1.3- Dienes insertion reactions

The diene polymerizations catalyzed by organometallic compounds are probably further examples of diene insertion reactions of metal alkyls (25). But there is little evidence on the detailed mechanisms of these polymerizations. 

Figure E shows some conjugated diene insertion reactions. As expected, these compounds react similarly to the olefins—the same reagents add. Manganese...

The diene insertion reaction will take place intramolecularly if the diene system is part of the acyl group in the acylcobalt tetracarbonyl (13). Thus, /ra j-2,4-pentadienoyl(triphenylphosphine)cobalt tricarbonyl, prepared by the acid chloride method, cyclizes on heating in ether solution in a closed vessel at 75° C for an hour, to 7r-cyclopentenonyl(triphenylphosphine)cobalt dicarbonyl in 30% yield. 

I.l.IJ Reactions nitlr 1,2-, 1.3-. ami 1.4-dienes. The reaction of conjugated dienes with aryl and alkenyl halides can be explained by the following mechanism. Insertion of a conjugated 1.3-diene into an aryl or alkenylpalladium bond gives the T-allvlpalladium complex 243 as an intermediate, which reacts further... 

For most dienes, 7r-bonds adjacent to quaternary centers do not undergo insertion reactions. One notable exception is 1,5-diene (75) (Scheme 17) [44], For this substrate, the observed selectivity is inconsistent with a chair-like (in this case a trans decalin-like) transition structure (76 top) leading to the insertion product, which is typically seen in diene cyclizations with 70 [36]. 

The reversal of the insertion reaction [Eq. (10)] is not normally observed [in contrast to nickel hydride addition to olefins, Eq. (9)]. An exception is the skeletal isomerization of 1,4-dienes (88, 89). A side reaction—the allylhydrogen transfer reaction [Eq. (5)]—which results in the formation of allylnickel species such as 19 as well as alkanes should also be mentioned. This reaction accounts for the formation of small amounts of alkanes and dienes during the olefin oligomerization reactions (51). 

Under the influence of nickel catalysts, 1,5- and 1,6-dienes undergo isomerization and cyclization, preferably to five-membered ring compounds. The cyclization takes place probably via an intramolecular insertion reaction ( , ) involving a ir-5-alken-l-ylnickel complex such as 33, Table III, and 34, Table IV formed by Ni — C, and Ni — C2 additions... 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

Metal-Halogen Compounds. An unusual example of the addition of a metal halide to a conjugated diene has been reported. The complex formed from palladium chloride and butadiene has been shown to be a dimer of 1-chloromethyl-7r-allylpalladium chloride, (85). Whether this is a true insertion reaction or some type of ionic reaction has not been determined, but its close analogy with the olefin-palladium chloride insertion reaction mentioned above would suggest an insertion mechanism for the diene reaction also. 

The facile isomerization of simple alkylcarbenes by alkyl or hydride shifts is so rapid that these species cannot be conveniently studied even by laser-flash photolysis.128- 130 a-Fluori-nation of alkylcarbenes dramatically reduces the rate of rearrangements.131-134 e.g. fluoro(methyl)carbeneis sufficiently stabilized that it can be trapped by cyclopenta-1,3-diene.135 This type of insertion reaction is quite typical of a-fluorinated carbenes. oc-Fluorinated earbenes are less prone to CH insertion, generally adding stereoselectively to alkenes.136... 

The transfer of hydrogen to the alkene also occurs by a radical process,55,50 and not by the insertion reaction which is much more common in homogeneous hydrogenation. These kinetic studies were carried out using cinnamic acid as alkene. The observation of alkenyl and allyl species in diene hydrogenation suggests that a different mechanism of hydrogen transfer may... 

A number of transition metal complexes react with alkenes, alkynes and dienes to afford insertion products (see Volume 4, Part 3). A general problem is that the newly formed carbon-metal bond is usually quite reactive and can undergo a variety of transformations, such as -hydride elimination or another insertion reaction, before being trapped by an electrophile.200 Usually, a better stability and lower reactivity is observed if the first carbometallation step leads to a metallacycle. It is worthy to note that the carbometallation of perfluorinated alkenes and alkynes constitutes a large fraction of the substrates investigated with transition metal complexes.20015... 

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 second step is insertion or transmetallation. An insertion reaction occurs when the palladium-carbon bond adds across a it bond to give a new organopal-ladium species. The types of it bonds normally reactive include alkenes, dienes, alkynes, carbon monoxide, and sometimes carbonyl it bonds. By far the most common reactions use alkenes and alkynes for the insertion reaction. This step results in a new carbon-carbon bond. 

The reaction mechanism can be explained by the formation of a dimethylsilylene intermediate which is able to undergo an insertion reaction into a Si—H bond (for more about this type of reaction, see Chapter 7 Silylenes). The formation of the silylene can be explained by an a elimination from the n metal-complexed disilane. The presence of the silylene has been proved by a capture reaction with diphenylacetylene, when the expected 1,1,4,4-tetramethyl-1,4-disilacyclohexa-diene could be isolated. 

A 1,4 addition seems to occur with dienes like 1,3-butadiene derivatives and benzene. The formation of a yellow compound by reaction of SiO with benzene results in a compound stable in air up to 500 °C. The appearance of Si-H vibrations in the IR spectrum suggests an insertion reaction in the C-H bond, similar to the reaction of SiF2 with benzene. 

A variety of simple insertion reactions are reported for the M—Sn bond, involving species as disparate as Te239, CS2337, SO2203 338, dienes and acetylenes... 

Scheme 20 Reaction mechanism of diene insertion with a catalyst system based on NdCl3 2 TBP and TIBA [131], reprinted with permission from Elsevier...

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