Rhodium complexes butadiene

A cationic rhodium complex-catalyzed codimerization of 1,3-dienes with alkynes gives the corresponding cyclohexadienes in good yields with high regioselectively, as exemplified in the reaction of 2-methyl-l,3-butadiene with phenylacetylene (Eq. 12) [31]. 

Metal-catalyzed [4 + 2 + 2] cyclotrimerizations of either heteroatom-containing enyne 62 with 1,3-butadiene (Eq. 17) [42] or heteroatom-containing dienyne 64 with an alkyne (Eq. 18) [43] are effected by cationic rhodium complexes generated in situ from a chlo-rorhodium complex modified with silver salts. These processes afford eight-membered ring products 63 and 65, respectively. In both processes, the nature and amount of the silver salt profoundly affect the outcomes. 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

Butadiene is polymerized by rhodium compounds in aqueous or alcoholic solution [178]. It is generally accepted that the active species is a TT-allyl rhodium complex of low valency [28, 179] which is not rapidly terminated by reaction with water or alcohol. No clear kinetic pattern was observed in the earlier papers but a recent investigation [180] has shown the rate and molecular weight data to be accommodated by a scheme involving monomer transfer and physical immobilization of the active centres in precipitated polymer. In the initial stages the polymerization is first order in rhodium and, at constant monomer concentration, is (pseudo) zero order E = 14.8 kcal mole" ). This is followed by a declining rate which is almost independent of temperature. Molecular weights rise slowly to a maximum value with time (ca. 4000 after 22 h at 70°C). 

Molecular oxygen has become a commonly used co-catalyst for inactive or weakly active transition metal complexes [1-5]. In addition, other oxidizing agents, mainly peroxides, have recently been used in active rhodium complexes in particular, but also in metal carbonyls, as catalysts for hydrosilylation. The catalytic activity of bis(triphenylphosphine)carbonylrhodium(I) in the hydrosilylation of C=C and C=0 bonds can be much increased by the addition of about a 50 % molar excess of tert-butyl hydroperoxide [100]. Chromium triad carbonyls M(CO)e, where M = Cr, Mo, W, have been tested to examine the effect of various organic peroxides on the hydrosilylation of 2,3-dimethyl-1,3-butadiene by triethyl-, triethoxy- and methyldiethoxysilanes [100]. The evidence for organic oxidant promotion of RhCl(cod)phosphine-catalyzed hydrosilylation of 1-hexene was demonstrated previously [101]. 

A remarkable example of the cooperation of different active sites in a polyfunctional catalyst is the one-step synthesis of 2-ethylhexanol, including a combined hydroformylation, aldol condensation, and hydrogenation process [17]. The catalyst in this case is a carbonyl-phosphine-rhodium complex immobilized on to polystyrene carrying amino groups close to the metal center. Another multistep catalytic process is the cyclooligomerization of butadiene combined with a subsequent hydroformylation or hydrogenation step [24, 25] using a styrene polymer on to which a rhodium-phosphine and a nickel-phosphine complex are anchored (cf Section 3.1.5). 

Butadiene Hydrogenation. Rhodium complexes of the type Rh(diene)(dppe)+, where dppe = 1,2-bis(diphenylphosphino)ethane, are catalyst precursors for overall 1,2 and 1,4 addition of hydrogen to 1,3-butadienes. In these reactions the distribution of terminal and internal olefin products is kinetically regulated by the reaction pathways of a common RhH(R)(dppe)+ intermediate (13). Under homogeneous reaction conditions, the thermodynamically more stable internal olefin products (1,4-addition) are favored over the synthetically more useful terminal olefin products (1,2 addition). However, significant increases in the yield of 1,2 addition products can be achieved by intercalation of the catalyst precursor in hectorite. (14)... 

Representative products are XLII and XLIV . With butadiene in benzene the rhodium complex XLIIIa reacts fast at RT, whereas the iridium complex XLIIIb requires 2 h at reflux. Other dienes give complex products with XLIIIa. The complex XLIIIb reacts with isoprene (12 h at reflux) but fails to react with cyclic dienes. 

Rhodium complexes catalyze the oligomerization of dienes. The isolation of the complex L from, e.g., RhClj, ethanol and butadiene, under conditions that yield an intermediate Rh(III) hydride, is therefore of mechanistic interest . 

The hydroformylation of conjugated dienes such as 1,3-butadiene, isoprene, and 1,3-pentadiene gives mixtures of regioisomers, isomerized aldehydes, and dialdehydes depending on the conditions and catalysts used. The reaction of 1,3-butadiene provides 1,6-hexanedial and has relevance to nylon produc-The reaction of 1,3-cyclohexadiene catalyzed by a rhodium complex... 

Table 12-3. Same characteristics of rhodium complexes attached to phosphynated CSSDVB (2% DVB) and their activities in butadiene and acetylene hydrogenation. ...

Very often, the kinetics of rhodium-catalyzed hydroformylation do not follow the expected first order in alkene concentration and the minus one order in GO pressure. Severe or slight incubation has also been observed. To those skilled in the art, it is known that impurities such as 1,3-alkadienes, enones, and terminal alkynes may be the cause of such behavior. In a mixture of 1-alkenes and butadiene for instance, the latter is much more reactive and will react preferentially with the rhodium hydride catalyst. The allylic rhodium species formed, however, reacts much more sluggishly with carbon monoxide than alkyl rhodium complexes, and thus the catalyst is tied up in this inactive sink. 

The trans isomer of 1,4-hexadiene is one of the required monomers for EPDM rubber. Although iron-, cobalt-, and nickel-based Ziegler-type catalysts can codimerize butadiene and ethylene, the selectivity to the desired trans isomer is low. A soluble rhodium complex can, however, catalyze the dimerization with high selectivity to the trans isomer. 

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