Dimetallic catalysts

The most important catalyst used industrially for the synthesis of high MW polyethers is potassium hydroxide (KOH) [1-14, 17, 49-53]. The second catalyst group is the group of dimetallic catalysts based on a nonstoichiometric complex of Zn3[Co(CN)6l3 ZnCl2 (DMC) with various ligands. DMC catalysts are the highest performance catalysts known at this time for PO polymerisation, being around 1000 times more active than potassium hydroxide (see Chapter 4.9). 

In the history of PU, some continuous processes for polyether polyol synthesis by anionic polymerisation were developed, but only at small scale (i.e., pilot plant). Tubular reactors with static mixing systems or a column with plate reactor types were used, but these technologies were not extended to industrial scale levels. The first continuous process for high MW polyether synthesis was developed by Bayer (IMPACT Technology) and is based on the very rapid coordinative polymerisation of alkylene oxides, especially PO, with dimetallic catalysts (DMC catalysts - see Chapter 5). A principle technological scheme of a polyether polyol fabrication plant is presented in Figure 4.30. 

For low MW oligomers, (e.g., polyether triols initiated by glycerol with a MW of 600-1000 daltons), the cationic catalysis is used successfully especially in the synthesis of the starters/ precursors for coordinative polymerisation with dimetallic catalysts (DMC) (see Chapter 5). 

A spectacular increase in all properties was observed in elastic polyurethanes (especially in polyurethane elastomers, but in flexible foams too), by using polyethers obtained with dimetallic catalysts (DMC) instead of potassium hydroxide. There are obtained directly from synthesis, polyethers with a very low unsaturation, in essence polyethers with a very low content of polyether monols. 

One important class of cluster catalysts are dimetallic catalysts such as Rh2(carboxylate)4, which effect additions and insertions of carbenoids derived from diazoalkanes. This class is not included below because the strict definition of a cluster as containing three or more metal atoms has been adopted. This area of homogeneous catalysis has been reviewed recently " and is treated in the current edition concerning transition metals in organic synthesis. 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

Very recently Chen and co-workers have applied the previously mentioned Ni-based dimetallic pre-catalyst 14 in the Negishi reaction. Remarkable results were obtained even when unactivated aryl chlorides were chosen as reaction partners providing an alternative to the more expensive Pd-based catalysts. The fact that dinuclear pre-catalyst 14 is more active than its mononuclear analogue 13 indicates a possible cooperative effect between the two metal centres [86] (Scheme 6.23). 

In contrast, synthesis of 3,4-diphosphorylthiophenes requires more elaboration because of low reactivity of 3,4-positions of thiophene and unavailability of 3,4-dihalo or dimetallated thiophenes. Minami et al. synthesized 3,4-diphosphoryl thiophenes 16 as shown in Scheme 24 [46], Bis(phosphoryl)butadiene 17 was synthesized from 2-butyne-l,4-diol. Double addition of sodium sulfide to 17 gave tetrahydrothiophene 18. Oxidation of 18 to the corresponding sulfoxide 19 followed by dehydration gave dihydrothiophene 20. Final oxidation of 20 afforded 3,4-diphosphorylthiophene 16. 3,4-Diphosphorylthiophene derivative 21 was also synthesized by Pd catalyzed phosphorylation of 2,5-disubstituted-3,4-dihalothiophene and converted to diphosphine ligand for Rh catalysts for asymmetric hydrogenation (Scheme 25) [47],... 

Three transmetallation reactions are known. The reaction starts by the oxidative addition of halides to transition metal complexes to form 206. (In this scheme, all ligands are omitted.) (i) The C—C bonds 208 are formed by transmetallation of 206 with 207 and reductive elimination. Mainly Pd and Ni complexes are used as efficient catalysts. Aryl aryl, aryl alkenyl, alkenyl-alkenyl bonds, and some alkenyl alkyl and aryl-alkyl bonds, are formed by the cross-coupling, (ii) Metal hydrides 209 are another partner of the transmetallation, and hydrogenolysis of halides occurs to give 210. This reaction is discussed in Section 3.8. (iii) C—M bonds 212 are formed by the reaction of dimetallic compounds 211 with 206. These reactions are summarized in Schemes 3.3-3.6. 

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

Metalametallations of alkenes and alkynes are useful methods for the construction of 1,2-dimetala-alkanes and 1,2-dimetala-l-alkenes, which react subsequently with suitable electrophiles to form substituted alkanes and alkenes. Metalametallation is carried out usually with bimetallic reagents of the type R Si-M R, or R Sn-M R in which M = B, Al, Mg, Cu, Zn, Si or Sn. Some metalametallations proceed without catalysts Cu, Ag and Pd compounds are good catalysts. The metalametallation with bimetallic compounds, such as Si-B, Si-Mg, Si-Al, Si-Zn, Si-Sn, Si-Si, Sn-Al or Sn—Sn bonds, catalysed by transition metal complexes, is explained by the oxidative addition of the bimetallic compounds to form 478, and insertion of alkene generates 479. Finally 1,2-dimetallic compounds 480 are formed by reductive elimination. Dimetallation of alkynes proceeds similarly to give 481. Dimetallation is syn addition. 

Peris and coworkers have also disclosed Ir and Rh complexes 33 and 34 which can catalyze the hydrosilylation of alkynes [77]. Again, poor selectivity was observed as mixtures of the -trans, [ -cis, and a addition products were obtained. Generally speaking, it was found that Rh catalysts were more reactive than the Ir catalyst and the dimetallic complexes were much more active than their monometallic counterparts. It is believed that the difference in reactivity between the dimetallic and monometallic complexes arises from the dimetallic species ability to oxidize to the corresponding M(III) species, thus preventing oxidative addition of the silane. 

The alkenylindium compounds, obtained by the addition of benzyl- and allylindium to alkynes, couple with organic halides in the presence of a palladium catalyst to give the three-component coupling products (Scheme 99).286 1,3-Dibromopropene or 3-bromo-l-iodopropene reacts with indium to give diindiopropene 87a,b.147,148 This dimetallic reagent reacts with two different electrophiles successively carbonyl compounds and imines are allowed to react with 87 as the first electrophile to give vinylic indium intermediates 88, which react with... 

Many organometallic compounds that have main group metal-hydrogen or metal-metal bonds undergo 1,2-hydrometallation or 1,2-dimetallation of alkynes. Pd complexes are good catalysts for these processes [118]. Since the resulting products contain one or two reactive carbon-metal bonds they are well suited for further transformations, particularly in a sequential fashion. 

Many metal complexes have been designed and synthesized as catalysts for transesterification of phosphate diesters to model mechanisms of RNases. Typical examples are monomeric Zn complexes (53) and (54) and dimetallic Zn complexes such as (55) (Scheme 35). For (53), the pseudo-first-order rate constant, k, for hydrolysis of BPP and for transesterification of... 

The cationic complex (VII, X = Cl) forms a symmetrical ( Pnmr i.r.) complex with diphenylacetylene, which is very labile. The complexes [Rh2Cl2(/Lt-CO)(/Lc-acet)(dppm)2], where acet is hexafluoro-2-butyne or acetylenedicarboxylic acid dimethyl ester, are also symmetrical, and the acet ligand is coplanar with the rhodium atoms. The C C bond length is increased to 1.32 A, and so the complex is truly described as a cis-dimetallated alkene. Comparison with the reactions with CO suggests that the activation of an alkyne (or alkene) to hydrogenation may occur when the alkyne is initially coordinated to the catalyst at a terminal position, e.g., (XII), with subsequent oxidative addition and hydrogen transfer steps. A complex of the type (XII) may instead be only a minor but active component of a solution of which the alkyne-bridged complex is the major but less active component. ... 

Breyfogle and co-workers, for the comparison of different metal ion activity with the same structure in cyclo ester ROP, prepared dimetallic chloro monoethoxide complexes of Mg, Co, and Zn, 24 [25]. Their catalysts did not show very high activity but confirmed ROP reaction rate difference of Mg Co>Zn in accordance with other literature reports. 

Peris and co-workers reported dimetallic complexes 87, where the neutral metal fragment was proposed to be the active site in the hydrosilylation of phenylacetylene or 1-hexyne (Figure 13.14). These catalysts produced a mixture of p-( ), P-(Z) and a-isomers, although the p-(Z) isomers were major (up to 5 93 2, p-( )/ i-(Z)/a) in all cases. Related 88 was also found active. In contrast with other cationic rhodium complexes, 89 produced (3-( )-vinylsi-lanes as the major product. ... 

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