Hexadiene complexes with rhodium

Hexadiene complexes with gold, 12 348 with iron, 12 263, 264 with platinum, 12 319 with rhodium, 12 297-299 with silver, 12 340 Hexafluorides, structures, 27 106-108 Hexafluoroacetone, 30 223-312, 44 317-318 metal complexes, 30 279-298 of cobalt, 30 286-287 of gold, 30 295... 

The 1,5-hexadiene complex, [RhCl(C6H,0)] 2, may be prepared by this method with a reaction time of 24 hours. The temperature should not exceed 40° to avoid the deposition of metallic rhodium. Under these conditions the yield is 85% of analytically pure product. 

Selective codimerization of ethene and 1, 3-butadiene is also possible because the diene first forms an f/ -allyl complex with the metal centre (Fig. 12.4). An industrial process along these lines has been developed in the USA by Du Pont. A rhodium catalyst is employed rhodium trichloride in ethanol itself gives 80% selectivity towards the desired product, irflns-1,4-hexadiene, which is used in the manufacture of an ethene-propene-hexadiene synthetic rubber. The diene introduces some double bonds into the polymer chains which are required for vulcanization. 

A similar insertion of ethylene into a C—M bond of a w-butenyl complex seems to occur in the industrial synthesis of hexadienes from butadiene and ethylene, at least with rhodium and nickel compounds. These catalysts give tra f-l,4-hexadiene as the initial product 178). Cobalt and iron catalysts give the cis isomer 179,180), probably by a different mechanism. 

The 5-phenyldibenzophosphole (25) complex [RhH(DBP)4] catalyzes the hydrogenation of terminal alkenes, 1,5-hexadiene and some substituted alkenes. With 1-hexene it was far more active than [RhCl(PPh3)3] or [RhH(PPh3)4]. The mechanism is again thought to involve dissociation of DBP followed by alkene insertion into the rhodium hydride bond and subsequent reaction with hydrogen.123... 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

The oldest syntheses of chrysanthemates are those starting from 2,5-dimethyl-2,4-hexadiene (238). There have been more papers on the use of rhodium or antimony to catalyze the addition of diazoacetate and chiral copper complexes to create asymmetry during the addition (see Vol. 4, p. 482, Refs. 219-222). The problem with this route is to avoid the use of diazo compounds. An old synthesis of Corey and Jautelat used the ylide addition of a sulfurane to a suitable precursor (in this case a C3 unit was added to methyl 5-methyl-2,4-hexadienoate, 239), and a recent paper gives details about the addition of ethyl dimethylsulfuranylideneacetate to 2,5-dimethyl-4-hexen-3-one (240). This led exclusively to the tran -isomer 241, from which ethyl trans-chrysanthemate (185, R = Et) was made. Other ylide additions are mentioned below. 

Ligands affect not only catalyst activity, but also stereoselectivity. Thus, hydrovinylation of norbornene with ethene using phosphane modified allylnickel complexes gives exclusively the exo-vinyl product4,5. In the rhodium-catalyzed codimerization of ethene and butadiene, the ratio of cis- and trans- 1,4-hexadiene is dependent on the type of donor used as the addend6. 

Similarly, intramolecular [4 4- 2] cycloaddition of unactivated dienynes is also catalyzed and dramatically accelerated by low-valent rhodium complexes, e.g., Wilkinson s catalyst [chloro-tris(triphenylphosphane)rhodium] and phosphite analogs, under mild conditions46. Thus, ( , )-l-(2-propynyloxy)-2,4-hexadiene (3, Z = O) and similar dienynes, with 5 mol% of chlorotris(tri-phenylphosphane)rhodium in 2,2,2-trifluoroethanol for 30 minutes at 55 C. give up to quantitative yield of the cycloadducts with excellent to complete diastereoselection. According to control... 

Bimetallic phase-transfer-catalysis is a process whereby a reaction that occurs using two different metal complexes, does not proceed in the absence of either metal species, or proceeds only at reduced rate. An apparent system of this class has been reported, in which Co2(CO)g and [RhCl(l,5-hexadiene)]2 mutually increased their reactivity when used as catalysts for the conversion of nitrobenzene to aniline in a biphasic system (benzene, aqueous NaOH, dodecyltrimethylammonium chloride) in a carbon monoxide atmosphere [73]. However, another member of the same research group later showed [74] that the apparent bimetallic promotion was due to the fact that the alkylammonium salt used as a phase-transfer agent actually inhibited the activity of the active rhodium complex (apparently a cluster, which is active in the absence of both the alkylammonium salt and the cobalt compound) by rendering it insoluble. The added Co2(CO)g reacts with the alkylammonium salt to generate... 

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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