Transition Metal-Alkene Complexes

A variety of synthetic routes to monoene and polyene tri-fluorophosphine-transition metal complexes have been devised. Direct photochemically induced reaction of a metal-PF3 complex with an activated alkene or diene (method A) has proved useful only for iron, the products being either [Fe(PF3)4(alkene)J or [Fe(PF3)3(diene)] (194). Mixed carbonyl-trifluorophosphine complexes of the type [Fe(PF3)x(CO)3 x(diene)] result from either thermal or photochemical reactions of dieneiron carbonyl complexes and PF3 (52, 53) (method B). The compounds are fluxional. 

There is one report (193) of the formation of a hexa-l,3-dienemetal complex formed by a coupling reaction of the ally] ligands (method C) in the reaction of [Fe(PF3)5] with allyl chloride. 

The most generally applicable route to diene or triene metal PF3 complexes is via metal vapor synthesis (method D), in which the metal vapors are cocondensed with PF3 and the polyene at liquid nitrogen temperatures. 

Similar reactions using 1,3-cyclooctadiene, chromium atoms, and PF3 led only to the t] 5-(cycloocta-l,3-dienyl) hydrido tris(trifluorophos-phine)chromium complex [Cr(C8Hn)(PF3)3H] (7), whose structure has been established by X-ray crystallography (125). (See also Section VIII). The chromium-phosphorus bond lengths [ave. 2.146(3) A] are, as expected, particularly short. 

The [Ni(alkene)(PF3)3] complexes (alkene = C1CH=CH2, CH3CH= CH2, CF3CH=CH2, and FCH=CH2) made by metal vapor synthesis are much less thermally stable than the polyene complexes (28) and decompose readily to give [Ni(PF3)J, alkene, and nickel metal. 

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Stable transition-metal-alkene complexes can be obtained readily from their salts and alkenes in water.141... 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

Figure 1.14. Bonding in transition metal-alkene complexes, (a) Orbitals involved (b) valence bond representations.

The derived transition metal-alkene complex can exist in two diastereomeric forms, exemplified in Eqs. (2a, b) with R)-S anti and syn-y reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers [compare Eqs. (2a) and (2b), or Eqs. (2b) and (2c)]. The data in Table 2 indicate that the mode of addition shown in Eq (2a) is preferred. 

Conjugated dienes can be dimerized or trimerized at their 1,4 positions (formally, [4 4- 4] and [4 4-4 4-4] cycloadditions) by treatment with certain complexes or other transition metal compounds. " Thus butadiene gives 1,5-cyclooctadiene and 1,5,9-cyclododecatriene. " The relative amount of each product can be controlled by use of the proper catalyst. For example, Ni P(OC6H4—o-Ph)3 gives predominant dimerization, while Ni(cyclooctadiene)2 gives mostly trimerization. The products arise, not by direct 1,4 to 1,4 attack, but by stepwise mechanisms involving metal-alkene complexes. " ... 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

Oppolzer, W. Transition Metal Allyl Complexes Intramolecular Alkene and Alkyne Insertions. In Comprehensive Organometallic Chemistry II Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds. Elsevier Oxford, 1995 Vol. 12, pp 905-921. 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

As expected, many unsaturated transition metal hydride complexes catalyse isomerisation. Examples include monohydrides of Rh(I), Pd(II), Ni(II), Pt(II), Ti(IV), and Zr(IV). The general scheme for alkene isomerisation is very simple for instance it may read as follows (Figure 5.1) ... 

In addition to catalytically active transition metal complexes, several stable, electrophilic carbene complexes have been prepared, which can be used to cyclopropanate alkenes (Figure 3.32). These complexes have to be used in stoichiometric quantities to achieve complete conversion of the substrate. Not surprisingly, this type of carbene complex has not attained such broad acceptance by organic chemists as have catalytic cyclopropanations. However, for certain applications the use of stoichiometric amounts of a transition metal carbene complex offers practical advantages such as mild reaction conditions or safer handling. 

The hydrosilylation of ethylene by the early-late transition-metal heterodinuclear complexes [CpTa( t-CH2)2lr(CO)2] has been studied mainly in a bid to recognize the mechanism of reaction, which occurs via a predominant alkene/Ir—H insertion pathway over a minor insertion of ethylene into the Ir—Si bonds [21]. 

The hydrocarboxylation can take place by insertion of the alkene into a metal-hydride bond followed by CO insertion and finally reaction of the acyl complex with solvent as illustrated in equation (36). Alternatively, a transition metal-carboxylate complex can be generated initially. Insertion of the alkene into the metal-carbon bond of this carboxylate complex followed by cleavage of the metal-carbon bond by solvent completes the addition, as shown in equation (37). Both sequences provide the same product. 

Among the carbonylative cycloaddition reactions, the Pauson-Khand (P-K) reaction, in which an alkyne, an alkene, and carbon monoxide are condensed in a formal [2+2+1] cycloaddition to form cyclopentenones, has attracted considerable attention [3]. Significant progress in this reaction has been made in this decade. In the past, a stoichiometric amount of Co2(CO)8 was used as the source of CO. Various additive promoters, such as amines, amine N-oxides, phosphanes, ethers, and sulfides, have been developed thus far for a stoichiometric P-K reaction to proceed under milder reaction conditions. Other transition-metal carbonyl complexes, such as Fe(CO)4(acetone), W(CO)5(tetrahydrofuran), W(CO)5F, Cp2Mo2(CO)4, where Cp is cyclopentadienyl, and Mo(CO)6, are also used as the source of CO in place of Co2(CO)8. There has been significant interest in developing catalytic variants of the P-K reaction. Rautenstrauch et al. [4] reported the first catalytic P-K reaction in which alkenes are limited to reactive alkenes, such as ethylene and norbornene. Since 1994 when Jeong et al. [5] reported the first catalytic intramolecular P-K reaction, most attention has been focused on the modification of the cobalt catalytic system [3]. Recently, other transition-metal complexes, such as Ti [6], Rh [7], and Ir complexes [8], have been found to be active for intramolecular P-K reactions. 

Athough transition metal alkylidene complexes are successfully used for the alkenation of carbonyl compounds, various 1,1-bimetalloalkanes, often prepared by the hydrometal-lation of alkenyl organometallics, are also useful reagents for the alkenation of carbonyl compounds. 

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