Reductive eliminations

Reductive elimination of a zirconacycle to give a four-membered ring is very rare. Only one example has been reported in the case of a-alkynylated zirconacyclopentenes, as shown in Eq. 2.75 [54]. 

The formation of cyclobutadiene derivatives from zirconacydopentadienes can be accomplished as follows (Eq. 2.76) [55]. 

Monoiodination of a zirconacyclopentadiene with one equivalent of iodine followed by the addition of one equivalent of CuCl gives the dimer of the cyclobutadiene and the Diels—Alder product in the presence of methyl maleate. This indicates the formation of a l-iodo-l,3-dienyl copper compound and the subsequent elimination of Cul to give a cyclobutadiene equivalent. Direct reductive elimination of zirconacydopentadienes affording cyclobutadienes has not yet been observed. 

Reductive elimination is simply the reverse reaction of oxidative addition the formal oxidation state of the metal is reduced by two (or one in a bimetallic reaction), and the total electron count of the complex is reduced by two. While oxidative addition can also be observed for main group elements, this reaction is more typical of the transition elements in particular the electronegative, noble metals. In a catalytic cycle the two reactions occur pairwise. At one stage the oxidative addition occurs, followed by, for example, insertion reactions and then the cycle is completed by a reductive elimination of the product. Reductive 

Examples of reductive elimination are present in the literature mostly for group 10 metals, e.g. Fig. 4.28. As outlined above, the reaction can be induced by the addition of electron-withdrawing ligands in this case electron-poor alkenes are very effective as ligands. 

In practice the reactions are less straightforward them is shown in Fig. 4.28, and several other decomposition pathways are available (see a-elimination below). 

Reductive elimination is the reverse of oxidative addition. To illustrate this distinction, consider the following equilibrium  

The forward reaction involves formal oxidation of the metal, accompanied by an increase in coordination number it is an OA. The reverse reaction is an example of RE, which involves a decrease in both oxidation number and coordination number. 

RE reactions often involve elimination of molecules such as 

The products eliminated by these reactions may be important and useful organic compounds (R—H, R—R, R—X). In some cases, the organic fragments (R, R ) undergo rearrangement or other reactions while coordinated to the metal. Examples of this phenomenon will be discussed later in this chapter. 

As might be expected, the rates of RE reactions are also affected by ligand bulk. An example of this effect is shown in Table 14-2. The three cA-dimethyl complexes shown undergo RE following replacement of a phosphine ligand by a solvent molecule (solv)  

A reductive elimination is the reverse of an oxidative addition. The coordination sphere of the metal is diminished, an organic molecule or other structure is eliminated, and the metal is reduced (Eq. 12.27). The study of these reactions is typically done independently of oxidative addition, and therefore these reactions have their own structure-function relationships. 

Reductive eliminations do not always lead to stable metal products, because the organo-metallic complex is losing electrons and therefore is typically dropping below 18 electrons. These reactions are normally very fast in catalytic cycles, and therefore difficult to observe. Hence, the study of reductive eliminations has not been as extensive as that of oxidative additions. However, this reaction is certainly just as important as oxidative addition in catalysis, because it represents the manner in which organic products are often released from the metal center. 

In a reductive elimination reaction a dialkyl transition metal complex, symbolized by 19.43, decomposes into an alkane and a coordinatively unsaluratcd complex. The reaction is synthetically useful under catalytic and stoichiometric conditions  

Eisensiein and R. Hoffmann, J. Am. Chem. Soc., 103,4308 (1981) H. Fiijimoto and N. Koga, Tet. Letts., 4357 (1982) . sec also the related case of nucleophilic substitution on TT-allyl complexes, S. Sakaki, M. Nishikawa, and A. Oliyoshi,. /. Am. Chem. Soc., 102, 4062 (1980) and coordinated carbon monoxide, S. Nakamura and A. Dedieu, Theorct. Chim, Acta.(), 5%l (1982). 

Hofmann,/IChem., 89, 551 (1977) Habilitationschrift, Universitat Krlangcn-Nurnberg (1978). 

Collman and L. S. icg,c ns. Principles and Applications oj Organotransition Metal Chemistry, University Science Books. Mill Valley, CA (1980), pp. 177-258. 

Hoffmann in, P rontiers of Chemistry. K.. T. Laidler, editor, Pergamon Press, New York (1982), pp. 247-263. 

The kinetics of reductive elimination of hydrogen from iridium(i) have been investigated, mainly in chloroform solution  

The mechanism of reductive elimination of triphenylsilane from the d manganese(iii) hydride complex (23) by the action of triphenyl-phosphine, 

The synthesis of /ra j -[IrCl(CS)(PPh3)2] from iridium(i) and carbon disulphide involves oxidative addition followed by reductive elimination. The reaction of alkyl bromides (RBr) with [Fe(CO)4] involves oxidative addition to give (25), which isomerizes to (26). Further reactions of (26), including addition of a hydrogen to the metal, are followed by reductive 

The investigation of intramolecular rearrangements of organometallic derivatives of the transition metals by n.m.r. techniques has been reviewed. A new system of nomenclature for such processes has been proposed, and the application of matrix representations and group theory to these systems discussed. 

A theoretical study on the reductive elimination square-planar phosphine complexes cis-[Pd(CH3)2L2] and cis-[Pd(CH3)(Cl)L2] shows that the activation energies depend on the a-donating ability of L [354]. For bulky phosphine ligands, the steric effect is also significant. 

Formation of T-shaped intermediates from square-planar complexes greatly accelerates the reductive elimination of [Pd(L)2RR j complexes [61]. 

On the other hand, for a series of [Pd(L-L)Me2] with L-L = dppp (bis(l,3-diphenylphosphino)propane), dppf, and dppr (l,l -bis(diphenylphosphino)ruthe-nocene), the fastest elimination was observed for the ligand with the largest bite angle [357, 358]. This effect on the reductive elimination was also found by Hayashi et al. [359] and van Leeuwen [360] in the palladium-catalyzed cross-coupling reaction of Grignard reagents with aryl halides. 

An ESI-MS/MS study by CID (collision-induced dissociation) of the Suzuki-Miyaura reaction with palladium-diene catalysts showed that in this system, the reductive elimination step determines the rate of the whole process [361], 

A new Pd(0) -catalyzed carboiodination reaction of alkenes with aryl iodides, which generates a C-C and a C-I bond, involves a rate-determining reductive elimination step to form the C(sp ) - I bond, which is facilitated by bulky monophosphine ligands by preventing the formation of tetracoordinated intermediates [365]. 

A large number of examples of reductive elimination of NHC ligands and adjacent metal alkyl, acyl or hydride ligands have appeared. One critical point that should be remembered when considering these decomposition pathways is that while they may be observed under stoichiometric conditions, they are not necessarily an issue under catalytic conditions.This is because under catalytic conditions, the rates of the productive catalytic steps can be significantly accelerated relative to the decomposition reactions.  

When they occur under catalytically relevant conditions, these reactions can lead to a loss or decrease of activity, or enantioselectivity if chiral ligands are employed. Even when small amounts of decomposition are observed, this can be problematic if the non-NHC-containing catalyst is more active than the NHC-containing species. 

When 35 was treated with AgBp4 and butyl acrylate under stoichiometric conditions, decomposition products resulting from reductive elimination from all of the key catalytic intermediates were observed. The stability of the catalyst under the actual reaction conditions was much higher, and was attributed to faster rates of p-hydride elimination from the arylated acrylate at the elevated reaction temperatures, which removed at least two of the decomposition pathways. However, it should be noted that these decomposition pathways could be responsible for the generation of catalytically active Pd complexes containing one or zero carbene ligands in this or other systems. 

In addition to alkyl and aryl groups, allyl substituents were shown to undergo facile reductive elimination, in a process that was used for the generation of catalytically active Pd species from Pd precursors (see Section 3.5.3). Although they generally showed increased stability, chelating NHC ligands also underwent reductive elimination reactions.  

Nickel-containing carbene complexes are also known to undergo reductive elimination reactions. Depending on the nature of the carbene and the ancillary ligands, this reaction can dominate their chemistry. In a study on the use of 

For example, in the carbonylation reaction the acylmetal complex 52, formed by the insertion of CO, undergoes reductive elimination to give the carbonyl compound 53 as a product, and the catalytic species M(n) is regenerated 

The reductive elimination of A—B proceeds if A and B are mutually cis. In other words, reductive elimination is possible from cis complexes. If the groups to be 

The catalytic cycle of the Ni-catalysed dimerization of ethylene to give 1-butene (65) is explained by the insertion of ethylene to the nickel hydride 62 twice to form the ethyl complex 63 and the butyl complex 64. The elimination of /1-hydrogen gives 1-butene (65), and regenerates the Ni—H species 62. The reaction is chemoselective. Curiously, no further insertion of ethylene to 64 occurs. 

The carbonyl compounds 67 are formed by the elimination of /1-hydrogen from the metal alkoxides 66. 

The elimination of a-hydrogen is not general and observed only with limited numbers of metal complexes. The elimination of a-hydrogen from the methyl group in the dimethylmetal complex 68 generates the metal hydride 69 and a carbene that coordinates to the metal. Liberation of methane by the reductive elimination generates the carbene complex 70. Formation of carbene complexes of Mo and Wis a key step in alkene metathesis. The a-elimination is similar to the 1,2-hydride shift observed in organic reactions. 

Another termination step in a catalytic cycle is syn elimination of hydrogen from carbon at jS-position to Pd in alkylpalladium complexes to give rise to Pd hydride (H-Pd-X) and an alkene. This process is called either jS-hydride elimination or y3-hydrogen elimination . Most frequently used is -hydride elimination , because the y3-H is eliminated as the Pd-hydride (H-Pd-X). The proper and unambiguous term of this process is dehydropalladation in a cis maimer. This is somewhat similar to a El or E2 reaction in organic chemistry, althought it is anti elimination. 

Organic chemists, particularly synthetic organic chemists including myself, prefer to use arrows in mechanistic discussion of organic reactions to show formation and fission of bonds. When arrows are used, the direction of the arrow is important. An El or E2 reaction may be simply stated. Dehydropalladation may be explained similarly, because Pd-X is regarded as a leaving group. 

However, we must consider dehydropalladation from a different point of view, although it may cause some confusion. The f-H is eliminated as a hydride by 

Moreover, two important variations of the Julia reaction were reported during the 1990s. As both procedures afford the final alkene product directly from the sulfone and carbonyl components, they shall be discussed in greater detail in a subsequent part of this chapter (Section 3.5). In this section, attention is focused on the reductive desulfonylation of various 8-oxygenated sulfones and of vinyl sulfones. Insights into the advantages and disadvantages of the various protocols, some mechanistic considerations, and the stereochemical outcome of the reductive elimination step are provided. 

EtOH or i-PrOH NaBRi/HCl/NaBHj NaBH4, BF3OEt2 or A1C13 Zn, H20, NH3 

RE is the reverse of OA, whereby oxidation state, coordination number, and electron count all decrease, usually by two units. According to the principle of microscopic reversibility, the mechanistic pathways for RE are exactly the same as those for OA, only now in the reverse sense (this principle corresponds to the idea that the lowest pathway over a mountain chain must be the same regardless of the direction of travel). Equation 7.47 shows an example of RE from a platinum complex to give a silylalkyne. RE here likely goes through a concerted, three-centered transition state with both M-Si and M-C(alkynyl) bonds breaking and the new Si-C bond starting to form. 

The schematic reaction shown in equation 7.48 could be considered a RE, although the halide attacks the CH3 ligand in an SN2 manner to give the alkyl halide and reduced metal complex. We saw the reverse of this reaction in Section 7-2-2.100 

Most synthetically-useful reductive eliminations occur by a concerted three-centered process (the reverse of three-center OA) outlined in equation 7.49. 

For RE of d 16- e f/. v-dihydrocarbyl complexes of the type L M(R)(R ), where metals in the group 10 triad have been most studied, there are three reasonable mechanistic paths 104 

Loss of an L ligand (usually a phosphine) to give a 14-e complex (T-or Y-shaped), followed by concerted reductive elimination to form R-R (see preceding paragraph)  

Sammes synthesized /5-bulnesene by employing a sodium reduction of chloroether 248 to effect the ring opening of the bridging C-0 bond in a [3.2.1] oxabicyclic system, Eq. 152 [56]. 

Wender incorporated this strategy into the synthetic plan for the first total synthesis of phorbol, whereby intermediate 249 was subjected to lithium-iodine exchange to yield alkenol 250, Eq. 153 [199]. 

A recent example of a ring opening based on the same principle is found in a series of synthetic studies toward taxol, in which model compound 251 has an oxabicyclo[2.2.1]heptane moiety derived from furfuryl alcohol as the precursor for ring-C of the target [200]. The hydroxymethyl group in 251 was converted to the iodide, and treatment with freshly activated zinc resulted in ring opening to the tricyclic system 252, Eq. 154. 

Samarium iodide has been used to reduce sulfonylated oxabicydic substrates leading to the elimination of the ft oxygen moiety. Molander used this strategy for the synthesis of substituted cycloheptenes and cyclooctenes, Eq. 155 [81]. 

NaBH EtOH or z-PrOH NaBH4/HCl/NaBH4 NaBH BF3 OEt2 or Ald3 Zn, H20, NH3 

Allyl complexes may react with ligands which stabilize low oxidation states with the formation of dienes and transition metal complexes. 

Reductive elimination also occurs in the case of unstable hydrido allyl complexes. 

Such reactions have considerable significance in oligomerization and diene polymerization  

This predicts a linear relationship between the reciprocal of fcobs and [bipy] and this is observed experimentally. 

The trimethylene moiety can also be displaced from [Pt(X)2(C3He)(L)2] by other ligands, particularly those with high trans effects, and generally cyclopropane is evolved. For the reaction 

The rate of the first type follows Eq. 6.21 (Q = H-I-), when protonation is the slow step. Switching from HX to HBF4 provides a test, because an intermediate, [LnMH]BF4, is then expected only the first step of Eq. 6.19 is viable, BF4 being noncoordinating. 

The rate of the second type (Eq. 6.20) usually follows Eq. 6.21 (Q = X ), suggesting that anion addition is the slow step. If so, this step should occur with LiCl alone, but no reaction is expected with HBF4 alone. 

Thermodynamics dictates if OA or RE will dominate, for example, Eq.6. 22 typically goes to the right for X = alkyl or aryl and Y = H, 

TABLE 6.3 Oxidative Addition One Reaction but Many Mechanisms 

Mechanistic Class Typical Metal, M Typical Substrate, A-B Config. AOS, Ae Remarks 

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The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

The most useful reaction of Pd is a catalytic reaction, which can be carried out with only a small amount of expensive Pd compounds. The catalytic cycle for the Pd(0) catalyst, which is understood by the combination of the aforementioned reactions, is possible by reductive elimination to generate Pd(0), The Pd(0) thus generated undergoes oxidative addition and starts another catalytic cycle. A Pd(0) catalytic species is also regenerated by /3-elimination to form Pd—H which is followed by the insertion of the alkene to start the new catalytic cycle. These relationships can be expressed as shown. 

Interestingly, some nucleophiles attack the central carbon of the 7r-allyl system to form a palladacyclobutane 316 and its reductive elimination gives... 

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. 

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]. 

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

Interesting formation of the fulvene 422 takes place by the reaction of the alkenyl bromide 421 with a disubstituted alkyne[288]. The indenone 425 is prepared by the reaction of o-iodobenzaldehyde (423) with internal alkyne. The intermediate 424 is formed by oxidative addition of the C—H bond of the aldehyde and its reductive elimination affords the enone 425(289,290]. 

Formation of ketones. Ketones can be prepared by the carbonylation of halides and pseudo-halides in the presence of various organometallic compounds of Zn, B, Al, Sn, Si, and Hg, and other carbon nucleophiles, which attack acylpalladium intermediates (transmetallation and reductive elimination). 

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 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]. 

The transmetallation of the siloxycyclopropane 751 with the aryl- or alke-nylpalladium 752 generates the Pd homoenolate 753. and subsequent reductive elimination gives the /3-aryl or alkenyl ketone 754[618]. It should be noted that the Pd homoenolate 753 generated in this reaction undergoes reductive elimination without d-elimination. 

A trialkylsilyl group can be introduced into aryl or alkenyl groups using hexaalkyidisilanes. The Si—Si bond is cleaved with a Pd catalyst, and trans-metallation and reductive elimination afford the silylated products. In this way, 1,2-bis-silylethylene 761 is prepared from 1,2-dichloroethylene (760)[625,626], The facile reaction of (Me3Si)2 to give 762 proceeds at room temperature in the presence of fluoride anion[627]. Alkenyl- and arylsilanes are prepared by the reaction of (Me3Si)3Al (763)[628],... 

Hydrogenolysis of aryl and alkenyl halides and triflates proceeds by the treatment with various hydride sources. The reaction can be explained by the transmetallation with hydride to form palladium hydride, which undergoes reductive elimination. Several boro hydrides are used for this purpose[680], Deuteration of aromatic rings is possible by the reaction of aryl chlorides with NaBD4681]. 

The reaction of benzoyl chloride with (Me3Si)2 affords benzoyltrimethylsi-lane (878)[626,749,750]. Hexamethyldigermane behaves similarly. The siloxy-cyclopropane 879 forms the Pd homoenolate of a ketone and reacts with an acyl halide to form,880. The 1,4-diketone 881 is obtained by reductive elimination of 880 without undergoing elimination of /7-hydrogen[751]. 

The stereochemistry of the Pd-catalyzed allylation of nucleophiles has been studied extensively[5,l8-20]. In the first step, 7r-allylpalladium complex formation by the attack of Pd(0) on an allylic part proceeds by inversion (anti attack). Then subsequent reaction of soft carbon nucleophiles, N- and 0-nucleophiles proceeds by inversion to give 1. Thus overall retention is observed. On the other hand, the reaction of hard carbon nucleophiles of organometallic compounds proceeds via transmetallation, which affords 2 by retention, and reductive elimination affords the final product 3. Thus the overall inversion is observed in this case[21,22]. 

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

Diphenylketene (253) reacts with allyl carbonate or acetate to give the a-allylated ester 255 at 0 °C in DMF, The reaction proceeds via the intermediate 254 formed by the insertion of the C = C bond of the ketene into 7r-allylpalla-dium, followed by reductive elimination. Depending on the reaction conditions, the decarbonylation and elimination of h-hydrogen take place in benzene at 25 °C to afford the conjugated diene 256(155]. 

It is known that tr-allylpalladium acetate is converted into allyl acetate by reductive elimination when it is treated with CO[242,243]. For this reason, the carbonylation of allylic acetates themselves is difficult. The allylic acetate 386 is carbonylated in the presence of NaBr (20-50 mol%) under severe conditions, probably via allylic bromides[244]. However, the carbonylation of 5-phenyl-2,4-pentadienyl acetate (387) was carried out in the presence of EtiN without using NaBr at 100 °C to yield methyl 6-phenyl-3,5-hexadienoate (388)[245J. The dicarbonylation of l,4-diacetoxy-2-butene to form the 3-hexenedioate also proceeds by using tetrabutylphosphonium chloride as a ligand in 49% yield[246]. 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

The steroidal 4/3-acetoxy-5/J,6/l/-epoxy-2-en-l-one system 546 was converted at room temperature into the 6/3-hydroxy-2,4-dien-l-one 547 by reductive elimination of the vicinal oxygen function, and the reaction has been applied to the synthesis of withanolide[352]. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

An active catalytic species in the dimerization reaction is Pd(0) complex, which forms the bis-7r-allylpalladium complex 3, The formation of 1,3,7-octa-triene (7) is understood by the elimination of/5-hydrogen from the intermediate complex 1 to give 4 and its reductive elimination. In telomer formation, a nucleophile reacts with butadiene to form the dimeric telomers in which the nucleophile is introduced mainly at the terminal position to form the 1-substituted 2,7-octadiene 5. As a minor product, the isomeric 3-substituted 1,7-octadiene 6 is formed[13,14]. The dimerization carried out in MeOD produces l-methoxy-6-deuterio-2,7-octadiene (10) as a main product 15]. This result suggests that the telomers are formed by the 1,6- and 3,6-additions of MeO and D to the intermediate complexes I and 2. 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

The unsaturated c.vo-enol lactone 17 is obtained by the coupling of propargylic acetate with 4-pentynoic acid in the presence of KBr using tri(2-furyl)-phosphine (TFP) as a ligand. The reaction is explained by the oxypalladation of the triple bond of 4-pentynoic acid with the ailenyipailadium and the carbox-ylate as shown by 16, followed by reductive elimination to afford the lactone 17. The ( -alkene bond is formed because the oxypalladation is tnins addition[8]. 

Tandem cyclization/3-substitution can be achieved starting with o-(trifluoro-acetamido)phenylacetylenes. Cyclization and coupling with cycloalkenyl trif-lates can be done with Pd(PPh3)4 as the catalyst[9]. The Pd presumably cycles between the (0) and (II) oxidation levels by oxidative addition with the triflate and the reductive elimination which completes the 3-alkenylation. The N-protecting group is removed by solvolysis under the reaction conditions, 3-Aryl groups can also be introduced using aryl iodides[9]. 

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