Cyclohexadienyliron complexes

Regiocontrol During Nucleophile Addition to Cyclohexadienyliron Complexes Applications ofAlkoxy- 674 and Methoxycarbonyl-substituted Complexes in Organic Synthesis... 

Although tricarbonylbutadieneiron (1) was prepared by Reihlen et a/.1 in 1930, some considerable time passed before the corresponding cyclohexadiene complex (2 equation 1) was reported.2 Fischer and Fischer described the conversion of (2) to the cationic cyclohexadienyliron complex (3 equation 1) by reaction with triphenylmethyl tetrafluoroborate in dichloromethane.3 This particular complex is extremely easy to prepare and isolate as the hydride abstraction reaction proceeds the product (3) crystallizes out. Precipitation is completed by pouring the reaction mixture into wet diethyl ether, the small amount of water present serving to destroy any excess triphenylmethyl tetrafluoroborate by conversion to triphe-nylmethanol. Filtration, followed by washing the residue with ether, gives pure dienyl complex. 

The earlier woik of Birch et a/.8-10-11 laid the foundations for much of the synthetic application of cyclohexadienyliron complexes. Of particular interest are the reactions of dihydroanisole derivatives8-10... 

A method for preparing specific alkyl-substituted dienyl complexes takes advantage of the propensity of (diene)Fe(CO)3 complexes to rearrange under acidic conditions, coupled with the acid-promoted dehydration illustrated earlier for the conversion of (9) to (8). Birch and Haas14 discovered that complexes derived from methylanisoles could be converted to methyl-substituted cyclohexadienyliron complexes, whose substitution pattern is defined by the relative positions of the methoxy and methyl substituents in the precursor. Several examples are given in Schemes 5 and 6 and equation (17). 

In those cases where a hydroxyethyl substituent is attached to the cyclohexadiene ting, an oxidative cyclization procedure has been developed to effect conversion to cyclohexadienyliron complexes.14 Thus, treatment of complex (52) with manganese dioxide gives the cyclic ether (53) which can be converted to the dienyl complex (54 Scheme 7) by treatment with tetrafluoroboric acid in acetic anhydride. 

Regiocontrol During Nucleophile Addition to Cyclohexadienyliron Complexes ... 

The addition of carbon nucleophiles to complex (27), followed by demetallation, is equivalent to the y-alkylation of cyclohexenone. This overall transformation can also be accomplished directly via addition of electrophiles to dienolsilanes, but it becomes nontrivial for cases where the cyclohexenone C-4 position is already substituted.37 On the other hand, 1 -substituted cyclohexadienyliron complexes, such as (30), react very cleanly with certain carbon nucleophiles, at the substituted dienyl terminus. This provides useful methodology for the construction of 4,4-disubstituted cyclohexenones, and has been employed in a variety of natural product syntheses. 

Steric hindrance to nucleophile addition becomes a major problem with 6-exo-substituted cyclohexadienyliron complexes, and the outcome of these reactions is highly dependent on the size of the substi-... 

The trichothecenes, a group of sesquiterpenes having a reasonably complex tricyclic structure, represent useful targets for the synthetic application of cyclohexadienyliron complexes. Several members of... 

Using cationic tricarbonyl(q5-cyclohexadienyl)iron complexes as starting materials, different synthetic routes to a large number of carbazole alkaloids have been developed [51, 58, 67]. The first step is an electrophilic substitution of a substituted arylamine using the cyclohexadienyliron complex and provides the corresponding 5-aryl-substituted cyclohexadiene-iron complexes (Scheme 1.29). 

J.F. Helling and W.A. Hendrickson, Pi-cyclohexadienyliron complexes bearing exocyclic double-bonds. J. Organomet. Chem. 1977, 141(1), 99-105. 

The use of cyclohexadienyliron complexes for preparing functionalized six-membered rings continues to prove of value. Spirocyclic enones are available by treating the complex (243) with methyl sodiocyanoacetate and decomplexa-tion. Additionally, in some cases, good diastereoselectivity is observed e.g. 9 1 when n = 3), so enhancing the value of the method. ... 

Both the required salts were easily accessible from 19 as described in Section 14.5.3. The viability of this approach has been shown by successful completion of a simple synthesis of (+/-)-0-methyljoubertiamine from 84 [4]. A detailed crystallographic study of conformational effects in an extended series [109,128] of 1-aryl-substituted cyclohexadienyliron complexes had established, as expected, that ortho substituents on the aromatic ring substantially impeded the nucleophile s approach, and could switch the control to predominantly a> control by the aryl group. However, with an o-alkoxy substituent, the conformation with the substituent below the plane of the dienyl system is easily accessible (and characterized in an X-ray structure [128]) opening the way for more ambitious synthetic applications [109]. This conformational effect has been put to use in a formal total synthesis (entry 19) of lycoramine (87) from 88 [106]. Electronic effects from additional donor substituents also flatten the ring and open up the aryl-substituted position [128], effects that will be put to work in ongoing work towards maritidine and crinine. The completed O-methyljoubertiamine and lycoramine syntheses make multiple use of the metal to form both bonds at the aryl-substituted quaternary center, and so can be classified as iterative ( —> f/ V V ) synthetic routes. 

The use of transition-metal complexes to activate haloarenes for S Ar reactions has been used as a strategy for important skeletal bond formation steps in a number of syntheses. An example of carbon-carbon bond formation in this way can be found in amino acid synthesis. The Shiffs base nucleophile (Me02C)(Ph2C=N)CH displaces fluorine from (fluorotoluene)Cr(CO)3 [316]. This type of substitution reaction has been applied in a model study for the diaryl ether section of SK F L-94901 (68) using a cationic cyclohexadienyliron complex (see Scheme 14.16)... 

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