Dienyl complexes

Dienyl and Arene Complexes 379 activated to nucleophilic attack 

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As mentioned earlier, metal complexation not only allows isolation of the QM derivatives but can also dramatically modify their reactivity patterns.29o-QMs are important intermediates in numerous synthetic and biological processes, in which the exocyclic carbon exhibits an electrophilic character.30-33 In contrast, a metal-stabilized o-QM can react as a base or nucleophile (Scheme 3.16).29 For instance, protonation of the Ir-T 4-QM complex 24 by one equivalent of HBF4 gave the initial oxo-dienyl complex 25, while in the presence of an excess of acid the dicationic complex 26 was obtained. Reaction of 24 with I2 led to the formation of new oxo-dienyl complex 27, instead of the expected oxidation of the complex and elimination of the free o-QM. Such reactivity of the exocyclic methylene group can be compared with the reactivity of electron-rich enol acetates or enol silyl ethers, which undergo electrophilic iodination.34... 

FORMATION OF DIENYL COMPLEXES BY REACTION OF OsHCI(CO)(P Pr3)2 WITH ENYNES... 

Table 6.1 Catalytic systems based on Group IV dicyclopenta-dienyl complexes.

Preliminary studies77 into the reactivity of complex endo-62 have shown that treatment with triethylamine regenerates the exo-(l,2,3-7]3)-trans-buta-dienyl complex exo-60 in almost quantitative yield, suggesting that the transformation endo-65 to endo-62 is reversible. The reactions of complex 62 with nucleophiles also suggest that this step is reversible, as this reactivity... 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

The checkers report that the reactions leading to dienyl complexes proceed best when heating is slowly commenced alternatively, the reactions can be carried out by stirring the mixtures at room temperature for 4-20 h. The time is determined primarily by particle size and the amount of excess carbonate that is used, indicating a heterogeneous reaction. As a result, very finely ground sodium or potassium carbonates may be used successfully. 

In addition, these fused-ring systems allow a one-step syntheses of mixed arene-dienyl complexes (136) ... 

Alkali metal alkyls, particularly n-butyl lithium, are the most frequently used reagents to form metallated intermediates.246 247 In certain cases (di- and triphenyl-methane, acetylene and 1-alkynes, cyclopentadiene) alkali metals can be directly applied. Grignard reagents are used to form magnesium acetylides and cyclopenta-dienyl complexes.248 Organolithium compounds with a bulky alkoxide, most notably M-BuLi-ferf-BuOK in THF/hexane mixture, known as the Lochmann-Schlosser reagent or LICKOR superbase, are more active and versatile reagents.249-252... 

The method has been extended to bimetallic complexes (equation 87) and to butadiene type compounds (equation 88).309 These dienes may be used in complex formation producing complexes analogous to butadiene, as shown by equation (89). The structure of (59b) and the diene [Cp(CO)2 Mo=P(R)CH=P(R)] (59c) have been proved by X-ray crystallography. A similar series of dienyl complexes have been synthesized by Huttner et a/.281 (see Section 14.5.1.3.i). 

Although tricarbonylbutadieneiron (1) was prepared by Reihlen et a/.1 in 1930, some considerable time passed before the corresponding cyclohexadiene complex (2 equation 1) was reported.2 Fischer and Fischer described the conversion of (2) to the cationic cyclohexadienyliron complex (3 equation 1) by reaction with triphenylmethyl tetrafluoroborate in dichloromethane.3 This particular complex is extremely easy to prepare and isolate as the hydride abstraction reaction proceeds the product (3) crystallizes out. Precipitation is completed by pouring the reaction mixture into wet diethyl ether, the small amount of water present serving to destroy any excess triphenylmethyl tetrafluoroborate by conversion to triphe-nylmethanol. Filtration, followed by washing the residue with ether, gives pure dienyl complex. 

A method for preparing specific alkyl-substituted dienyl complexes takes advantage of the propensity of (diene)Fe(CO)3 complexes to rearrange under acidic conditions, coupled with the acid-promoted dehydration illustrated earlier for the conversion of (9) to (8). Birch and Haas14 discovered that complexes derived from methylanisoles could be converted to methyl-substituted cyclohexadienyliron complexes, whose substitution pattern is defined by the relative positions of the methoxy and methyl substituents in the precursor. Several examples are given in Schemes 5 and 6 and equation (17). 

Related to this reaction is the acid treatment of the (l-hydFoxymethylcyclohexadiene)tricarbonyliron (45), prepared via DIBAL-H reduction of ester (26) or borane reduction of the corresponding carboxylic acid, which leads to the 1-methylcyclohexadienyliron complex (46 equation 18).11 Using these methods, it is therefore possible to prepare a range of alkyl-substituted dienyl complexes having a defined substitution pattern. 

In those cases where a hydroxyethyl substituent is attached to the cyclohexadiene ting, an oxidative cyclization procedure has been developed to effect conversion to cyclohexadienyliron complexes.14 Thus, treatment of complex (52) with manganese dioxide gives the cyclic ether (53) which can be converted to the dienyl complex (54 Scheme 7) by treatment with tetrafluoroboric acid in acetic anhydride. 

Given that nucleophile addition to (dienyl)Fe(CO)3 complexes proceeds stereospecifically trans to the metal, the question arises as to whether this can be used to control relative stereochemistry during multiple functionalization of cyclohexadienes and cycloheptadienes. A hypothetical example is shown in Scheme 29, where nucleophile addition is followed by a second hydride abstraction, or its equivalent, to generate a substituted dienyl complex. Addition of a second nucleophile, assuming steieocontrol from the metal, would generate a disubstituted derivative with defined relative stereochemistry. 

There are two approaches for obtaining enantiomerically pure, unsymmetrically substituted, dienyliron complexes (i) Resolution of either the dienyl complex itself or its dieneiron complex precursor and (ii) asymmetric synthesis of the diene or dienyl complex. 

The first method, resolution, is unattractive unless both enantiomers are useful in synthesis. In some cases, such as the resolution of dienecarboxylic acid derivatives mentioned earlier (via the phenylethyl-ammonium salt), the resolution is efficient and provides optically pure materials in good yield.39 60,63 In certain cases, the dienyliron complex can be treated with a chiral nucleophile to give a mixture of dia-stereomers which are separated and then reconverted to enantiomerically pure dienyl complex.64 An example of this method is the resolution of complex (27 Scheme 33), via the menthyl ethers (195) and... 

A common observation is that electrophiles add readily to complexes containing uncomplexed double bonds (normally at the uncomplexed double bond) to give 18-electron species as the initially generated complex. For example, acylation of [Fe(cycloheptatriene)(CO)3] yields an 18-electron dienyl complex, as shown in equation (1). 

Dienyl carbonyl compounds, reductive cyclization, 10, 522 Dienyl complexes, with vanadium, 5, 49... 

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