Oxidative addition facile

During a study of Pd/P(t-Bu)2Me-catalyzed Suzuki cross-couplings of primary alkyl bromides with boronic acids, Fu examined the stoichiometric reaction of Pd(P(t-Bu)2Me)2 with an alkyl bromide (Eq. 16) [17]. Interestingly, not only was oxidative addition facile at 0 °C, but the adduct (4) could be isolated in excellent yield and even crystallographically characterized. 

TT-Aliylpalladium chloride reacts with a soft carbon nucleophile such as mal-onate and acetoacetate in DMSO as a coordinating solvent, and facile carbon-carbon bond formation takes place[l2,265], This reaction constitutes the basis of both stoichiometric and catalytic 7r-allylpalladium chemistry. Depending on the way in which 7r-allylpalladium complexes are prepared, the reaction becomes stoichiometric or catalytic. Preparation of the 7r-allylpalladium complexes 298 by the oxidative addition of Pd(0) to various allylic compounds (esters, carbonates etc.), and their reactions with nucleophiles, are catalytic, because Pd(0) is regenerated after the reaction with the nucleophile, and reacts again with allylic compounds. These catalytic reactions are treated in Chapter 4, Section 2. On the other hand, the preparation of the 7r-allyl complexes 299 from alkenes requires Pd(II) salts. The subsequent reaction with the nucleophile forms Pd(0). The whole process consumes Pd(ll), and ends as a stoichiometric process, because the in situ reoxidation of Pd(0) is hardly attainable. These stoichiometric reactions are treated in this section. 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

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. 

Facile oxidative addition is possible with iodides and bromides. The reactions of iodides can be carried out even in the absence of a phosphine ligand,... 

Aromatic acyl halides and sulfonyl halides undergo oxidative addition, followed by facile elimination of CO and SO2 to form arylpalladium complexes. Benzenediazonium salts are the most reactive source of arylpalladium complexes. 

Oxidative addition of alkyl halides to Pd(0) is slow. Furthermore, alkyl-Pd complexes, formed by the oxidative addition of alkyl halides, undergo facile elimination of /3-hydrogen and the reaction stops at this stage without undergoing insertion or transmetallation. Although not many examples are available, alkynyl iodides react with Pd(0) to form alkynylpalladium complexes. 

Oxidative addition of the sulfonyl chlorides 144 is followed by facile generation of SO2 to form arylpalladium complexes which undergo alkene inser-tion[112,113]. 

The Pd-catalyzed elimination of the mesylate 909 at an anomeric center, although it is a saturated pseudo-halide, under mild conditions is explained by the facile oxidative addition to the mesylate C—O bond, followed by elimination of /3-hydrogen to give the enol ether 910[767],... 

It IS likely that the syn selectivity exhibited in cycloadditions of fluoroallene IS due to electrostatic interactions [23 25] As in the case of difluoroallene the reactions of fluoroallene with diazoalkanes and nitrile oxides are facile, but such reactions, other than that shown in equation 18, are neither regio nor stereospeutic [23, 25] Indeed, the addition of phenylnitrile oxide to fluoroallene occurs with preferential anti addition for both regioisomenc products (equation 20)... 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

The reaction proceeds at room temperature and is rationalized invoking oxidative addition of a Pd(0) species upon the allylic C - O bond of 67, followed by decarboxylation to form an oxapalladacyclopentane intermediate 66 (Pd in place of Ni), which undergoes a facile b-C elimination to finally give an co-dienyl aldehyde 68 (Scheme 17). Recently, it has been revealed that a combination of Ni(cod)2 and a phosphine ligand also catalyzes the same... 

There are many biomimetic model Co complexes of the cobalamins.1149 The primary criterion for an effective B12 model has been that the complex may be reduced to the monovalent state and undergo facile oxidative addition to generate a stable alkylcobalt(III) complex. The two main classes of B12 model complexes that have been investigated are Co oximes and Schiff base complexes. The former class shares the planar CoN4 array of their biological analogs whereas the majority of effective Schiff base Bi2 model complexes comprise equatorial czj-N202 donor sets. 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

The observation of extremely facile formation of an acetyl complex and the finding that oxidative addition is the rate-determining step are almost certainly related to the high selectivity observed in the reaction. Thus, the extremely short lifetime of any CH3—Rh species makes it unlikely that it would be reacted off to methane in the presence of hydrogen (and/or metal hydrides). 

The Ge(TMTAA) complex and the well known Sn(TMTAA) complex undergo facile oxidative addition reactions and reverse ylide formation with Mel and C6F5I because of the reactive M(II) (M = Sn, Ge) lone pair of electrons. In case of the oxidation with Mel it was assumed that, in solution, an ionic-covalent equilibrium exists (equation 48)95. 

In the second instance, two approaches seem to be worthy of special note. The synthetic utility of elemental phosphorus based on it acting as a radical trap appears to be quite valuable, but additional effort is required to determine the variability of the source of the organic free radicals. (Is there some other, more efficacious, source of organic free radicals that works better with this system than acylated iV-hydroxy-2-pyridones ) The other approach that appears ripe for development is the hydrolysis/elimination with "phosphorates" derived from the oxidative addition of white phosphorus to alkenes. We look forward to the continued development of such facile approaches toward the preparation of fundamental phosphonic acids. 

Since nucleophilic addition to a metal-coordinated alkene generates a cr-metal species bonded to an -hybridized carbon, facile 3-H elimination may then ensue. An important example of pertinence to this mechanism is the Wacker reaction, in which alkenes are converted into carbonyl compounds by the oxidative addition of water (Equation (108)), typically in the presence of a Pd(n) catalyst and a stoichiometric reoxidant.399 When an alcohol is employed as the nucleophile instead, the reaction produces a vinyl or allylic ether as the product, thus accomplishing an etherification process. 

For the oxidative addition pathway, however, it is not obvious why the C-H bond cleavage reaction should be more facile if the hydrocarbon first binds in the coordination sphere of the metal (Scheme 5, c). One argument could be that the equilibrium between the Pt(II) alkane complex and the five-coordinate Pt(IV) alkyl hydride has an intrinsically low activation barrier. Insight into this question together with detailed information about the mechanisms of these Pt(II) a-complex/Pt(IV) alkyl hydride interconversions has been gained via detailed studies of reductive elimination reactions from Pt(IV), as discussed below. 

Recently, the groups of Fu and Buchwald have coupled aryl chlorides with arylboronic acids [34, 35]. The methodology may be amenable to large-scale synthesis because organic chlorides are less expensive and more readily available than other organic halides. Under conventional Suzuki conditions, chlorobenzene is virtually inert because of its reluctance to oxidatively add to Pd(0). However, in the presence of sterically hindered, electron-rich phosphine ligands [e.g., P(f-Bu)3 or tricyclohexylphosphine], enhanced reactivity is acquired presumably because the oxidative addition of an aryl chloride is more facile with a more electron-rich palladium complex. For... 

Carbonylation of chloropyridines is less straightforward than that of bromo- and iodopyridines. The a and y positions are sufficiently activated to undergo facile oxidative addition to Pd(0), whereas the reactivity is greatly diminished at (3 positions. Thus, 2,6-dichloropyridine was converted to the corresponding dimethyl ester in good yield under normal Pd-catalyzed carbonylation conditions [152], and 2-chloropyridine was readily carbonylated to furnish methyl 2-pyridinecarboxylate (191), but 3-chloropyridine gave no carbonylated products under the same conditions [153,154],... 

The transformation of RhCl(PH3)2(HC=CH) to RhCl(PH3)2(C=CH2) has been calculated (MP2) to be exothermic by 7.8 kcal.mol"1. The intraligand 1,2-hydrogen shift mechanism found in the Ru11 system is not relevant to the present rhodium case. Starting from a T 2 C=C complex, both systems give a metal-( T]2 C-H) species in a subsequent step. In the case of the d6 Ru system this ri2 C-H complex is an intermediate. In contrast, the T 2 C-H coordinated state is a transition state in the d8 Rh1 system, the oxidative addition being a very facile process. 

These authors propose as the mechanism for this reaction a reversible oxidative addition of the aryl-phosphido fragments to a low valent rhodium species. A facile aryl exchange has been described for complexes Pd(PPh3)2(C6H4CH3)I. The authors [35] suggest a pathway involving oxidative additions and reductive eliminations. The mechanism outlined below, however, can also explain the results of these two studies. 

Oxidative addition of the carbon-carbon bond of cyclopropanes to zero-valent cobalt species is not in general a facile process. It is assumed that in this reaction the alkynyl part of the molecule works as an anchor for the cobalt carbonyl, which enables an efficient insertion of the cobalt moiety into the proximal carbon-carbon bond of the cyclopropane to proceed. It therefore became a matter of interest to see whether direct connection of the alkynyl part with the cyclopropanol is essential or not for this type of reaction. 

Whether this condition can be fulfiUed depends on the electron count of the metal, and the stereochemistry of the elimination. For instance, in m-elimination from octahedral d , or square planar d , systems, metal ndipP -y ) acts as acceptor, and this should be a facile process ( e Fip. 1, 2). For /rans-elimination, on tiie other hand, the lowest empty orbital of correct symmetry is (n + l)p. Such elimination Kerns energetically less Ukely, unless a non-concerted pathway (such as successive anionic and cationic loss) is available. The same arguments apply, of course, to oxidative additions. It foUows that the many known cases of traits oxidative addition to square planar t/ systems are unlikely to take place by a concerted mechanism, and this conclusion is now generally accepted There are special complexities in reductive elimination from trigonal systems, and these are discussed furdier in Part III. 

The transformations of 136 proceed cleanly upon treatment with a catalytic amount of Pd(PPh3)4, in the presence of triethylamine and molecular sieve (MS) 4 A it apparently is initiated by oxidative addition of the N(sp )-0 bond of 136 to the Pd(0) complex, and this is succeeded by two or even three intramolecular carbopalladations followed by / -hydride elimination. This Heck-type reaction is not affected by the configuration of the oxime derivatives probably due to a facile enough if/Z-isomerization of the alkylideneaminopalladium intermediate. 

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