Examples of 1,2 Insertions

FIGURE 14.14 Mechanisms of Reverse Reactions for CO Migration and Alkyl Insertion (2). C indicates the location of C. 

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Two examples of 1,2 insertions have been shown in Figure 14-7. An important application of 1,2 insertions of alkenes into metal-alkyl bonds is in the formation of polymers. 

Simple examples of 1,2-insertion reactions of NO like reaction (a) are unknown, probably because of the instability of the expected products. 

The insertions of imines into late transition metal-carbon bonds are even less common. In one case, the insertion of an imine into Ni- and Pd-acyl bonds occurs with 2,1-regiochemistry to form an aminoalkyl product (Equation 9.80). Tlus reaction is likely to occur through a polar transition state formed by attack of a nucleophilic nitrogen at the electrophilic acyl carbon. One set of examples of 1,2-insertions of imines into late metal-carbon bonds have been reported. This example involves insertion of N-aryl aldimines into rhodium-aryl complexes containing a labile pyridine ligand (Equation 9.81). The rates of these reactions were inverse order in added pyridine, suggesting that the reaction occurs by an intramolecular migratory insertion mechanism after replacement of the coordinated pyridine by the imine. 

These reactions are examples of 1,2-addition. In addition to insertion in the M-H bond as shown earlier, this type of reaction also occurs with metal-halogen and metal-carbon bonds. The following equation shows an example of a reaction in which insertion occurs between a metal and a halogen ... 

Hydride elimination reactions are characterized by the transfer of a hydrogen atom from a ligand to a metal. Eifectively, this may be considered an oxidative addition, with both the coordination number and the formal oxidation state of the metal being increased (the hydrogen transferred is formally considered as hydride, H ). The most common type is p elimination, with a proton in a p position on an alkyl ligand being transferred to the metal by way of an intermediate in which the metal, the a and P carbons, and the hydride are coplanar. An example is shown in Figure 14-11. p Elimination is the reverse of 1,2-insertion. 

In the inserted product, M and R can be bonded to the same atom of L (1,1-insertions) or to two different atoms (l,n-insertions, with n designating the number of atoms constituting the inserted sequence). Thus, for example, 1,1-insertions [see reaction (b)] are observed with CO and with isocyanides in the more frequent type of coordination, carbon being the bridging atom in both cases, while 1,2-insertions are found with olefins, acetylenes, and carbon dioxide [reactions (c)-(f)] the -bond type, which is frequently encountered with early transition metal elements and with / element compounds for both carbon monoxide and isocyanides, is in fact a case of 1,2-insertion (see 11.3.2.1.5-11.3.2.2). 

We and others have revealed that syndiospecific propylene polymerization is exclusively initiated by 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization [64]. This is the first example of a predominant 2,1-insertion mechanism for chain propagation exhibited by a group 4 metal-based catalyst. The unusual preference for 2,1-regiochemistry displayed by the Ti-FI catalysts compared with the Zr- and Hf-FI catalysts is apparently inconsistent with the crys-tallographically characterized structures, which indicate that the Ti is shielded more by the phenoxy-imine ligands and thus possesses higher steric compression. The reason for the unusual preference in the regiochemistry of Ti-FI catalysts is unclear at the present time. 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

The interaction may not be quite as strong as in the case of 2,1 insertion discussed above, but there will always be a tendency of the growing chain to arrive at an isotactic stereochemistry when 1,2 insertion occurs. One example of chain-end control leading to isotactic polymer was reported by Ewen [13] using Cp2TiPh2/alumoxane as the catalyst. The stereoregularity increased with lower temperatures at -45 °C the isotactic index as measured on pentads amounted to 52 %. The polymer contains stereoblocks of isotactic polymer. At 25 °C the polymerisation gives almost random 1,2 insertion and an atactic polymer is formed. 

Carbenes do not usually undergo intermolecular C-C bond insertions [1172], Intramolecular 1,2-insertions into C-C bonds are, however, frequently observed [976], and have, e.g., been used for the synthesis of strained bridgehead olefins [1173]. Further examples are listed in Table 4.10. 

The synthesis and chemistry of metal complexes of thiophenes have been reported including the electrophilic additions to osmium-thiophene complexes <9902988> and nucleophilic additions to ruthenium-thiophene complexes <99JOMC242>. The selectivity for the insertion of ruthenium into 3-substituted thiophenes was studied <99CC1793>. For example, treatment of 3-acetylthiophene (84) with Ru(cod)(cot) led to a regioselective 1,2-insertion of ruthenium giving thiaruthenacycle 85. 

In all of the examples cited in Section 1.2.2.3.2.3.1, the diazo compounds are arranged such that none has a 3-hydride available. It could be expected that if a simple a-diazo ketone with fi-C — H bonds were exposed to the rhodium catalyst, metallocarbene formation would proceed as usual, but that /S-hydride elimination would compete with the desired 1,5-insertion. Such a /3-hydride elimination could, in fact, be viewed as a 1,2-insertion, i.e., 1 to 2. 

The reported addition of triphenylaluminum to diphenylacetylene to form 1, 2, 3-triphenylbenzaluminole 22) is another clear example of an acetylene insertion, this one being followed by a cyclization reaction. 

Achiral catalysts derived from VCl4/Al(C2H5)2 or V(acac)3/A1(C2H5)2 are the best known examples, giving syndiotactic polymers [54]. Termination experiments have proved that this polymerization involves a 2,1-insertion instead of the expected 1,2-insertion. This is to say that the intermediate vanadium alkyl contains a branched or secondary alkyl group. Two simple assumptions now... 

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