Metal complexes nucleophiles

C. Chiral Silyl-Transition Metal Complexes Nucleophilic Displacement of... 

Oxidation of free phosphines was mentioned above as a reaction leading to phosphine loss. Here we will discuss three further ways of phosphine decomposition oxidative addition of phosphines to low-valent metal complexes, nucleophilic attack on coordinated phosphines, and aryl exchange via phosphonium species. Interestingly in all cases the metal serves as the catalyst for the decomposition reaction ... 

This review will concentrate on the detailed mechanistic study of the reactions between metal-complex nucleophiles and organic molecules including the stereochemical consequences of the reactions at both the carbon centre (Sections 6 and 7) and the metal (Sections 8 and 9). Furthermore, the influence of substituents on the carbon centre (Sections 11 and 12) and the nature of the solvent (Section 5) will be discussed. However, neither the nucleophilicity of ligands towards organic substrates nor the synthesis of nucleophilic complexes will be covered. The reader interested in the latter is recommended to the reviews by King (1975) and Vaska (1968). 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Preparation and Properties of Organophosphines. AUphatic phosphines can be gases, volatile Hquids, or oils. Aromatic phosphines frequentiy are crystalline, although many are oils. Some physical properties are Hsted in Table 14. The most characteristic chemical properties of phosphines include their susceptabiUty to oxidation and their nucleophilicity. The most common derivatives of the phosphines include halophosphines, phosphine oxides, metal complexes of phosphines, and phosphonium salts. Phosphines are also raw materials in the preparation of derivatives, ie, derivatives of the isomers phosphinic acid, HP(OH)2, and phosphonous acid, H2P(=0)0H. 

Fe+ + has been deprotonated, but the reaction is complicated by further nucleophilic attack of the methylene unit with the starting material [17]. Enhanced acidity of the ring hydrogens in arene-metal complexes is shown [21] by the formation of complexes of alkyllithium by proton abstraction. 

A very few coordination complexes of tetramethylene sulphone [(CH2)4S02] with transition metal ions have been prepared, and the coordinative ability of sulpholane is generally regarded as quite weak224,225. Sulpholane metal complexes should therefore serve as excellent precursors of the coordination compounds containing other weakly nucleophilic ligands. 

The surprising stability of N-heterocyclic carbenes was of interest to organometallic chemists who started to explore the metal complexes of these new ligands. The first examples of this class had been synthesized as early as 1968 by Wanzlick [9] and Ofele [10], only 4 years after the first Fischer-type carbene complex was synthesized [2,3] and 6 years before the first report of a Schrock-type carbene complex [11]. Once the N-heterocyclic ligands are attached to a metal they show a completely different reaction pattern compared to the electrophilic Fischer- and nucleophilic Schrock-type carbene complexes. 

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

Neutral transition-metal complexes that are not fully coordinatively saturated possess nucleophile metal centers capable of coordinating to electrophiles. On the other hand, group-IIIB halides serve as typical electron-pair acceptors and are, therefore, able to interact coordinatively with basic metal complexes. 

In the skeleton of many chelating diphosphines, the phosphorus atoms bear two aryl substituents, not least because the traditional route to this class of compounds involves the nucleophilic substitution with alkali metal diarylphosphides of enantiopure ditosylates derived from optically active natural precursors, approach which is inapplicable to the preparation of P-alkylated analogs. The correct orientation of these aryl substituents in the coordination sphere has been identified as a stereo chemically important feature contributing to the recognition ability of the metal complex [11,18-20]. 

Electrophilic and nucleophilic phosphinidene complexes have been related to the corresponding carbene complexes of which the Fischer-type is usually considered as a singlet-singlet combination and the Schrock-type as a triplet-triplet combination. However, both the strongly preferred triplet state of R-P and the M=P bond analysis suggest this schematic interpretation to be less appropriate for transition metal complexed phosphinidenes. 

The following chapter concerns another kind of low-valent organophosphorus compounds, namely phosphinidenes. Little is known about free phos-phinidenes in contrast to the corresponding transition metal complexes. Many new reagents have been generated exhibiting either electrophilic or nucleophilic properties. The reactivity of these carbene-like reagents is evaluated (K. hammer tsma). 

The subjects of structure and bonding in metal isocyanide complexes have been discussed before 90, 156) and will not be treated extensively here. A brief discussion of this subject is presented in Section II of course, special emphasis is given to the more recent information which has appeared. Several areas of current study in the field of transition metal-isocyanide complexes have become particularly important and are discussed in this review in Section III. These include the additions of protonic compounds to coordinated isocyanides, probably the subject most actively being studied at this time insertion reactions into metal-carbon bonded species nucleophilic reactions with metal isocyanide complexes and the metal-catalyzed a-addition reactions. Concurrent with these new developments, there has been a general expansion of descriptive chemistry of isocyanide-metal complexes, and further study of the physical properties of selected species. These developments are summarized in Section IV. 

The reactions of nucleophilic reagents with cationic and uncharged metal carbonyl complexes have received much attention in the past, and it is not surprising that these studies have now been extended to isocyanide metal complexes. Different products in these reactions can arise by three general routes these include ligand substitution, reactions involving attack at a ligand, and reduction of the metal complex. All have been observed in reactions with metal isocyanide complexes. 

The susceptibility of a metal complex to nucleophilic attack is enhanced by a positive charge on the complex. This fact, and the fact that most metal isocyanide complexes are cationic, probably explains why no nucleophilic reactions of uncharged isocyanide complexes have yet been reported. It is... 

Few quantitative data are available on the relative nucleophilicities of L toward various alkyl carbonyls. The rates of the reaction of CpMo(CO)3Me with L in toluene (Table II) decrease as a function of the latter reactant P( -Bu)3 > P( -OBu)j > PPhj > P(OPh)j, but the spread is relatively small (<8). The above order is that customarily observed for 8 2 reactions of low-valent transition metal complexes (J, 214). Interestingly, neither CpMo(CO)3Me nor CpFe(CO)2Me reacts with 1 or N, S, and As donor ligands 28, 79). This is in direct contrast to the insertion reactions of MeMn(CO)5 which manifest much less selectivity toward various L (see Section VI,B,C,D for details). 

The use of chiral transition-metal complexes as catalysts for stereoselective C-C bond forming reactions has developed into a topic of fimdamental importance. The allyhc alkylation is one of the best known of this type of reaction. It allows the Pd-catalyzed substitution of a suitable leaving group in the allylic position by a soft nucleophile. 

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