt. 

The situation is different with monophosphines as far as the prevailing isomer is concerned. In fact it appears (see Table III) that the branched isomer prevails over the linear one (17), contrary to the predictions of the model formulated for the palladium-(-)DIOP catalytic system. The available experimental data are not sufficient to allow us to formulate a model for the transition state in the case of palladium monophosphine-catalytic systems. To attempt a preliminary explanation of the isomeric composition found, we suggest a transition state having a geometry approximating a square pyramid in which the olefinic bond of the substrate interacts with three and not with four substituents (see Figures 2a and 2a ). 

An efficient aqueous phase Suzuki-Miyaura reaction of activated aryl chlorides with aryl boronic acids has been reported. The method uses a new D-glucosamine-based dicyclohexylarylphosphine ligand for the palladium catalyst and works well with nitro-and cyano-activated chlorides.32 The aryl fluoride bond has been considered inert to palladium-catalysed substitution reactions. However, a computational study, backed up by experiment, shows that the presence of a carboxylate group ortho to fluorine will allow reaction both with phenylboronic acids in a Suzuki-type reaction and with organotin reagents in a Stille-type reaction the presence of the adjacent oxyanion stabilizes the transition state.33... 

Pd-catalysed chelate-directed acetoxylation of meta -substituted arenes has been studied.61 Many substituted groups are tolerated by this process and the reaction shows a high degree of regioselectivity for the less sterically hindered ortfto-position. For example, 2-(3-nitrophenyl)pyridine forms 2-(2-acetoxy-3-nitrophenyl)pyridine. Finally, density functional calculations62 on the palladium acetate-promoted cyclomet-allation of dimethylbenzylamine suggest that reaction occurs via an agostic C-H complex rather than a Wheland intermediate. An intramolecular H-transfer to a coordinated acetate via a six-membered transition state follows. 

A palladium(0)-catalysed cascade cyclization-Suzuki coupling reaction of various 1,6-enynes (72) with ArB(OH)2 has been developed as a new approach to the synthesis of stereodefined a-arylmethylene-y-butyrolactones, lactams, multifunctional tetrahy-drofurans, pyrrolidines, and cyclopentanes (73) (X = O, H2, Y = O, CH2, NR3). A 7r-allylpalladium intermediate and a chair-like transition state were suggested to account for the stereochemistry of this reaction.86... 

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