Tricarbonyl iron complexes hydride abstraction

Hydride abstraction from dienyl tricarbonyl iron complexes furnishes cationic dienyl tricarbonyl iron complexes. For example, reaction of the diene-iron tricarbonyl complex (115) with triphenyhnethyl hexafluorophosphate followed by trimethylsilyl cyanide furnished with excellent regio- and stereoselectivity a new diene iron tricarbonyl complex (116) (Scheme 170). Excellent regio- and stereoselectivity is seen upon reaction of the cationic complex (116) with trimethylsilyl cyanide (TMS-CN) (Scheme 170). Reduction of the nitrile affords a spirocyclic lactam complex. Intramolecular cyclization of in situ formed enols furnishes spirocyclic compounds again with excellent stereoconfrol (Scheme 171). An interesting example of hydride transfer from a cyclohexadiene ring to a pendant aldehyde followed by nucleophilic addition is seen in Scheme 172. 

Tricarbonyl iron complexes of a number of substituted dihydroanisic esters have been prepared. Several undergo hydride abstraction to yield [(cyclo-hexadienyl)Fe(CO)3]+ salts, which react with malonate in the expected manner to give the 5-substituted (l,3-chd)Fe(CO)3 derivative. Hydrolysis of the parent salt (63) yields (cyclohexa-2,4-dienone)Fe(CO)3 containing the keto-isomer of phenol, and (63) may also be alkylated with dimedone to yield the 5-substituted (l,3-chd)Fe(CO)s complex. The bicyclic analogue (64) and... 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Iron pentacarbonyl and l-methoxy-l,4-cyclohexadiene react as shown by Birch and oo-workera, but in dibutyl ether this solvent has been found superior. The tricarbonyl(methoxy-l,3-cyclohexadiene)iron isomers undergo hydride abstraction with triphenylmethyl tetrafluoro-borate to form the dienyl salt mixture of which the 1-methoxy isomer is hydrolyzed by water to the cyclohexadienone complex. The 2-methoxy isomer can be recovered by precipitation as the hexafluoro-phosphate salt. By this method the 3-methyl-substituted dienone complex has also been prepared from l-methoxy-3-methylbenzene. The use of the conjugated 1-methoxy-1,3-cyclohexadiene in Part B led to no increase in yield or rate and resulted chiefly in another product of higher molecular weight. An alternative procedure for the dienone is to react tricarbonyl(l,4-dimethoxycyclohexadiene)iron with sulfuric acid. ... 

The iron-mediated construction of the carbazole framework proceeds via consecutive C-C and C-N bond formation as key steps [70,71]. The C-C bond formation is achieved by electrophilic substitution of the arylamine with a tricarbonyliron-coordinated cyclohexadienyl cation. The parent iron complex salt for electrophilic substitutions, tricarbonyl[/j -cyclohexadienylium] iron tetrafluoroborate 6a, is readily available by azadiene-catalyzed complexation and subsequent hydride abstraction (Scheme 9). 

Similarly protonation of diphenylfulveneiron tricarbonyl generates a substituted 7r-cyclopentadienyl cation (241, 242). Hydride abstraction from cyclopentadieneiron tricarbonyl releases the 77-cyclopentadienyl cation complex (172). The Mossbauer spectra of the [CpFe(CO)3]1 cation and related iron carbonyl cations have been determined (121). 

Apart from the di- and oligoolefm iron tricarbonyl complexes, which nowadays are frequently used in organic synthesis [71, 72], the chemistry of the readily accessible cyclohepatriene chromium and molybdenum tricarbonyls 2 and 3 was the focus of intense research efforts as well. Only a few months after the synthesis of 2 and 3 was published [58,59], both Hyp Dauben and Peter Pauson reported that these compounds react with triphenylmethyl tetrafluoro-borate in methylene chloride to give the tropylium complexes 4 and 5 in excellent yield (Scheme 7.1) [73, 74]. Later this method of hydride abstraction was also used for the preparation of the tropylium cation itself and subsequently led to the generation of several cationic rc-complexes of iron, manganese and cobalt [71, 72], The reactions of the cations of 4 and 5 with nucleophilic... 

Tricarbonyl(cyclohexadiene)iron complexes are stereospecifically attacked by trityl cation on the rear face. If there is no hydrogen available on that face, hydride abstraction does not occur [Eqs. (187) and (188) A. J. Birch, B. J. Chauncy, and D. J. Thompson, unpublished work.)]... 

Triphenylcarbenium salts (PhsC ) readily abstract a hydride from V cyclo-hexadiene iron tricarbonyl complexes to form cationic tj -cyclohexadienyl salts, the hydride being removed from one of the methylene groups adjacent to the V-diene unit. When the cyclohexadiene ligand is monosubstituted, or has two or more different functionaUties attached, a mixture of regioisomers may potentially result. For example, hydride abstraction from 38 results in a 1.5 1 ratio of the two possible regioisomers 39 and 40. 

Cyclohexadienyl iron tricarbonyl cations are best isolated as their relatively stable hexafluorophosphate salts. These are easily formed by addition of a saturated aqueous solution of ammonium hexafluorophosphate to an aqueous solution containing the cationic complex arising from either hydride or methoxide abstraction. The following protocol for the hydride abstraction from tricarbonyl(l-methoxy-4-methylcyclohexa-l,3-diene)iron 34 is representative. 

Thus raM -l,3-pentadiene-Fe(CO)3 fails to react with the triphenyl-methyl fluoroborate under conditions favorable for hydride abstraction in the cyclic series. On the other hand ctf-l,3-pentadiene-iron tricarbonyl (LXXXV) reacts readily under the identical conditions to yield pentadienyl-iron tricarbonyl fluoroborate (LXXXVI) 32). The same cation is obtained from reaction with strong acids (37). Hydrolysis of the cation (LXXXVI) regenerates the alcohol complex while reduction with NaBH4 produces (LXXXV) this latter reaction constitutes the only means to date of making the ar-pentadiene complex (32). These reactions are given in Eq. 14. 

Tricarbonyl(Ti -cyclohexadienylium)iron tetrafluoroborate was described first by Fischer in 1960. The required tricarbonyl(T -cyclohexadiene)iron complex can be conveniently obtained in high yield from cyclohexa-1,3-diene and pentacarbonyliron using catalytic amounts of a 1-azabuta-1,3-diene (see Section 2.5.1). Hydride abstraction from the tricarbonyl(ri -cyclohexadiene)iron complex using triphenylmethyl tetrafluoroborate affords the Ti -dienyliumiron complex salt almost quantitatively (Scheme 4-1... 

The reactions of cationic (ii -dienyl)iron-tricarbonyl complexes with nucleophiles have been studied extensively by Birch and Pearson. These complexes are prepared by abstraction of an allylic hydride from riMiene complexes, which are readily available from the reaction of 1,3- or 1,4-dienes with Fe fCO),. The parent pentadienyl complex reacts with a variety of nucleophiles exclusively at the termineil carbon, and this reaction generates a new T -diene complex (Equation 11.50). ... 

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