Iron complexes, silyl

A few studies of isolated metal-silyl complexes and ttie computational study of rhodium-sUyl complexes illustrate the insertion of olefins into metal-silicon bonds. Wrighton studied the photochemical reaction of iron-silyl complexes witti ettiylene (Scheme 9.13). Photolysis of Cp FefCOl fSiMej) in the presence of ettiylene forms Cp Fe(CO)(CjHJ(SiMej). This complex appears to insert ethylene, but ttie 16-electron insertion product is unstable and forms the corresponding vinylsilane and iron hydride complexes as products. Photolysis of Cp Fe(CO)j(SiMe3) in the presence of ethylene and CO forms ttie p-silylaDcyl complex containing two CO ligands. 

Turculet L, Feldman JD, Tilley TD (2003) Coordinatively and electronically unsaturated zwitterionic iron silyl complexes featuring the tripodal phosphine ligand [PhB(CH2PiPt2)3] Organometallics 22 4627... 

The lithium enolate of the oc-silyl-substituted iron-acyl complex 19 reacts with aldehydes, however, products of the Peterson elimination process (E)- and (Z)-22 are usually isolat-ed22- 23,36.37 for t[1js anc other preparations of a,/t-unsaturated iron-acyl complexes see Section I.3.4.2.5.I.3.). 

Whereas, plahnum complexes are used predominantly as efficient catalysts in the hydrosilylation of carbon-carbon mulhple bonds, cobalt and iron triad complexes play a cmcial role in the catalysis of other processes, such as the hydrosi-lylahon of C=0 and C=N, dehydrogenative silylation, sUylcarbonylahon, and silylation with vinylsilanes and disilanes. 

When the salt [BU4NHAUX2] (X = Cl, Br) was reacted with 2 equiv of the dppm-substituted iron silyl metallate K[Fe Si(OMe)3 (CO)3(dppm)], instead of the expected [Fe—Au—Fe]- silyl derivative anion chain, the complex [AuFe Si(OMe)3 (CO)3(/r-dppm)]2 CH2Cl2 (30) was obtained with a Au—Au and two Au—Fe bonds. It is interesting to point out that complex 30 forms a 10-membered-ring structure in which the Au—Au interaction is reminiscent of the transannular interactions that occur in organic cycles of middle size (8-12). The similar reaction with HgCl2 yielded the expected Fe—Hg—Fe complex (31)96. 

An excess of alkene favored the formation of the unsaturated product 44 whereas the silylated compound 45 dominated athigher silane to alkene ratios. The described reaction was also carried out in the presence of colloidal iron and led to results similar to those obtained with Fe(CO)5. Hydrosilylation of vinyltrimethylsilane 46 with chlorosilanes R3SiH using the modified iron carbonyl complex (CH2=CHSiMe3)Fe(CO)4 [44, 45]... 

A few dinuclear complexes are known, in which the a-CH bond of a metal-bonded alkyl group is tj2 coordinated to another metal moiety, similar to the previously discussed dinuclear silyl complexes. In the iron complex 22 (75) both metal moieties are the same, and therefore the Fe—C distances [202.5(3) and 211.3(3) pm] can be directly compared without correction. The relative lengthening of the M—C distance of the Fe—H—C three-center bond (4.5%) is comparable to that in the silane-bridged titanium complex 13 (6.5%), corresponding to a similar stage of the oxidative addition. 

Transition metal derivatives are known to stabilize unusual molecules in their coordination sphere. First studies on silanediyl- and silyl complexes of iron carbonyls have been carried out [7-9],... 

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. ... 

Cyclobutadiene iron tricarbonyl complexes also stabilized carbocations on an adjacent carbon. The cation reacts with silyl enol ethers to afford alkylated complexes such as (127) (Scheme 187). A samarimn diiodide -mediated intermolecular radical cychzation of iron tricarbonyl complex (128) is depicted in Scheme 188. An excellent stereocontrol at three contiguous centers is observed. 

Synthesis and Reaction Chemistry of a,p-Unsaturated Acyl Complexes Derived from (2). Two methods for the preparation of optically active ( )- and (Z)-a,p-unsaturated iron acyls from (2) have been reported." One method involves aldol condensation of (2) with aldehydes followed by 0-methylation to produce diastereomeric acyls (18). This mixture (18) is then treated with Sodium Hydride to produce predominantly ( )-a,p-unsaturated acyl complexes (19) (eq 13). Alternatively, (2) can be depro-tonated and treated with Chlorotrimethylsilane to produce the C-silylated complex which is subsequently deprotonated and treated with an aldehyde. This Peterson alkenation produced mixtures... 

Photolysis of bulky permethylated Fp complexes such as FpSi[Si(CH3)3]3 does not cause deoligomerization, possibly because stable intermediate iron-silyl(silylene) complexes are not formed (27). Other less bulky transi-tion-metal-oligosilane complexes are also unreactive under the photolysis conditions. For example, the ruthenium analogues of the iron complexes, [( ri -C5H5)Ru(CO)2-Si ], are essentially photostable (23). Whether this behavior is due to the strength of the Ru-CO bond or to the enhanced stability of the Si-Si bond is not clear, and this problem is currently under investigation. 

The reductive elimination process that yields the hydrosilylation product has been recently observed in the decomposition of cis-alkyl (trimethylsilyl)iron carbonyl complexes 133 (212) (eq. [46]). To account for the overall retention of configuration at silicon the hydrosilylation process, the last step should occur with retention. Although not demonstrated, this is very likely, since reductive elimination of hydrosilanes in silyl hydride transition metal complexes occurs with retention of configuration (211,213). 

All of the steps in this mechanism have precedent in stoichiometric reaction chemistry. It is assumed that alkene adds before silane, but these steps may be reversed in some cases. The reversibility of two steps in the mechanism accounts for both the observed isotopic exchange between the alkene and silane, and the accompanying isomerization of alkenes. The fact that hydrosilylation occurs with retention of configuration at silicon266 is consistent with this mechanism, since oxidative addition of silane to a metal center is known to proceed in a cis manner and with retention199,267. The product-releasing step (elimination from an alkyl/silyl complex) has recently been observed in the thermal decomposition of an iron alkyl/silyl derivative (equation 100)268. Hydrosilylation as catalyzed by Co2(CO)8 appears to proceed by a somewhat different pathway8,35,262. 

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