Benzene reductive elimination

While all of the substrates discussed above are not shown in Fig. 2, the same analysis can be performed with all of them (alkynes, substituted methanes). One caveat that we encountered was that many of these substituted derivatives proved to be very stable. Loss of alkane from the n-pentyl hydride complex has a half-hfe of about an hour at 25°C. Methane loss from 3 has a half-life of about 5 h. Loss of benzene from 2, however, is extremely slow (months), and therefore, the rate of benzene reductive elimination at 25°C was determined by extrapolation from the rate at higher temperatures. The Eyring plot of hi( /T) vs. 1/T gave activation parameters for reductive elimination of benzene A// = 37.8 (1.1) kcal/mol and = 23 (3) e.u., which can be used to calculate the rate at other temperatures. As mentioned above, the substituted derivatives are much more stable. Reductive elimination of the alkynyl hydrides was examined at lOO C, as was the elimination of many of the substituted methyl derivatives. In these cases, the rate of benzene elimination was calculated from the Eyring parameters at the same temperature as that where the rate of reductive elimination was measured, so that the barriers could be directly compared as in Fig. 2. The determinatimi of AG° for all substrates allows Eq. 7 to be used to determine relative metal-carbon bond strengths for these compounds. Table 1 summarizes these data, giving A AG, AG°, and Drei(Rh-C) for all substrates. 

As with the other hgands, reductive ehmination studies were carried out in CgDe solvent to generate hydrocarbon and Tp Rh[P(OMe)3](CgD5)D 9-d. The eliminations were carried out at temperatures between 20 and 140°C. To compare these elimination barriers with those of benzene, reductive elimination of CeHs from 9 was carried out at 70-100°C and activation parameters measured for the reductive elimination. An Eyring plot gave A// = 30.7(6) kcal/mol and A5 = 10.3(3) e.u. and permitted comparison of barriers in Fig. 1 at the same temperature. Table 3 summarizes the barrier heights measured and the temperature at which they were measured. 

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]. 

Along similar lines, Schwartz and Gell later reported that tertiary phosphines would also induce reductive elimination in bis(i7-cyclopenta-dienyl) (cyclohexylmethyl) (hydrido)zirconium resulting in high yields of zirconocene bis(phosphine) complexes (53-55). Carbon monoxide was found to readily react with a benzene solution of Cp2Zr(PMePh2)2... 

By using hydrogen at high pressure, M.A. Green et al. were able to show that the first step in the photolysis of 0sH,L3 (L PMe Ph) is the reductive elimination of H. The 16-electron intermediate can react with excess phosphine, or can dimerize, or can exchange hydrogen with the benzene solvent /46/. 

These compounds contain a furan ring fused to a benzene moiety in the 2,3-position. This synthesis was also described by Flynn et al. [73] and is shown in Scheme 25 involved the coupling of 2-iodo-5-methoxyphenol 104, 4-methoxyphenylethyne 105 to form the intermediate o-alkynylphenolate 106. Aryl iodide 107 was added to the phenolate in DMSO with heat. Oxidative addition, palladium(II)-induced cyclization and reductive elimination resulted in the product 108 with an 88% yield. 

Further insight into the carbon-oxygen reductive elimination from Pt(IV) and the involvement of five-coordinate Pt(IV) intermediates has been provided recently. The first direct observation of high-yield C-0 reductive elimination from Pt(IV) was described and studied in detail (50,51). Carbon-oxygen coupling to form methyl carboxylates and methyl aryl ethers was observed upon thermolysis of the Pt(IV) complexes ( P2 )PtMe3(OR) ( P2 =bis(diphenylphosphino)ethane or o-bis(diphenyl-phosphino)benzene OR=carboxylate, aryl oxide). As shown in Scheme 47, competitive C-C reductive elimination to form ethane was also observed. 

The proposed reaction mechanism is as follows (Scheme 16.83). Zinc metal reduces Ni(II) species to Ni(0). A nickelacyclopentadiene may be produced via coordination of two molecules of propiolates and regioselective head-to-head oxidative cyclometallation. Coordination and subsequent insertion of an allene into the Ni(II)-carbon bond give rise to a nickelacycloheptadiene intermediate. Finally, a benzene derivative is produced via reductive elimination followed by isomerization. 

Murakami and colleagues132 studied the Diels-Alder reactions of vinylallenes with alkynes catalyzed by a rhodium complex. When a vinylallene lacking substituents at the vinylic terminus was reacted with a terminal alkyne, 1,3,5-trisubstituted benzenes were obtained, the reaction between vinylallene 197 and 1-hexyne (198) being a representative example (equation 55). The reaction was proposed to proceed via a rhodacycle which afforded the primary Diels-Alder adduct via reductive elimination. Aromatization via isomerization of the exocyclic double bond led to the isolation of 199. 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

This sequence of events may be illustrated by the homogeneous hydrogenation of ethylene in (say) benzene solution by Wilkinson s catalyst, RhCl(PPh3)3 (Ph = phenyl, CeH5 omitted for clarity in cycle 18.10). In that square-planar complex, the central rhodium atom is stabilized in the oxidation state I by acceptance of excess electron density into the 3d orbitals of the triphenylphosphane ligands but is readily oxidized to rhodium (III), which is preferentially six coordinate. Thus, we have a typical candidate for a catalytic cycle of oxidative addition and subsequent reductive elimination ... 

Me3SiSiMe3 reacts similarly with benzene and toluene to yield the corresponding trimethylsilyl derivatives. The reductive elimination of arylsilane from the arylsi-lylrhodium intermediate was postulated. 

Similar octahedral facial silyl methyl hydride complexes of the type IrL3H(SiR3)Me have been shown to induce competitive C—H/Si—C reductive elimination depending on the electronic properties of the silyl ligand, thus affording a novel example of a metallation of silyl ligands or the metallation of the sp3C—H bond of the ethyl moiety when R = OEt. For the complex with R = Et, mixtures of different complexes are formed by the thermolysis with benzene (Scheme 32)198,199. 

Similar competitive reductive elimination of C—H and Si—H bonds have been observed for the complex IrH(SiHPh2)(Mes)(CO)(dppe) in benzene, leading to the iridium complexes Ir(Mes)(CO)(dppe) and Ir(SiHPh2)(CO)(dppe) following first-order kinetics for both eliminations200. 

The fluoride ion-induced reductive / -elimination makes it possible to generate highly strained olefins [Eq. (31)]. /J-Silyl A3-iodane 40 generates five-mem-bered cumulene with remarkable reactivity at room temperature and affords Diels-Alder adduct 40a (7 % yield) by the reaction with benzene [63]. 

Aryl-A3-iodanes bearing an electron-deficient alkyl ligand such as aryl(sul-fonylmethyl)-A3-iodanes (Section 3.2.7) and aryl(perfluoroalkyl)-A3-iodanes are relatively stable. A series of (perfluoroalkyl)phenyl-A3-iodanes 96 were synthesized in good yields by treating bis(trifluoroacetoxy)-A3-iodanes with benzene in the presence of triflic acid [47]. The AModanes 96 transfer the perfluoroalkyl groups to a variety of nucleophiles with reductive elimination of iodobenzene. The nucleophiles involve Grignard reagents, alkyllithiums, enolate anions, alkenes, alkynes, trimethylsilyl enol ethers, arenes, phenols, and thiols. In these reactions, the AModane 96 serves as a source of the perfluoroalkyl cation and, in... 

The other major dehalogenation pathway involves elimination of two halogens, leaving behind a pair of electrons that usually goes to form a carbon-carbon double bond. Where the pathway involves halogens on adjacent carbons, it is known as vicinal dehalogenation or reductive -elimination. The major pathway for reductive transformation of lindane involves vicinal dehalogenation, which can proceed by steps all the way to benzene (28). Recently, data has shown that this pathway not only can convert alkanes to alkenes, but can produce alkynes from dihaloalkenes (29). 

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from... 

Vaska s complex trans-IrCl(CO)(PPh ) has served as an important model for mechanistic investigation of catalytically relevant reactions such as the oxidative addition and reductive elimination of small molecules(15). The latter processes have also been the subject of some photochemical investigation. For example, the reductive elimination of H2 depicted in Equation 5, which is a relatively slow thermally activated process (k = 3.8 x 10- s l in 25° benzene solution (15)), has been shown to occur readily when the dihydride complex was subjected to continuous photolysis with 366 nm light(16). However, Vaska s compound itself was reported to be... 

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