Enyl complexes isomerization

Unlike the alkyl and aryl-substituted methylenecyclopropanes discussed above, both cis- and franj-Feist s esters undergo chloropalladation with proximal 1,2-ring opening, to give isomeric n3-[3-chloro-l,2-bis(methoxycarbonyl)but-3-enyl]palladium complexes (equation 331)397. Formation of the but-3-enyl complexes is rationalized by sequential... 

Note that the cis isomer lacks an improper axis of rotation and is tbereftire chiral, but that the trans isomer has a plane of symmetry and will be achiral in the absence of an asymmetric carbon in the phoq hine ligand. As in the case of the previously encountered cyclopentat enyl complex (page 476), it can be argued whether the co ination number is S or 9. fn either semantic interi etatiDn these compounds are of considerable intoest since isomerism in nine-coordinate complexes is even less well documented than in those with coordination number S. 

In reaction (30), the addition of the Fe—H to butadiene gives a cis-trans isomeric mixture of the Fe-but-2-enyl complex which is the 1 4 addition product. There is no evidence for the formation of either the 1 3 or 1 2 addition product (102). 

The enyl-palladium bond formed can further be transformed by different means i.e. hydrogen transfer to the enyl system creates an alkene system and free alkene or hydrogen transfer from the enyl complex yields the starting or isomerized diene. 

Abstraction of chloride from a vinylpalladium complex by silver acetate has been reported,45 as well as halide abstraction by silver triflate from arylpalladium halides.46 More interestingly in the context of creating C-C bonds, silver perchlorate was able to promote the reaction of (r -aryl)palladium bromide with dienes. Silver-promoted bromide abstraction led to the formation of (r 1-r 2-enyl)palladium complexes, which evolved further through the regioselective formation of a C-C bond between the aryl group and the former diene. Reactions with nonconjugated dienes suggested that the reaction proceeds via carbometallation of the less crowded double bond. Isomerization and (3 elimination led to a (ri3-allyl)palladium complex (Scheme 10.26).47... 

When the cyclopropenyl rhenium complex (CO)5Re( 7 -C3Ph3) was refluxed in hexane for 3-4 h, the isomeric / -oxocyclobutenyl and tricarbonyl( 7 -hexaphenylcyclohexadi-enyl)rhenium complexes are obtained, in addition to the tetracarbonyl rhenacyclobutadi-ene complex. In the presence of excess Me3NO, a mixture of two interconvertible triphenyl rhenafuran complexes is obtained. Furthermore, if the thermal reaction is conducted in the presence of phosphorus ligands, addition and ring expansion occur, affording octahedral -cyclobutenone complexes (equation 206) . ... 

The reaction of 3,3-disubstituted cyclopropenes with mono- and 1,2-disubstituted alkenes proceeds only with difficulty and leads to low yields of cyclopropanes. In the case of but-l-ene, an 8% yield, with hex-1-ene and hept-l-ene between 5 and 10% yield, and with cyclooctene about 10% of the cyclopropane product is formed. In these cases, the major product is the formal dimer of the intermediate ethenylcarbene complex, i.e. the corresponding (fj-hexatriene. When copper(I) chloride is used as catalyst rather than the copper halide/phosphane or phosphite system, about half the yield of the [2-f-1] cycloadduct is obtained along with an increased amount of the hexatriene. Mechanistically, these acyclic trienes could also be formed from an (alk-l-enyl)bicyclo[1.1.0]butane intermediate without any carbene being involved. Bicyclo[1.1.0]butanes are low yield (< 20%) byproducts of the thermal dimerization reaction of methyl 3,3-dimethylcyclopropenecarboxylate (1). On the other hand, bicyclo[l. 1. Ojbutanes, such as 3, are known to undergo isomerization to form 1,3-dienes. ... 

Within the last few years there has been a growing tendency to draw the analogy between the chemistry of organometallic complexes and that of hydrocarbons adsorbed on metal atoms in surfaces. For example, it is postulated that the isotopic exchan between deuterium and alkanes, catalysed by thin films of nickel, palladium or rhodium, occurs via an equilibrium between a- and 7r-bonded metal-organic species [109, 110, 111, 112]. i-Bonded olefins are postulated in Zie er catalysis, see above, in the catalytic deuteration of aromatic complexes [113], and in a variety of isomerization reactions [112, 114, US]. Other intermediate metal-carbon systems which have been proposed in heterogeneous catalytic systems include carbenes , e.g. M= H2 [109], Ji-enyl systems [114] and of course the metal-carbonyl M—CO and bridging carbonyl M- O—M. 

---