Allyl-Fe-complex

Scheme 7.15] or S -type mechanism [Equation (7.9)]. Depending on the nature of the nucleophile and catalyst employed, the subsequent nucleophilic substitution of the metal can follow either via a-elimination [path A, Equations (7.8) and (7.9), Scheme 7.15], via an SN2 reaction (path B) or via an SN2 -type reaction (path C). For reasons of clarity, only strictly concerted and stereospecific SN2- or SN2 -anti-type mechanistic scenarios are shown in Scheme 7.15. The situation might, however, be complicated if, e.g., the initial S l -anti ionization event is competing with an Sn2 -syn reaction. Erosion in stereo- and regioselectivity can be the result of these competing reactions. Furthermore, fluxional intermediates such as 7t-allyl Fe complexes are not shown in Scheme 7.15 for reasons of clarity. These intermediates are known for a variety of late transition metal allyl complexes and will be referred to later. Moreover, apart from these ionic mechanisms, radicals might also be involved in the reaction. So far no distinct mechanistic study on allylic substitutions has been published.

Although this catalytic reaction appeared to be of synthetic interest, it has since then neither been applied in synthesis nor further developed. This might be attributed in part to problems with reproducibility and catalyst stability under the reaction conditions, although the Hieber complex was used in a stoichiometric manner for the preparation of a variety of 7i-allyl-Fe complexes. These latter compounds served as starting materials for a plethora of subsequent reactions [34]. The results obtained by Nakanishi and coworkers on the stability and reactivity of n-allyl-Fe-nitrosyl complexes proved such intermediates to be reactive towards a variety of nucleophiles however, the Fe complexes formed upon nucleophilic substitution were catalytically inactive. Hence, in order to maintain the catalytic activity, the formation of intermediate 7i-allyl-Fe complexes had to be circumvented. About 3 years ago we started our research in this field and envisioned the use of a monodentate ligand to be a suitable way to stabilize the proposed catalytically active G-allyl complex. The replacement of one CO by a non-volatile basic ligand was thought to prevent the formation of the catalytically inactive 7t-allyl-Fe complex (Scheme 7.21). 

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. 

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. 

Years earlier, Nicholas and Ladoulis had found another example of reactions catalyzed by Fe2(CO)9 127. They had shown that Fe2(CO)9 127 can be used as a catalyst for allylic alkylation of allylic acetates 129 by various malonate nucleophiles [109]. Although the regioselectivites were only moderately temperature-, solvent-, and substrate-dependent, further investigations concerned with the reaction mechanism and the catalytic species were undertaken [110]. Comparing stoichiometric reactions of cationic (ri -allyl)Fe(CO)4 and neutral (rj -crotyl ace-tate)Fe(CO)4 with different types of sodium malonates and the results of the Fe2(CO)9 127-catalyzed allylation they could show that these complexes are likely no reaction intermediates, because regioselectivites between stoichiometric and catalytic reactions differed. Examining the interaction of sodium dimethylmalonate 75 and Fe2(CO)9 127 they found some evidence for the involvement of a coordinated malonate species in the catalytic reactions. With an excess of malonate they... 

C, complex 185 isomerizes to the (a fi-allyl)Fe(CO)3" anion (186), which may be trapped by reaction with Me3SnCl to give the corresponding ( nt/-crotyl)Fe(CO)3SnMe3 complex (187). Isomerization of 187 to the thermodynamically more stable (svn-crotyl) isomer (188) occurs only at a higher temperature (55 °C). 

Examination of the reactivity of acyclic (diene)Fe(CO)3 complexes indicates that this nucleophilic addition is reversible. The reaction of (C4H6)Fe(CO)3 with strong carbon nucleophiles, followed by protonation, gives olefinic products 195 and 196 (Scheme 49)187. The ratio of 195 and 196 depends upon the reaction temperature and time. Thus, for short reaction time and low temperature (0.5 h, —78 °C) the product from attack at C2 (i.e. 195) predominates while at higher temperature and longer reaction time (2 h, 0 °C) the product from attack at Cl (i.e. 196) predominates. This selectivity is rationalized by kinetically controlled attack at the more electron-poor carbon (C2) at low temperature. Nucleophilic attack is reversible and, under conditions where an equilibrium is established, the thermodynamically more stable (allyl)Fe(CO)3" is favored. The regioselectivity for nucleophilic attack on substituted (diene)Fe(CO)3 complexes has been reported187. The... 

Complementary to the conjugate substitution reaction in which the nucleophile is transferred directly from the tetraalkyl ferrate to the allylic ligand, preformed low-valent Fe complexes can form reactive allyl-iron complexes via an SN2 -type mechanism (path C, Equations (7.8) and (7.9), Scheme 7.16], These complexes react with incoming nucleophiles and electrophiles in a substitution reaction. Depending on the nature ofthe iron complex employed in the reaction, either o- or Jt-allyl complexes are generated. 

The linear dimers 89-91 are formed by Ni [32], Co [33], Fe [34] and Pd [35] catalysts. Linear dimers 90 and 91 are produced via the formation of metal M—H, accompanying migration of hydrogen. The formation of 89 is discussed later. The mechanism of the formation of 91 was studied by an experiment using butadiene 92 deuterated at the terminal carbons. In the formation of the branched dimer 91 from the deuterated butadiene 92, catalysed by Co or Fe complexes, insertion of the second butadiene occurs at the substituted side of the 7r-allyl complex 93 to give 94. Finally, the triene 96 is formed from 95 and Fe—H(D) is regenerated. 

The chlorination of alkyl aromatics by sulfuryl chloride promoted by free-radical initiators, which was originally discovered by Kharasch and Brown990, can be modified by incorporation of transition metal complexes. Matsumoto and coworkers have observed that, upon addition of Pd(PPh3)4, in place of a radical initiator, the side-chain monochlorination of toluene is substantially more selective991. Davis and his colleagues992 have extended this study and report that Pt(0) and Pd(0) are effective initiators for side-chain chlorination of toluene by sulfuryl chloride and dichlorine. Mn, Re, Mo and Fe complexes, on the other hand, behave more like Friedel-Crafts catalysts. Gas-phase chlorination of olefins to allyl chlorides is catalyzed by PdCl2 or by PtCl2993. 

Cyclopentadienyl) (or related hgand). Treatment of (ry -allyl)dicarbonylnitrosyliron complexes (see Nitrosyl Complexes) with phosphite or phosphine gives five-coordinate ()] -allyl) complexes (22) of limited stability. The reaction of Fe(t-BuNC)5 with both aUyl bromide and chloride gives a product formulated as (23). Compounds of the general stracture (24) were prepared by reaction of [(CO)4Fe(SiR3)] and HFe(CO)4(SiR3) with allyl bromide and isoprene, respectively. ... 

Allyl)Fp complexes are also subject to attack, at C-3, by radicals. The mechanism of allylic transposition of ()] -allyl)Fp complexes, as well as the mechanism of phosphite substitution for CO, has been ascribed to attack by Cp(CO)(L)Fe- on the original Fp-aUyl. The reaction of (12) with CCI4 proceeds by a radical chain mechanism, ultimately between CCI3 and the Fp-allyl. The substitution of a-halo ketones and esters most likely proceeds similarly. A radical cation coupling mechanism has been proposed for the dimerization of (jj -allyl)Fp and (j7 -propargyl)Fp complexes. ... 

Acyclic ( -pentadienyl)Fp complexes also behave as Diels-Alder dienes in some cases, although two other reaction pathways compete. With TCNE and maleic anhydride, (17 R = H, Me) undergoes cycloaddition in moderate yields. Highly electron-deficient alkynes, on the other hand, undergo Fe bond insertion reactions reminiscent of SO2 reactions with ( -allyl)Fp complexes, to give (30). Finally, when the pentadiene is disubstituted at C-5, the Diels-Alder route is effectively blocked, and [3 + 2] cycloadditions result with TCNE at the 2,3-double bond. ... 

Nucleophilic attack on ( -alkene)Fp+ cations may be effected by heteroatom nucleophiles including amines, azide ion, cyanate ion (through N), alcohols, and thiols (Scheme 39). Carbon-based nucleophiles, such as the anions of active methylene compounds (malonic esters, /3-keto esters, cyanoac-etate), enamines, cyanide, cuprates, Grignard reagents, and ( l -allyl)Fe(Cp)(CO)2 complexes react similarly. In addition, several hydride sources, most notably NaBHsCN, deliver hydride ion to Fp(jj -alkene)+ complexes. Subjecting complexes of type (79) to Nal or NaBr in acetone, however, does not give nncleophilic attack, but instead results rehably in the displacement of the alkene from the iron residue. Cyclohexanone enolates or silyl enol ethers also may be added, and the iron alkyl complexes thus produced can give Robinson annulation-type products (Scheme 40). Vinyl ether-cationic Fp complexes as the electrophiles are nseful as vinyl cation equivalents. ... 

Chelated complexes related to (138) may be made by closely analogous methods. If the allylic alcohol in question contains a remote, coordinated alkene, protonation can result in () -allyl)Fe(CO)3(alkene)+ cations (141). Electrophilic attack on ( -cyclooctatetraene)Fe (CO)3 gives reorganization of the carbon framework, to afford... 

Cationic phosphine/phosphite -allyliron complexes are more rare. Aside from the previously mentioned report of the preparation of (139a), complex (139b) has been made from the reaction of allyl halides and Fe[P(OMe)3]5. Additionally, complexes with mixed CO/phosphine ligands (139c) have been prepared by the reaction of either (135 X = Br) or [( -allyl)Fe(CO)3]2 with two equivalents of phosphine, followed by NaBPlu addition. ... 

Substitution of the CO ligand for others in (143) has proved to be fairly facile. The mixing of equimolar amounts of (143) and a number of phosphines or phosphites results in the formation of complexes (145), through the intermediacy of ( -allyl)Fe(CO)2(NO)(PR3) complexes. Selected compounds of type (145) have also been prepared by the allyl halide attack method. Even in compound (145 L = P(OMe)3) the remaining CO ligand has been shown to be capable of replacement by an NO hgand from NO+PFe" to give cationic complexes (146). ... 

Other reports of (jj -allyl)Fe(CO)3R complexes are much more scattered and much less systematic. At least three general types of reactions have been observed in more than one case. First, the reaction of (diene)Fe(CO)3 complexes with electrophilic alkenes gives aUyhron complexes in two different ways. If the diene in question is acylic, electrophilic attack at C-1 of the diene gives compounds of type (158) (equation 33). In the case of substituted rj -cycloheptatriene-or azepine-Fe(CO)3 complexes, reaction... 

Each of ()] -allyl)Fp, () -pentadienyl)Fp, and ( -cyclopentadienyl)Fp complexes undergo photolytic loss of CO to afford their respective Cp(CO)Fe( ) complexes. In both the cases of allyl and acyclic pentadienyl complexes, the -complexes are stable compounds, isolated as a mix-tme of exo and endo complexes, with the exo complex being more thermodynamically stable. In the pentadienyl case a significant amount of the ( -pentadienyl)FeCp complex is also isolated. In the case of ( -cyclopentadienyl)Fp complexes the -complexes are observable (again as an exolendo mixtme) only at 77 K, by IR spectroscopy. At room temperatme, only the ( -cyclopentadienyl)FeCp is realized. ... 

In ( -allyl)Fe(CO)3X complexes (135), the molecnles are oriented in what is normally considered an octahedral assembly of hgands around iron (allyl being bidentate) with a fac arrangement of the carbonyls. The allyl unit may be oriented in an endo or exo manner. Spectroscopic studies show that the two isomers are in equilibrium, although the coalescence temperature was above the decomposition temperature of the sample. Furthermore, the endo isomer predominates, although the endolexo isomer ratios may vary from very large (minor isomer not observable) to 1.3 1. ... 

In the case of [( -allyl)Fe(CO)4]+ cations (138), there is no possibility for endolexo isomerism. Crystal structures of this type of complex have been reported, and are similar to those of (135). Noteworthy in the character of these complexes is their geometric stability the syn and anti isomers do not interconvert unless heated to 60-70 °C for extended periods of time. ... 

A report of the X-ray crystallographic studies of enan-tiopure ( -allyl)Fe(CO)2(NO) complex (170) has appeared, though httle detail was provided. The same report described the CD spectrum of this complex in more detail the negative band at ca. 350 mn and the positive band at ca. 450 nm can be used to assign the configuration of the complex. Diastere-omeric complexes exhibit the opposite Cotton effect. The crystal structures of corresponding monophosphine complexes (145) have been determined. ft is possible to consider these complexes as either trigonal bipyramidal (bidentate allyl) or tetrahedral see Tetrahedral) (monodentate allyl), with the central carbon of the allyl closer to the iron atom (2.084 A) than the terminal carbon atoms (2.117 and 2.142 A). These complexes are chiral at the iron atom, and it has proved possible to separate the diastereomeric complexes formed by enantiomerically pure aminophosphines. 

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