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1 Compounds with Transition Metal–Carbon π-Bonds and Compounds of Groups 10 - 8 (Ni, Pd, Pt, Co, Rh, Ir, Fe, Ru, Os)

DOI: 10.1055/sos-SD-001-00001

Lautens, M.Science of Synthesis, (200111.

This volume of Science of Synthesis describes compounds with transition-metalcarbon π-bonds for groups 108. It is the first volume in the category Organometallics and the first volume of the Science of Synthesis series. The elements covered are nickel, palladium, platinum, cobalt, rhodium, iridium, iron, ruthenium, and osmium.

The goal of Science of Synthesis is to be selective in its coverage rather than comprehensive. Each author has critically evaluated the literature and found the most general and useful methods for the synthesis of a particular target class of π-complex. In general, one author, or one senior author and coauthors, has written on each product class in order to ensure uniform coverage of the topic. In two instances, i.e. Sections 1.2 and 1.7 (Pd and Fe, respectively), the sheer volume of information required that an additional author contribute to a subsection of the topic.

The parent structures of representative complexes containing π-bonds that are covered in this volume are shown in Table 1.

Table 1 π-Bonded Complexes Covered in Volume 1

Product Subclass Representative Structure(s) Hapticity
arene complexes η6
pentadienyl complexes η5
cyclopentadienyl complexes η5
diene complexes η4
cyclobutadiene complexes η4
allyl complexes η3
alkyne complexes η2
alkene complexes η2
allene/cummulene complexes η2

The organization of the volume follows the general pattern x.y.z., where x: volume number=1, y: product class, and z: product subclass (methods and variations are described under the subclass heading).

These are now discussed in turn in order to show how the volume can be used.

The product class y refers to the individual chapters on each metal. The relative size of each product class is roughly proportional to the proven importance of this metal in synthetically useful processes. Thus, the discussion of Product Class 2 (Organometallic Complexes of Palladium) comprises more than 30% of the volume and reflects the widespread use of palladium complexes and catalysts in organic synthesis.

The product classes appear in the following order:

1.1 Organometallic Complexes of Nickel

1.2 Organometallic Complexes of Palladium

1.3 Organometallic Complexes of Platinum

1.4 Organometallic Complexes of Cobalt

1.5 Organometallic Complexes of Rhodium

1.6 Organometallic Complexes of Iridium

1.7 Organometallic Complexes of Iron

1.8 Organometallic Complexes of Ruthenium

1.9 Organometallic Complexes of Osmium

The product subclass descriptor z refers to the metalcarbon π-bonds of a specific subtype (see Table 1) listed in order of descending hapticity of the ligand.

The number of subclasses varies from metal to metal and reflects the range of complexes that are known or were judged to be important species in synthetically important reactions. As a result, the subclass designator is not connected to a particular class of π-bond. Two examples are shown below for palladium (Product Class 2) and ruthenium (Product Class 8). It can be easily seen that for palladium, Subclass 1 refers to diene complexes, but for ruthenium, Subclass 1 refers to arene complexes.

1.2.1 PalladiumDiene Complexes

1.2.2 PalladiumAllyl Complexes

1.2.3 PalladiumAlkyne Complexes

1.2.4 PalladiumAlkene Complexes


1.8.1 RutheniumArene Complexes

1.8.2 RutheniumPentadienyl Complexes

1.8.3 RutheniumDiene Complexes

1.8.4 RutheniumAllyl Complexes

1.8.5 RutheniumAlkyne Complexes

1.8.6 RutheniumAlkene Complexes

Within each product subclass, methods for the synthesis of a particular member of the subclass are described. Charged complexes are ordered within the methods in the following way: (1) neutral, (2) anionic, (3) cationic. For example, in Product Class 1 (Organometallic Complexes of Nickel), Synthesis of Product Subclass 2 (NickelAllyl Complexes), the use of Ni(0) is presented before Ni(II) complexes: Method 1: Oxidative Addition of Nickel(0) with Allylic Electrophiles Method 2: Addition of Allylmagnesium Halides to Nickel(II) Salts

The number of these methods varies widely depending on the metal and the particular subclass of π-complex. For instance, in the case of palladiumallyl complexes there are 66 methods described covering all of the important processes for generating this important subclass of π-complexes.

Within each method an introduction is included followed by a comparison with other methods, a discussion of the scope of the method with representative examples, safety concerns (when warranted), a table of examples (for selected methods), and reaction schemes. Finally, selected experimental procedures are provided along with spectroscopic or physical data.

Each method is further subdivided into variations of the method. An example of the method and variation systems is shown for rutheniumarene complexes.

1.8 Product Class 8: Organometallic Complexes of Ruthenium

1.8.1 Product Subclass 1: RutheniumArene Complexes

Synthesis of Product Subclass 1 Method 1: Preparation of Ruthenium(II)Arene Complexes Variation 1: From Dienes Variation 2: By Ligand Exchange Method 2: Preparation of Ruthenium(0)Arene Complexes Variation 1: From Ruthenium(II) Complexes Variation 2: By Ligand Exchange

Applications of Product Subclass 1 in Organic Synthesis Method 3: Reactions Involving RutheniumArene Complexes

As can be seen from this example, methods and their variations are discussed for both the Synthesis of the Product Subclass and Application of the Product Subclass in Organic Synthesis. The former category falls directly within the scope of Science of Synthesis (which is based on products), whereas the latter category has been created in order to include metal-catalyzed processes in which a π-complex is a proposed and likely intermediate species, but is not isolated. The need to capture the multitude of catalytic reactions where the organic product is not the key feature, but rather an intermediate, differentiates Volumes 18 of Science of Synthesis from the other volumes in the series. This point warrants further discussion and expansion of the principles underlying this approach are described below.

For some metals and classes of π-bond, the synthesis of the π-complex generates a stable entity that can be used in a subsequent reaction where the π-bond is apparently unchanged at the end of the reaction. In other words, the product contains a π-bond of a certain subclass as does the starting material. These transformations fit precisely within the scope of Science of Synthesis. Many examples illustrating this point are found in Section 1.7.8 (Ferrocenes) and a few are abstracted below to illustrate the point (Scheme 1).[‌1‌,‌2‌] It is important to note that while some of the transformations in this section undoubtedly occur through the π-bond (in analogy with electrophilic aromatic substitution) the product still contains the cyclopentadienyl ring and is grouped under cyclopentadienyl complexes.

Scheme 1 Reactions of Ferrocene Complexes[‌1‌,‌2‌]

Many of the ferrocene complexes described in this section are useful in the synthesis of ligands for catalytic reactions and subsequently appear under the heading Application of the Product Subclass in Organic Synthesis. For example: Method 12: Catalytic Enantioselective Hydrogenation Method 13: Catalytic Enantioselective Hydroboration Method 14: Catalytic Enantioselective Hydrosilylation Method 15: Catalytic Enantioselective Allylic Substitution Method 16: Catalytic Enantioselective Aldol Reactions

In other examples, a metal π-bonded complex can be easily isolated and reacted in a subsequent stoichiometric process to generate a new class of metal π-bond. The reactions of ironarene complexes with a nucleophile are illustrative of this class of reactions (Scheme 2).[‌3‌,‌4‌] In this instance, methods for the preparation of the ironarene complex are included in the volume (as a method, under Synthesis of Product Subclass) as are examples of making dienepentadienyl complexes by reaction with the nucleophile (also a method, under Synthesis of Product Subclass). Subsequent reactions and demetalation produces purely organic products and these results are discussed under the heading of Application of the Product Subclass in Organic Synthesis.

Scheme 2 Reactions at the π-Bond Generating a New Class of π-Bond[‌3‌,‌4‌]

The final category of transformations include those reactions where the π-bonded metal complex is never isolated and is only assumed based on mechanistic studies using known π-complexes and their reactions as a point of reference. The majority of catalytic reactions fall into this category where an organic substrate is converted into an organic product through the auspices of a metal. More than one type of π-complex is typically formed during a cycle, but the reaction is classified based on the π-complex of highest priority that is proposed (η6>η5>η4).

An example to illustrate the point is the palladium-catalyzed functionalization of allylic acetates and carboxylates (Scheme 3). The most likely mechanism involves complexation of the alkene with palladium (η2-complex) followed by ionization to form a π-allyl palladium species (η3-complex). Subsequent reaction with a nucleophile to form a π-bonded palladium product (η2-complex) ensues. The complex of highest priority in this cycle is the η3-allyl species and so reactions of this type are found in Section 1.2.2 (PalladiumAllyl Complexes).

Scheme 3 Mechanism of the Catalytic Reaction of an Allylic Acetate with a Nucleophile

It is important to note that the organic products of each catalytic reaction will also appear in the appropriate volume of Science of Synthesis dedicated to that product class. For example, palladium-catalyzed allylic amination of an allylic acetate would appear in Volume 1 under Product Class 2, Subclass 2 PalladiumAllyl Complexes and also in Science of Synthesis, Vol.40 (Amines, Ammonium Salts, Haloamines, Hydroxylamines, Hydrazines, Triazines, and Tetrazanes).

In the following section, selected examples abstracted from the individual chapters are presented in order to highlight the diversity of important reactions of π-complexes covered in this volume.

Section 1.1.1 covers the general topic of nickeldiene complexes and in Section a nickeldiene species is proposed to be involved in a reductive-coupling process promoted by nickel(0). (2E,4E)-Hexa-2,4-dienoate reacts with an aldehyde in the presence of triethylborane to give the hydroxy ester in 91% yield (Scheme 4).[‌5‌]

Scheme 4 Reductive Coupling with Triethylborane[‌5‌]

As mentioned above, Section 1.2 covers the π-complexes of palladium. Section outlines the formation and reactions of alkene π-complexes including one of the most important synthetic transformations, the Wacker oxidation. A specific example to highlight the chemoselectivity and synthetic utility is shown below (Scheme 5).[‌6‌]

Scheme 5 The Chemoselective Palladium-Catalyzed Oxidation of an Isopropyl-Substituted 2-Allylcyclohexanone to a 1,4-Dicarbonyl Compound[‌6‌,‌20‌]

Section 1.3.4 is devoted to the synthesis of alkyne complexes of platinum as well as catalytic processes where a platinumalkyne species is a likely intermediate. In Section, a carbonylative process is described where a terminal alkyne reacts with an alkylthiol and carbon monoxide in the presence of tetrakis(triphenylphosphine)platinum(0) to yield the product arising from addition to the internal position of the alkyne (Scheme 6).[‌7‌]

Scheme 6 Platinum-Catalyzed Carbonylative Thiolation of an Alkyne[‌7‌]

Cobalt π-complexes are important in both stoichiometric and catalytic processes. In Section, a reaction between a cationic dienecobalt complex and a bis(silyl) enol ether is reported which provides access to fused ring systems in good to excellent yield (Scheme 7).[‌8‌]

Scheme 7 Addition of Bis(trimethylsiloxy)dienes to Tricarbonyl(η4-diene)cobalt(I) Tetrafluoroborate Complexes[‌8‌]

Rhodium π-complexes (Section 1.5) are widely used as catalysts for laboratory and industrial-scale processes of many different types. Among the most useful are diene complexes where norbornadiene or cycloocta-1,5-diene are the ligands. Section outlines the most direct approach to chiral diene complexes of rhodium which are then used in catalytic amounts for hydrogenation, hydroacylation, cycloaddition, and isomerization processes (Scheme 8).[‌9‌,‌10‌]

Scheme 8 Synthesis of Cationic Chiral Diene Complexes[‌9‌,‌10‌]

Rhodiumallyl complexes are becoming increasingly important species in a variety of catalytic processes. One such reaction is the metallo-ene process which is known for palladium, nickel, and, more recently, rhodium, (Section and Scheme 9).[‌11‌]

Scheme 9 Metallo-Ene Cyclization[‌11‌]

Iridium π-complexes (Section 1.6) are valuable species as precatalysts in organic synthesis and are frequently proposed as intermediates in catalytic cycles involving iridium. An iridiumdiene complex, [Ir2(μ-Cl)2(η4-cod)2], reacts with chiral ligands to form complexes that enantioselectively reduce ketones, alkenes, and imines. Some examples of imine reduction are illustrated in Section (Table 2).[‌12‌‌15‌]

Table 2 Asymmetric Hydrogenation of Imines with [Ir2(μ-Cl)2(η4-cod)2] and Chiral Phosphine Ligands as the Catalyst Precursors[‌12‌‌15‌]

Substrate Ligand Additive Solvent, Temp Product Config ee (%) Yield (%) Ref
(2S,4S)-BCPM BiI3 MeOH, benzene, 30°C (+) 91 92 [‌12‌]
(2S,4S)-BCPM phthalimide toluene, 25°C S >85 95 [‌15‌]
(2S,4S)-BPPM BiI3 MeOH, benzene, 10°C S 90 96 [‌13‌]
(S)-Tol-BINAP BnNH2 MeOH, 20°C R 90 100 [‌14‌]
(R)-BINAP BnNH2 MeOH, 20°C S 87 100 [‌14‌]

a Ligand structures:

Section 1.7 covers a multitude of iron π-complexes which have been synthesized and reacted, among them conjugated dienes and heterodienes. In Section, iron η4-azadiene synthesis is discussed (Scheme 10),[‌16‌] while in Section, the use of chiral azadienes in the synthesis of chiral cyclohexadienes is outlined.[‌17‌]

Scheme 10 Preparation of an Iron η4-Azadiene Complex[‌16‌]

Ruthenium π-complexes are discussed in detail in Section 1.8. Many classes of π-complexes are isolable species that are subsequently used in catalytic processes due to the high cost of ruthenium. For example, in Section a η3-ruthenium complex is used as a precatalyst in the regio- and stereoselective anti-Markovnikov addition of acetic acid to a terminal alkyne (Scheme 11).[‌18‌] Complexation of the alkyne is proposed as a key step in the reaction.

Scheme 11 The Ruthenium-Catalyzed Anti-Markovnikov Addition of Carboxylic Acids to Terminal Alkynes[‌18‌]

The synthesis of osmiumη2-arene and η2-heteroarene complexes is discussed in Sections and The most direct method for simple pentaammineosmiumarene complexes is shown below (Scheme 12).[‌19‌]

Scheme 12 Synthesis of Pentaammineosmium(II)Arene Complexes[‌19‌]

In addition, other methods for the conversion of these compounds into more highly substituted derivatives such as electrophilic aromatic substitution and electrophilic addition are presented in this section of the volume.