Iron complexes, with pentadienyl

Pentadentate 2,2 6, 2"-terpyridine, 30 107 Pentadiene complexes with iron, 12 255-257 with rhodium, 12 295 with silver, 12 340 Pentadienyl complexes with group VIB metals, 12 224, 225 with group VIIB metals, 12 240 JV,Af,/V,lV",N"-Pentaethyldiethylenetriamine, 42 76... 

Lithium alkynylcuprates react with haloallenes to give similar skipped diacetylenes (see below). The related skipped enynes can be prepared by treatment of (pentadienyl)iron(tricarbonyl) halide complexes with dilithium trialkynylcuprates, the compounds being isolated as the iron(tricarbonyl)(diene) complexes (Scheme 4). Further examples of alkylation reactions of copper alkynides are illustrated in Scheme 5. Reaction between a lithium cyanoaikynecuprate and an iodoallene leads to a skipped diacetylene. This useful reaction has been used by Corey in his synthesis of hybridalactone (Scheme 6). °... 

Recent developments include asymmetric polyene cyclisations but also include new ways to initiate cyclisations. Iron tricarbonyl stabilises pentadienyl cations and its complexes with dienes are chiral. Addition of the achiral lithium derivative 268 to the chiral complex 269 gives the polyene precursor 270 that cyclises with Lewis acid to give a single diastereoisomer of the bicyclic compound43 271. 

As discussed earlier (Section 6.3.4), acyclic (pentadienyl) iron cations are intermediates in reactions involving protonation or Lewis acid activation of iron dienol or dienol acetate complexes. With appropriate functionalization, these cations may also be trapped intramolecularly (Scheme 81). In the case of the formation of tetrahydrofurans (278) and (279), equilibration of the transoid and cisoid cations is observed. However, this equilibration is not observed in the stereoselective cyclization to provide (racemic) oxocene (280). Analogous intramolecular cyclizations of sulfur and nitrogen... 

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). ... 

Iron carbonyl complexes containing 77 -alkyl-77 -allyl coordinated hydrocarbon ligands are obtained in several ways. Nucleophilic addition to cationic iron complexes containing 77 -pentadienyl ligands yields (pentenediyl)iron complexes. Oxidatively-induced reductive elimination of these complexes can be utilized as a means to generate 1,2,3-trisubstituted cyclopropanes.The reaction of cationic cycloheptadienyl complexes (Scheme 22) with appropriate nucleophiles also yields the alkyl-allyliron carbonyl complexes. Fe(CO)s also reacts with a- or /3-pincnc in refluxing dioxane (Scheme 22) to produce an alkyl-allyliron complex. Recently, 1,2- and 1,4-disubstituted [(pentadienyl)Fe(CO)3] cations were shown to react with carbon nucleophiles, such as sodium dimethylmalonate, to yield 77 77 -allyl complexes as products. 

Gycloheptatrienes in protic solvents are reported to react with Fe(GO)s and a catalytic amount of NaBH4 to produce ( 7" -l,3-diene)iron tricarbonyl complexes. Pearson and Ghidu have demonstrated that stereospecific cyclization of iron tricarbonyl diene complexes with pendant alkenes and arenes proceeds via protonation of a double bond vicinal to the iron tricarbonyl diene moiety. This methodology has been used to diastereoselectively produce polycycles from iron tricarbonyl-stabilized pentadienyl carbocations. " ... 

Dehydration of (pentadienol)- or (pentadienylether)Fe(GO)3 complexes with a strong acid is the most common method used for the preparation of acyclic (ry -pentadienyljiron cations. It has been demonstrated that ionization of the hydroxyl group occurs with anchimeric assistance from iron, and that isomerization of the initially generated transotd pentadienyl cation to the more stable cisoid cation occurs with retention of configuration about the C1-G2 bond." ... 

Complexes (160) (see equation 35) undergo conversion to phenols upon thermolysis. Alternatively, treatment with AICI3 under a CO atmosphere gives an j)" -iron cyclohexadienone complex thermolysis or treatment with CuCl2 affords a phenol as well. Complexes (162), derived from nucleophihc addition to ( ) -pentadienyl)Fe(CO)3+ cations (see equation 37), can be converted to vinyl cyclopropanes by using CAN (equation 41). ... 

Substitution of complexed dienols (244) or dienol acetates with carbon or heteroatom nucleophiles, in the presence of a Lewis acid, occurs with retention of configuration (Scheme 69). (Alkyl aluminum reagents act as both nucleophile and Lewis acid in this process). This reaction is believed to proceed via stereospecific ionization, with anchimeric assistance from the iron, to generate the transoid pentadienyl cation (247) followed by attack of the weak nucleophile on the face opposite to iron. The cross-conjugated pentadienyl cation can also be generated the substitution of (2-acetoxymethyl-l,3-butadiene)Fe(CO)3 (193) has previously been discussed (Section 6.1.1). 

As described in Section 6.3.4, activation of coordinated dienols (by protonation) or coordinated dienol acetates (by treatment with Lewis acid) affords the corresponding (pentadienyl)iron cations (248) (Scheme 69). Since the acyclic pentadienyl cationic complexes do not have the geometrical constraints of their cyclic counterparts, they can potentially adopt either a cisoid ( U ) or transoid ( S , sickle) conformation. Nearly all of the (pentadienyl)iron cations prepared appear to be in the cisoid conformation by H NMR spectroscopy. Only a single, sterically biased transoid pentadienyl cation (269) has been spectroscopically observed, but not isolated (equation 66). Owing to their potential to undergo U to S conformational inversion in solution, and owing to the considerably higher reactivity of the less stable S conformer, the (pentadienyl)iron cations are considerably more sensitive to moisture than their cyclic counterparts. 

Acid mediated elimination of cyclic (dienyl ether)- and (dienol)Fe(CO)2L complexes leads to the formation of (cyclodienyl)Fe(CO)2L cations (equation 26 and 27) . Protonation of (pentadienol)- or (pentadienyl ether)Fe(CO)3 complexes generates the corresponding (pentadienyl)Fe(CO)3+ cations 167 (Scheme 41). Lillya and coworkers have demonstrated that ionization of the hydroxyl substituent occurs with anchimeric assistance from iron, and that isomerization of the initially generated transoid pentadienyl cation 168 to the more stable cisoid cation occurs with retention of configuration about the Cl—C2 bond . The in situ generated transoid pentadienyl cations may also undergo reaction with heteroatom, hydride or carbon nucleophiles to afford substituted ( , -diene)Fe(CO)3 products (169) . Acyclic (pcntadicnyl)MCp cations (M = Rh, Ir) may be prepared by acidic dehydration of (dienol)MCp complexes. ... 

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