Insertion reactions olefins

In this reaction one ligand is inserted between the metal and another ligand, creating a site of coordinative unsaturation so that another reactant ligand can be associated with the metal. The insertion reaction accounts for the chain-growth steps of olefin polymeri2ation reactions. 

Mioskowski et al. have demonstrated a route to spirocyclopropanes. As an example, treatment of epoxide 100 with n-BuLi in pentane stereoselectively gave tricyclic alcohol 101, albeit in only 47% yield (Scheme 5.21) [29]. With a related substrate, epoxide 102 stereoselectively gave dicydopropane 103 on treatment with PhLi uniquely, the product was isolable after column chromatography in 74% yield [35]. As was also seen with attempts to perform C-H insertion reactions in a non-transannular sense, one should note that steps were taken to minimize the formation of olefin products, either by the use of a base with low nudeophilicity (LTM P) and/or by slow addition of the base to a dilute solution (10-3 m in the case of 102) of the epoxide. 

A large amount of the work on palladium isocyanide complexes has been mentioned earlier, in discussions on insertion reactions 30,74,108,169,170) and on addition reactions of coordinated isocyanides 25, 33, 34, 49) the reactions of [Pd(CNBu )2] with oxygen 107) and with various olefins 29, 110) were noted. 

Much of the recent interest in insertion reactions undeniably stems from the emphasis placed on development of homogeneous catalysis as a rational discipline. One or more insertion is involved in such catalytic processes as the hydroformylation (31) or the polymerization of olefins 26, 75) and isocyanides 244). In addition, many insertion reactions have been successfully employed in organic and organometallic synthesis. The research in this general area has helped systematize a large body of previously unrelated facts and opened new areas of chemistry for investigation. Heck 114) and Lappert and Prokai 161) provide a comprehensive compilation and a systematic discussion of a wide variety of insertion reactions in two relatively recent (1965 and 1967) reviews. 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

It might be expected that the aromatic character of (14) would lend unique characteristics to its chemistry. Studies of the olefin addition and C—H insertion reactions of the carbene, however, indicate that it is poorly stabilized and highly reactive.<26)... 

The olefinic products which formally correspond to C—H insertion reactions are thought to arise by stepwise abstraction of hydrogen by triplet carbene and subsequent recombination ... 

Carboalkoxymethylenes, like acylmethylenes, undergo rearrangement to ketenes as well as the olefin addition and C—H insertion reactions characteristic of methylenes.<37> Thus the photolysis of ethyl diazoacetate in olefinic solvents leads to substantial yields of products, which can be rationalized in terms of a Wolff rearrangement of the carboethoxymethylene followed by cycloaddition of the resulting ethoxyketene to the olefin ... 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

Billions of pounds of polyolefins are produced annually in the world [1], Through simple insertion reactions, inexpensive and abundant olefins are transformed into polymeric materials for a wide range of applications including plastics, fibers, and elastomers. Despite its long history, the polyolefin industry continues to grow steadily and remains technologically driven because of continuous discovery of... 

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). 

The rate also varies with butadiene concentration. However, the order of the rate dependence on butadiene concentration is temperature-de-pendent, i.e., a fractional order (0.34) at 30°C and first-order at 50°C (Tables II and III). Cramer s (4, 7) explanation for this temperature effect on the kinetics is that, at 50°C, the insertion reaction to form 4 from 3, although still slow, is no longer rate-determining. Rather, the rate-determining step is the conversion of the hexyl species in 4 into 1,4-hexadiene or the release of hexadiene from the catalyst complex. This interaction involves a hydride transfer from the hexyl ligand to a coordinated butadiene. This transfer should be fast, as indicated by some earlier studies of Rh-catalyzed olefin isomerization reactions (8). The slow release of the hexadiene is therefore attributed to the low concentration of butadiene. Thus, Scheme 2 can be expanded to include complex 6, as shown in Scheme 3. The rate of release of hexadiene depends on the concentra-... 

In this codimerization reaction, the predominant complex is 3, which should lead to the ethylene-butadiene codimerization product. If the ethylene in complex 3 is displaced by butadiene to form 7 before the insertion reaction takes place, then a C8 or higher olefin could be formed... 

The hydrogens within the octahedral olefin-dihydride intermediate are transferred consecutively with overall cis addition, and the rate-determining step (k9) is olefin insertion to give the alkyl- hydride. Kinetic and thermodynamic parameters for nearly all the steps of Fig. 1 have been estimated for the cyclohexene system. Because the insertion reaction is generally believed to require a cis disposition of the hydride and olefin... 

Among typical carbon-carbon bond (C-C) formation reactions with carbenes, the cyclopropanation reaction with olefins has been well studied including its application to industrial processes. The second typical reaction of carbenes is the insertion reaction into the carbon-hydrogen bond (C-H) which seems to be a direct and efficient C-C bond forming reaction. However, its use for synthetic purpose has often been limited due to low selectivity of the reactions.3... 

Intermediates corresponding to the coordination step are considered as sufficiently close to transition states of the insertion reaction, and hence as suitable preinsertion intermediates, only if the insertion can occur through a motion of the nuclei that is near to the least—principle of least nuclear motion.13,30,31 For instance, for alkene polymerizations preinsertion intermediates correspond to geometries with (a) a double bond of the olefin nearly parallel to the metal growing chain bond and (b) the first C-C bond of the chain nearly perpendicular to the plane defined by the double bond of the monomer and by the metal atom (50° < Gi < 130°, rather than 0i 180° see below). 

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... 

In concluding this paper dedicated to our distinguished octogenarian, it is especially appropriate to mention the heuristic practical value which our ring-expansion theory has had, since he has always been intent upon applications and practical uses. It was the ring-expansion theory which led the senior author to imagine that the acid-catalysed reactions of cyclic formals with olefins would be insertion reactions [21]. In his view they involve the insertion of an olefin into the 0-1, C-2 bond of a protonated (or alkylated) 1,3-dioxacycloalkane ... 

The electronics behind the insertion reaction is generally explained in terms of a simple three-orbitals four-electrons scheme. Hoffmann and Lauher early recognized that this is an easy reaction for d° complexes, and the relevant role played by the olefin n orbital in determining the insertion barrier [26], According to them, the empty Jt orbital of the olefin can stabilize high energy occupied d orbitals of the metal in the olefin complex, but this stabilization is lost as the insertion reaction approaches the transition state. The net effect is an energy increase of the metal d orbitals involved in the d-7t back-donation to the olefin n orbital. Since for d° systems this back-donation does not occur, d° systems were predicted to be barrierless, whereas a substantial barrier was predicted for dn (n > 0) systems [26],... 

The polymerization of 1-olefins introduces the problems of regioselectivity and stereoselectivity. In this section we will focus on the origin of the regioselectivity of the insertion reaction (primary vs. secondary 1-propene insertion, see Figure 9), while the origin of the stereoselectivity will be discussed in the next section. 

Instead of having the olefin insertion reactions, the calculations indicate that M2b and M2c can only proceed uphill with the reductive elimination of HB(OH)2, leading to the formation of M3, an olefin complex which could be in principle obtained directly from the addition of olefin to the catalyst Rh (PH3)2C1. The olefin complex M3 then could undergo a-bond metathesis processes with HB(OH)2, giving two isomeric products M4 and M5 depending on the orientation of the HB(OH)2 borane. The a-bond metathesis processes are however found to be unfavorable because of the very high reaction barriers (Figure 4). 

---