Allylic hydride complex

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

The isomerization mechanism is clearly established by labeling experiments. The rearrangement of a to c via a 7i-allyl hydride complex b in the coordination sphere of the metal is a key step in this cayalytic cycle (Scheme 54) [174, 175]. hi case of polyunsaturated derivatives, formation of a stable q" complexes (Scheme 55) is preferred over the rearrangement (a c). 

Complex a is readily converted into a Fe-y-H agnostic complex b within an early picosecond timescale and then the 7i-allyl hydride complex c is generated by hydride abstraction. The energy level of the 2-alkene isomer d, which is calculated by DPT experiments, is similar to that of the 1-alkene complex b. In the next step, Fe (CO)3(t -l-alkene)(ri -2-alkene) f, which is generated via intramolecular isomerization of the coordinated 1-alkene to 2-alkene and the coordination of another 1-alkene, is a thermodynamically favored product rather than formation of a Fe(CO)3(ri -l-alkene)2 e. Subsequently, release of the 2-aIkene from f regenerates the active species b to complete the catalytic cycle. 

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

A different reaction occurs when propylene is employed. A 7r-allyl hydride complex is the observed stable product, thus suggesting the following equilibrium ... 

The working mechanism involves a [2 + 2] cycloaddition between the Ru=C bond of ruthenium vinylidene and olefin to form the metallacyclobutane 92, which subsequently undergoes P-hydride elimination leading to the 7i-allyl hydride complex 93 and reductive elimination to furnish the conjugated trienes 89 (Scheme 6.31), and eventually to give the observed aromatic product 90. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

Allyl hydride complexes of osmium are isolated from treatment of phosphine-substituted osmacyclobutane complex 114 with a thallium salt (Scheme 21). The reaction initially provides a mixture of an unstable -benzyl hydride complex 115 and the rf -allyl hydride complex 116 the -benzyl intermediate ultimately isomerizes to the latter complex, a very rare instance of isolable allylic hydride products generated from a starting metallacyclobutane <2004OM4858>. 

Bergman has also found an iridium allyl hydride complex that reacts with arenes and alkanes, the allyl group being converted to an n-propyl group in the process. Butane and isobutane give methyl-substituted allyl derivatives under exchange with the coordinated allyl group (Eq. 23) [93]. 

The allylic hydride complex Tp RhH( -C3H5) (349) is noteworthy, not for its existence, but because it is formed from Tp RhBr( -allyl) (344) and the... 

Several transition-metal phosphine complexes, including RuCl2(PPh3)3, are effective for the selective reduction of dienes to monoolefins.It is generally believed that this selectivity arises from the exceptional stability of an intermediate 7r-allyl complex. The key intermediate in this process may be a 7r-allyl hydride complex. Complex 6, as we indicated in the Introduction, is the only known example of such a complex. The hydride ligand is cis to the allyl group. 

Finally, alkenes containing allylic hydrogens can undergo oxidative addition of the C—H bond in what is effectively a cyclometallation to give an allyl hydride complex. In the example shown, a base is also present so as to remove HCl from the metal and trap the allyl product."... 

Complexes 79 show several types of chemical reactions (87CCR229). Nucleophilic addition may proceed at the C2 and S atoms. In excess potassium cyanide, 79 (R = R = R" = R = H) forms mainly the allyl sulfide complex 82 (R = H, Nu = CN) (84JA2901). The reaction of sodium methylate, phenyl-, and 2-thienyllithium with 79 (R = R = r" = R = H) follows the same route. The fragment consisting of three coplanar carbon atoms is described as the allyl system over which the Tr-electron density is delocalized. The sulfur atom may participate in delocalization to some extent. Complex 82 (R = H, Nu = CN) may be proto-nated by hydrochloric acid to yield the product where the 2-cyanothiophene has been converted into 2,3-dihydro-2-cyanothiophene. The initial thiophene complex 79 (R = R = r" = R = H) reacts reversibly with tri-n-butylphosphine followed by the formation of 82 [R = H, Nu = P(n-Bu)3]. Less basic phosphines, such as methyldiphenylphosphine, add with much greater difficulty. The reaction of 79 (r2 = r3 = r4 = r5 = h) with the hydride anion [BH4, HFe(CO)4, HW(CO)J] followed by the formation of 82 (R = Nu, H) has also been studied in detail. When the hydride anion originates from HFe(CO)4, the process is complicated by the formation of side products 83 and 84. The 2-methylthiophene complex 79... 

The proposed catalytic cycle is shown in Scheme 31. Hence, FeCl2 is reduced by magnesium and subsequently coordinates both to the 1,3-diene and a-olefin (I III). The oxidative coupling of the coordinated 1,3-diene and a-olefin yields the allyl alkyl iron(II) complex IV. Subsequently, the 7i-a rearrangement takes place (IV V). The syn-p-hydride elimination (Hz) gives the hydride complex VI from which the C-Hz bond in the 1,4-addition product is formed via reductive elimination with regeneration of the active species II to complete the catalytic cycle. Deuteration experiments support this mechanistic scenario (Scheme 32). 

Allyl methylcarbonate reacts with norbornene following a ruthenium-catalyzed carbonylative cyclization under carbon monoxide pressure to give cyclopentenone derivatives 12 (Scheme 4).32 Catalyst loading, amine and CO pressure have been optimized to give the cyclopentenone compound in 80% yield and a total control of the stereoselectivity (exo 100%). Aromatic or bidentate amines inhibit the reaction certainly by a too strong interaction with ruthenium. A plausible mechanism is proposed. Stereoselective CM-carboruthenation of norbornene with allyl-ruthenium complex 13 followed by carbon monoxide insertion generates an acylruthenium intermediate 15. Intramolecular carboruthenation and /3-hydride elimination of 16 afford the -olefin 17. Isomerization of the double bond under experimental conditions allows formation of the cyclopentenone derivative 12. 

The mechanism of this reaction was considered on the basis of hydropalladation (Scheme 14). To minimize steric repulsions, the palladium hydride complex approaches the C=CH2 moiety of the allene in the anti-Markovnikov mode from the opposite side of the substituent. This addition gives a 7t—allyl palladium complex with the (Z)-configuration,18 which is converted to the (Z)-product by C-P bond formation, with regeneration of the Pd(0) catalyst. 

By this complex formation with monomer the A1X3 is effectively deactivated. It is important to realise that these experiments not only provide convincing evidence for the initiation by ions derived from the initiator, but also an almost insuperable obstacle to Kennedy s allylic hydride theory [9] and any other theory which involves a reaction of monomer with A1X3 molecules as the initiation step. 

Recendy, we found that A -allyl-o-vii rlaniline 44 gave 1,2-dihydroquinoline 45 by normal RCM and developed silyl enol ether-ene metathesis for the novel synthesis of 4-siloxy-1,2-dihydroquinoline and demonstrated a convenient entry to quinolines and 1,2,3,4-tetrahydroquinoline [13], We also have found a novel selective isomerization of terminal olefin to give the corresponding enamide 46 using rathenium carbene catalyst [Ru] and silyl enol ether [14], which represented a new synthetic route to a series of substituted indoles 47 [12], We also succeeded an unambiguous characterization of mthenium hydride complex [RuH] with ACheterocyclic carbene... 

The EAN of iron in this complex is 34, but it may be a solvated ion. Treatment of the salt with water gives 2-butanone, which was presumed to have been formed via nucleophilic attack on the cation to give a TT-allyl alcohol complex. This complex was then assumed to rearrange via the tricarbonyl hydride to an enol complex, which collapses to the ketone ... 

Some of the evidence for such structures comes from the change in product distribution of the butenes as a function of cyanide concentration when butadiene is hydrogenated with pentaeyanocobaltate(II) catalyst or when the a butenyl complex is reduced with the hydride complex [HCo(CN)5] . Thus 1-butene is the major product in the presence of excess CN, and major product in the absence of excess cyanide. The 1-butene presumably arises from the cleavage of a tr complex, and the 2-butene via an intermediate w-allyl complex. The Tr-allyl complexes of cobalt tricarbonyl are well-characterized and can be prepared either from butadiene and HCo(CO)4 or from methallyl halide and NaCo(CO)4 [49). 

When heated under reflux in benzene or methanol, in the absence of hydrogen, isomerisation of the terminal olefin of the pent-4-enyl phosphine n=3) is completed in less than 5 hours to yield the cis-pent-3-enyl phosphine complex. For the but-3-enyl complex ( =2) the isomerisation to the but-2-enyl phosphine complex is incomplete even after 88 hours. The mechanism involves a i-allyl hydride intermediate, whose stability, as in the case of the hydrogenation studies, is controlled by the... 

The isomerisation of the pentenyl and butenyl phosphine [PhzP-(CH2) CH=CH2 (n = 2,3) (see above and ref 43)] complexes of rhodium has been held to go through a ji-allyl hydride intermediate. Such an intermediate would be equally applicable to the reactions of the group VI metals with ap. Fig. 43b. 

Yttrium-catalyzed enyne cyclization/hydrosilylation was proposed to occur via cr-bond metathesis of the Y-G bond of pre-catalyst Cp 2YMe(THF) with the Si-H bond of the silane to form the yttrium hydride complex Ig (Scheme 8). Hydrometallation of the C=G bond of the enyne coupled with complexation of the pendant G=G bond could form the alkenylyttrium alkyl complex Ilg. Subsequent / -migratory insertion of the alkene moiety into the Y-C bond of Ilg could form cyclopentylmethyl complex Illg. Silylation of the resulting Y-C bond via cr-bond metathesis could release the silylated cycloalkane and regenerate the active yttrium hydride catalyst. Predominant formation of the /ra //j--cyclopentane presumably results from preferential orientation of the allylic substituent in a pseudo-equatorial position in a chairlike transition state for intramolecular carbometallation (Ilg —IHg). 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Similarly, condensation of a mixture of CsHe and C3D6 with nickel vapor resulted in isotopic scrambling. The intraligand isomerizations may be explained in terms of a 1,3-hydrogen shift via a nf-allyl nickel hydride complex in equilibrium with a ir-propene complex, e.g.,... 

Esters of allylic alcohols with resin-bound carboxylic acids can be converted into allyl palladium complexes, which react with carbon nucleophiles and with hydride sources to yield the formally reduced allyl derivatives (Entries 3 and 4, Table 3.47). Alkyl sulfonates have been reduced to alkanes with NaBH4 (Entry 5, Table 3.47). Aryl sulfonates (Entry 6, Table 3.47) and aryl perfluoroalkylsulfonates [814] can be reduced to alkanes by treatment with catalytic amounts of Pd(II) and formic acid as a hydride source. 

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