Tricarbonyl(pentadienyl)iron Tetrafluoroborate


[12079-72-0]  · C8H7BF4FeO3  · Tricarbonyl(pentadienyl)iron Tetrafluoroborate  · (MW 293.81)

(organometallic electrophile towards a variety of carbon and heteroatom nucleophiles)

Physical Data: yellow solid, mp 163-165 °C (dec) (PF6- salt).

Solubility: sol polar solvents (e.g. MeNO2, Me2CO); slightly sol CHCl3, CH2Cl2; reacts with water, alcohols.

Form Supplied in: not commercially available.

Analysis of Reagent Purity: NMR, elemental analysis.

Preparative Methods: was first prepared by the protonation of tricarbonyl(h4-pentadienol)iron with Tetrafluoroboric Acid in the presence of Acetic Anhydride (eq 1).1

Hexafluorophosphoric acid or perchloric acid may also be used to generate the corresponding PF6- and ClO4- salts. A wide variety of substituted pentadienyl cations, which may be of greater synthetic interest, have been prepared by protonation of substituted dienol and dienyl ether complexes.2-17 Bis[(dienyl)Fe(CO)3] dications have been prepared by protonation of bis-dienol complexes.18 Protonation of tricarbonyl(1,3,5-hexatriene)iron generates the tricarbonyl(1-methylpentadienyl)iron cation.19 Hydride abstraction from (cis-pentadiene)Fe(CO)3 (but not the trans isomer) with Triphenylcarbenium Tetrafluoroborate gives the title cation (eq 2).1 This methodology has been used to prepare a few substituted cations.20-22

Purification: by reprecipitation from MeNO2/ether.

Handling, Storage, and Precautions: the solid may be stored under inert atmosphere for extended periods. Perchlorate salts of the cation are explosive.

Reaction with Metal Hydrides.

Reaction of the (pentadienyl)Fe(CO)3+ cation with Sodium Cyanoborohydride gives (cis-pentadiene)Fe(CO)3 (eq 2),8 while reduction of the cation with Sodium Borohydride or Lithium Triethylborohydride gives mixtures of (cis- and (trans-pentadiene)Fe(CO)3.1,8 In general, reaction of substituted (pentadienyl)Fe(CO)3+ cations with metal hydrides gives mixtures of 1,3-diene complexes, resulting from nucleophilic addition at either the C-1 or C-5 terminus (eq 3);2,8,13,15,16,22 however, reaction of (1,5-diphenylpentadienyl)Fe(CO)3+ with NaBH3CN gives nearly equal proportions of products resulting from attack at the terminal (C-1) and internal (C-2) positions (eq 4).5

Reaction with Heteroatom Nucleophiles.

Reaction of (pentadienyl)Fe(CO)3+ cation with water results in formation of (trans-pentadienol)Fe(CO)3 (eq 1).1 In general, reaction of substituted (pentadienyl)Fe(CO)3+ cations with water or alcohols results in the formation of trans-dienol or trans-dienyl ether complexes, presumably via nucleophilic attack on the transoid form of the pentadienyl ligand (eq 5).2,4,7,10,13-17 An exception is the (1-methoxycarbonylpentadienyl)Fe(CO)3+ cation, which reacts with water to give (5-methoxycarbonyl-(2Z,4E)-pentadien-1-ol)Fe(CO)3 (eq 6).11

The reaction of (pentadienyl)Fe(CO)3+ cations with substituted anilines and with benzylamines gives the corresponding dienylamine and bis(dienyl)amine complexes. More basic amine nucleophiles give (Z)-dienylamine complexes, while less basic amines give (E)-dienyl complexes.23,24 The reaction of (pentadienyl)Fe(CO)3+ cations with Triphenylphosphine gives (Z,E)-dienylphosphonium salts.9,13,15-17,24 Upon deprotonation, the ylides isomerize to the corresponding (E,E)-dienylphosphorane complexes, which can be used in Wittig condensations with aldehydes.25 Nucleophilic attack by halide ions occurs at iron.26

Reaction with Metals.

The reaction of (pentadienyl)Fe(CO)3+ cations with Zinc results in reductive dimerization of the ligand to afford a bis-complexed (h8-1,3,7,9-decatetraene) (eq 7).2,18,21 The reaction of (pentadienyl)Fe(CO)3+ with CpMo(CO)3- is believed to proceed via rapid, reversible formation of a (Z)-diene bimetallic complex at low temperature (-78 °C). Upon warming to rt, only an (E)-diene complex is isolated (eq 8).27 This bimetallic complex eventually decomposes on standing to give the bis-complexed (h8-1,3,7,9-decatetraene).

Reaction with Carbon Nucleophiles.

The parent (pentadienyl)Fe(CO)3+ cation reacts with organocadmium reagents28 and with functionalized organocuprates29 to afford the corresponding substituted (Z)-diene complex (eq 9). Likewise, substituted (pentadienyl)Fe(CO)3+ cations react with organocuprates by attack at the less hindered terminus.13,15,30,31 In contrast, reaction of (pentadienyl)Fe(CO)3+ cations with organolithium reagents or alkynylcerates results in attack at both C-1 and at C-2.32 In general, attack by malonate anion on (pentadienyl)Fe(CO)3+ cations gives (Z)-diene complexes (eq 10).10,13-15,20,31,33 For substituted pentadienyl cations the regioselectivity is variable and depends on the substituents present on the pentadienyl ligand. Attack at a substituted pentadienyl terminus occurs in a stereospecific fashion, presumably by approach of the nucleophile on the face of the ligand opposite to iron. A few examples of pentenediyl complexes, formed by nucleophilic attack of malonate at C-2, have been reported.14,31,33 With weak carbon nucleophiles, such as electron-rich aromatics,6 and Allyltrimethylsilane,13,17,34 the acyclic (pentadienyl)Fe(CO)3+ cation reacts via its less stable transoid isomer (eq 11).

Applications to Natural Product Synthesis.

(Pentadienyl)Fe(CO)3+ cations react as organometallic electrophiles in the the formation of (diene)iron complexes. In order for these cations to be useful in organic synthesis there must exist methods for excising the metal. The diene ligand may be liberated by oxidation of iron.1 Photochemical reduction of (diene)Fe(CO)3 complexes in AcOH leads to the corresponding alkene,35 and nucleophilic attack on neutral (diene)Fe(CO)3 complexes has been reported.36 Substituted (pentadienyl)Fe(CO)3+ cations have been used in the synthesis of D3-lepelin,13 AF toxin IIa,37 5-HETE methyl ester,30c and lasiol.31

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William A. Donaldson

Marquette University, Milwaukee, WI, USA

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