Alkenes complexes with transition metals

Hydrocarbon monomers containing n bonds form n complexes with transition metals. Figure 2.1 shows the overlap of the molecular orbitals involved in the formation of a n bond between the simplest unsaturated hydrocarbon monomer, such as alkene, and transition metal [3],... 

Preparation of Complexes with Transition Metals. 3-Trimethylsilyl-l-prop3me reacts with octacarbonyldicobalt to give the complex shown in eq 27. This complex can react with alkenes to lead to 2-trimethylsilylmethylcyclopentenones (28-30%). 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

Scheme 6 Unified scheme for carbometallation of alkenes with transition metal complexes.

In addition to copper and rhodium catalysts commonly used in the generation of metal carbene complexes, other transition metals have also been explored in the diazo decomposition and subsequent ylide generation.Che and co-workers have recently studied ruthenium porphyrin-catalyzed diazo decomposition and demonstrated a three-component coupling reaction of a-diazo ester with a series of iV-benzylidene imines and alkenes to form functionalized pyrrolidines in excellent diastereoselectivities (Scheme 20). ... 

In contrast to the normal-scXcctwc hydroformylation mainly developed in industry, asymmetric hydroformylation, which requires /i o-aldehydes ( branched aldehydes) to be formed from I-alkenes, was first examined in the early 1970s by four groups independently, using Rh(i) complexes of chiral phosphines as catalysts. " Since then, a number of chiral ligands have been employed for asymmetric hydroformylation and used in combination with transition metal ions, especially Pt(ii) and Rh(i). Asymmetric hydroformylation of I-alkenes is most extensively studied. 

Many complexes of alkenes, cycloalkenes, alkynes, and cycloalkynes with transition metals are now known. Some examples are ... 

A number of transition metal complexes react with alkenes, alkynes and dienes to afford insertion products (see Volume 4, Part 3). A general problem is that the newly formed carbon-metal bond is usually quite reactive and can undergo a variety of transformations, such as -hydride elimination or another insertion reaction, before being trapped by an electrophile.200 Usually, a better stability and lower reactivity is observed if the first carbometallation step leads to a metallacycle. It is worthy to note that the carbometallation of perfluorinated alkenes and alkynes constitutes a large fraction of the substrates investigated with transition metal complexes.20015... 

The term oxidative cyclization is based on the fact that two-electron oxidation of the central metal occurs by the cyclization. The same reaction is sometimes called reductive cyclization . This term is based on alkene or alkyne bonds, because the alkene double bond in 13 is reduced to the alkane bond 14, and the alkyne 15 bond is reduced to the alkene bond 16 by the cyclization. Cyclizations of alkynes and alkenes catalyzed by transition metal complexes proceed by oxidative cyclization. In particular, low-valent complexes of early transition metals have a high tendency to obtain the highest possible oxidation state, and hence they react with alkynes and alkenes forming rather stable metallacycles by oxidative addition or oxidative cyclization. 

The reaction of alkenes and other unsaturated substances with transition metal hydrido or alkyl complexes is of prime importance in catalytic reactions such as hydrogenation, hydroformylation, and polymerization (see Chapter 22). It is one of the major methods for synthesizing metal-to-carbon bonds. The reverse reaction, the /3-hydride or /3-alkyl transfer-alkene elimination reaction has already been discussed (Section 21-3). 

Sulfur dioxide reacts generally with transition metal alkyl, aryl, and a-allyl complexes to give sulfinate complexes. The reaction, first described in 1964 by Wojcicki and Bibler, resembles well-known insertion reactions of CO, C2F4, SnCl2, tetracyanoethy-lene, and other unsaturated species into metal-alkyl bonds, but there are important stereochemical and mechanistic differences Sulfur dioxide insertion into metal-alkene and metal-alkyne bonds have not been reported. However, PdCl2 has been used as a catalyst for copolymerization of ethylene and SO2 to polysulfones and insertion into a Pd-ethylene bond is a conceivable reaction step. 

The 7c-complexing ability of the carbon groups can affect the thermodynamics of Eq. (a). While 7t-complexation by alkenes and alkynes with main-group metals is weak, that with transition metals leads to stable n-complexes. With such transition-metal n-complexes as products, transmetallation can be favorable, e.g., in the reaction of cyclopentadienyldiethylalane with FeClj the cyclopentadienyl rather than the Et group is transferred ... 

Although zirconium is only one out of over 50 potentially usable metals in this class (including the lanthanides and actinides), virtually all synthetic applications of hydrometallation with transition metals involve zirconium Why is this so The primary reason derives from the near requirement of a d -metal center for hydrometallation of a general alkene or alkyne. For later transition metals, hydrometallation to give a stable organometallic product can usually be achieved only for special cases—conjugated dienes, alkenes with electronegative substituents, etc. This is due to the relative stability of the ti -complex, as discussed previously. 

Diarylcarbene complexes of transition metals are considerably less reactive toward alkenes than the analogous monoarylcarbene complexes and, therefore, have been little used for preparation of gew-diarylcyclopropanes. Some examples, however, have been reported. When a solution of pentacarbonyldiphenylmethylidenetungsten in ethyl vinyl ether was heated to 37 °C for 3 hours, l-ethoxy-2,2-diphenylcyclopropane (2) was obtained in 61% yield together with 1,1-diphenylethene (3) and 2-ethoxy-l,l-diphenylethene If the reaction is carried in... 

Another important route for the preparation of 1-organo-l-organooxycyclopropanes is the reaction of alkenes with transition-metal carbonylcarbene complexes (Fischer complexes). ... 

When the Pt(0) complex 239, prepared from isomerically pure olefin 176, was recrystallized from refluxing ether, a 7 3 mixture of 239 and 240 could be obtained. The relevance of this and complementary experiments to the transition metal catalyzed isomerization of olefins has been discussed (178a). Although smaller bridgehead alkenes could be generated and trapped at low temperature, attempts to stabilize bicyclo[2.2.1]hept-l-ene (68) by complex-ation with transition metals have hitherto failed (179). The Pt(II) complex of bicyclo[3.3.1]non-l-ene (69) has been prepared by displacement of ethylene in 241 (180). Whereas 69 dimerizes at room temperature with a half-life time of about 3 days, the complex 242 is stable for several months. In reactions of 69 with Pd(II)-acetate or (CH3CN)2PdCl2, only bicyclo[3.3.1]non-2-ene rather than appropriate complexes could be detected. 

New Applications of TCNE in Organometallic Chemistry, A. J. Fatiadi (1987). Selected reactions used in organometallic synthesis are reviewed. 311 references are given. Structure and bonding of metal-TCNE complexes as well as reactions of TCNE with main-group organometallics, with transition-metal complexes, with metal-coordinated alkenes and alkynes, and reactions of platinum-family complexes are discussed. 

Vibrational spectra of r-bonded complexes of alkenes and alkynes with transition metals have been reviewed by Davidson. In contrast to c-bonded complexes (Sec. lV-2), the C=C and C=C stretching bands of jr-bonded complexes show marked shifts to lower frequencies relative to those of free ligands. 

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