Unsaturated compounds insertion reactions

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

The insertion reaction is the addition of a covalent metal compound, M-X, to a neutral unsaturated molecule, Y, forming a new complex where the unsaturated molecule has inserted itself between the metal and the atom wThich wras initially bonded to the metal. 

The insertion reaction is usually more complicated than equation (1) would indicate. The evidence now available suggests that M-X must be coordinately unsaturated in order to react with Y. Therefore, before the insertion reaction can occur, a preliminary step is often required to form M-Z from a coordinately saturated species. Furthermore, the insertion reaction may not go to completion or may not even go at all, unless there is another ligand molecule present to form a stable, coordinately saturated compound from M-Y-Z as the final product. 

There is considerable evidence that at least many of these reactions require coordinately unsaturated compounds to proceed. In those cases, a dissociation step may be the first part of the reaction. Then the.se coordinately unsaturated compounds react with an unsaturated molecule—it can be most anything as long as it has an available pair of electrons—and this inserting molecule goes in between the metal atom and one of the groups initially bonded to the metal. If the starting material is coordinately unsaturated, so is the product. A final step must be the formation of a coordinately saturated product by some final reaction, either with another ligand or by decomposition of this insertion product. 

Insertion Reactions of Compounds of Metals and Metalloids Involving Unsaturated Substrates M. F. Lappert and B. Prokai Olefin Oxidation with Palladium (II) Catalyst in Solution A. Aguil6... 

An unsaturated compound in the course of its isomerization in magnesium-containing films performs two functions (i) it forms a catalyst when the Mg4 cluster is inserted into the activated C-H bond, and (ii) it acts as a reaction substrate. The stages of the synthesis of the catalyst and isomerization can be separated. For example, anthracenyltetramagnesium hydride can be preliminarily obtained in anthracene-magnesium films, and then it can be used as a catalyst. The introduction of this cluster compound into a solution of allylbenzene or methylindene at room temperature ensures high yields of multiple bond migration products. 

In addition to carbon monoxide, other unsaturated compounds, for example isonitriles and acetylenes, can also insert into C-H bonds to give aldimines and substituted alkenes, respectively [12, 13]. Similar to carbonylation, high terminal selectivity for n-alkanes were also observed in these reactions. 

Allylic C-H insertions have been used in key steps of the enantioselective synthesis of the pharmaceuticals (+)-ceitedil (26) [21] and (+)-indatraline (27) [22] (Scheme 11). The allylic C-H insertion reaction is an exciting alternative to the Claisen rearrangement as a rapid method for the synthesis of y,c>-unsaturated ester [23 ]. Similarly, the allylic C-H insertion with vinyl silyl ethers generates protected 1,5-dicarbonyl compounds, a complimentary reaction to the Michael addition [24]. Both types of C-H insertion can be achieved with high diastereoselectiv-ity and enantioselectivity [23, 24]. 

This is the second chapter of a two-part review concerned with insertion reactions of transition metal-carbon a-bonded compounds. The first chapter, which appeared in Volume 11 of this series (137), provided a broad introduction to the subject of insertion reactions in general and a detailed treatment of the carbon monoxide insertion and decarbonylation. Presented herein are the insertion and elimination reactions of sulfur dioxide and of a few other unsaturated molecules. The reactions of sulfur dioxide are accorded a complete literature coverage, whereas those of the other inserting species are treated selectively. Metal-carbon a-bonded compounds of the main group elements are discussed only in the context of comparisons with their transition metal analogs. 

These reactions are considered to involve insertion of the unsaturated compounds to arylpalladium species followed by the formation of palladacycle intermediates. Oxidative addition of another halide molecule to them leads to the products. In the reaction with norbornene [105 -108] and diphenylacety-lene [109],the corresponding 3 1 and4 1 products and 3 1 product,respectively, are also formed under somewhat different conditions. The mechanisms to account for the formation of these unusual products involving multiple C-H cleavage steps have been proposed. It is noted that, in contrast to Eq. (49), treatment of aryl bromides with aliphatic internal alkynes gives allene derivatives (Eq.50) [110]. 

Insertion reactions of compounds of 9S metals and metalloids involving (294) unsaturated substrates Complex acetylides of transition 6 metals (29)... 

There are three general mechanisms for insertions concerted, free radical, and heterolytic addition. In the 1,2-insertion, the concerted mechanism proceeds via interaction of the 7t system of the unsaturated compound directly with the intact E-H bond, with each end of the n system directed at either the E or the H atom (Scheme 1). This interaction may or may not be preceded by precoordination of the unsaturated molecule to the element. The transition state for this reaction is considered to be four-centered, and yields products that are cis-substituted on the reduced unsaturated substrate. 

Radical addition of E-H to unsaturated compounds involves initial homolysis of the E-H bond into free radicals, followed by attack of one or the other radical on the unsaturated compound, then collapse. The heterolytic mechanism is similar to free radical, only in this case the initial E-H rupture generates charged species, which then attack the unsaturated n system. In both the latter systems, the stereochemistry of the insertion reaction can be either syn or anti, the differentiation being made by the selectivity of the second attack step on the unsaturated moiety. The steric and electronic factors that influence this selectivity are quite system specific, and often these reactions yield a mixture of products. 

Insertion into element-hydrogen bonds tend to be less favored thermodynamically than insertions into other bonds (e.g., element-carbon). This is often attributed to the high element-hydride bond strength, which is lost upon insertion. Since the insertion reaction is also entropically disfavored, the reverse deinsertion of the unsaturated moiety to generate an element-hydride bond can be thermodynamically favored. When the hydride exists in the P position of the inserted product, this process is commonly referred to as /S-hydride elimination. Nevertheless, there are many examples of insertions into element-hydride bonds that generate stable compounds, and when this insertion reaction is an uphill process, chelation to the element or subsequent chemistry (i.e., catalytic cycles) can be employed to facilitate the initial insertion step. 

The subsections that follow illustrate examples of the insertion reactions that have been noted for unsaturated substrates with element-hydride compounds. These are broken down by unsaturated species, with a strong concentration on reactions that lead to stable insertion products. 

Oxidative additions are a special class of insertion reactions. In addition to the categories mentioned in Section 10, which covers this topic, insertions of alkylidenes, silylenes, etc., into M-H bonds fall into an ambiguous domain they are insertion reactions of the unsaturated species into the M-H bond, yet oxidative additions at the C, Si, etc., atom. A similar ambiguity exists regarding the reverse reactions, namely /i-hydride and a-hydride eliminations from element-alkyls compounds to yield hy-drido-olefin and hydrido-alkylidene complexes, respectively. The former reaction is a reverse insertion if the product is viewed as an olefin complex, but an oxidative addition if it is viewed as a three-membered metallocycle. The latter reaction is a reverse insertion if the alkylidene is viewed as neutral, but an oxidative addition of a C-H bond to the metal centre. The tautomerization of phosphorous acid and of dialkylphosphites ... 

The possibility of using C02 for the synthesis of fine chemicals that are now derived from petroleum has prompted efforts to obtain a broader understanding of the coordination chemistry of CO2 during the past 20 years.1-21 Carbon dioxide utilization will inevitably center on metal complexes and their ability to bind C02. In the past decade, many C02—metal complexes have been prepared and the ligand has demonstrated a remarkable variety of coordination modes in its complexes. The sections below outline the synthesis, characterization by X-ray crystallography and IR spectroscopy, and some characteristic reactions of these compounds. Also discussed are C02 insertion reactions into M—X bonds and oxidative coupling reactions between C02 and unsaturated substrates which occur at some metal centers. Finally, a profile of the research on catalytic reductions of C02 is provided. Where possible, references are made to reviews rather than to the primary literature. 

---