Insertion Reactions

There are many reactions of transition metal-alkyl or -aryl complexes which may be represented thus  

The nature of interactioDS between transition metals and o bonded organic substituents 

In addition to the cr-bond which must be present there is evidence that some substantial t-bonding may occur in the metal-aryl, -acyl and -alkynyl bonds (see p. 273). There is considerable evidence that transition metals may form double bonds with =CR2 (carbene) systems. 

Numerous examples of insertion of carbon dioxide into metal to carbon bonds are known. This reaction usually affords ester products. However, treatment of liganded diphenyltita-nium gives rise to the formation of the mono- 98 and the diinsertion product 99 . 

A similar carboxylation of the benzene ring occurs in the reaction of L2Ti(Me)Ph with carbon dioxide . 

The photolysis of L2TiMe2 in the presence of carbon dioxide affords the mono insertion product 100 .  

Mono insertion of carbon dioxide also occurs into the intermediate 101 of the reaction between quadricyclane and [(bipy)Ni(cod)L to give the metallacycle 102 .  

Insertion reactions of carbon dioxide into P-N, As-N, Si-N and Hg-O bonds are also reported. For example, addition of carbon dioxide to the amides of trivalent phosphorous 

Heterocumulenes undergo insertion reactions with numerous substrates. In general, carbodiimides react faster than isocyanates and isothiocyanates, in that order. Insertions of carbodiimides into metal-hydrogen, metal-halogen, metal-mitrogen, metal-oxygen and metal-sulfur bonds are reported. Also insertions of carbodiimides into carbon-hydrogen bonds are known. 

Examples of insertion reactions include Grignard or alkyl lithium compounds which react with carbodiimides to give formamidines 389 after hydrolysis.  

The 1 1 reaction between the magnesium amide 390 (R = i-Pr) and diisopropylcarbo-diimide affords dinuclear amidinate complexes 391.  

Amidino-bridged mixed aluminum-magnesium complexes are obtained in the reaction of Al-Mg complexes with carbodiimides.  

Aluminium amidinate complexes 392 are synthesized by addition of aluminum alkyls to aliphatic carbodiimides. Also, alkylation of carbodiimides with MeLi, followed by reaction with AICI3 affords aluminum amidinate complexes 

A useful review has been written by Kuhlmann and Alexander on the migratory insertion of carbon monoxide into metal-carbon a bonds. It includes a section on kinetics and mechanism. 

Extended CNDO calculations support the alkyl migration mechanism for the well-known process in  

The observed sequence of increasing reaction rate for the forward reaction, R = CF3 CH3 C2H5, is calculated. It is further shown that a mechanism involving P-H elimination is reasonable for the side reaction  

The kinetics of the water-organic-solvent biphasic reaction of PhCH2Br, Na [Co(CO)4]-, [Bu4N] C1 , and CO to give [PhCH2COCo(CO)4] have been analyzed in terms of reactions (49) and (50), which occur in the organic phase. 

The first step is first order in each reactant, rate determining, and slightly dependent on the organic solvent. 

The CO insertion can be an important step in carbon-carbon bond-forming reactions that are catalyzed by organometallic complexes. At first sight, this appears to be an insertion of the entering CO into the Mn—CHj bond and the name insertion has continued to be used. However, phosphines and other nucleophiles, L, bring about an analogous transformation, as shown in the following reaction  

This could still be an insertion of a coordinated CO into the M—CHj bond, but it might also be migration of the CHj to a coordinated CO. The classic IR study of Noack and Calderazzo using C-labeled CO showed that the 

Brunner et al. have shown that methyl migration occurs in the system studied by Flood if the reaction is catalyzed by BF3. This catalysis is consistent with BFj complexing with the oxygen of CO, thereby decreasing the electron density on the C and promoting CHj migration. 

For the analogous CHj derivative of Pd(II), CHj migration was found. Qualitative rate observations indicated that the least stable isomer, based on the trans effect, is the most reactive in these systems. 

The pseudo-first-order rate constant ([L] [Mn]) is given by 

When a group enters a complex by being inserted between the metal and another ligand in the complex, the reaction is known as an insertion reaction. This type of reaction can be shown as follows  

Some molecules that will undergo insertion reactions are CO, SnCl2, S02, R-NC, C2H4, and (CN)2C=C(CN)2. The following equations show examples of insertion reactions  

This reaction proceeds via an intramolecular oxidative addition, followed by reductive elimination of HC1. But many closely-related reactions apparently do not involve oxidative addition. Consider, for example, the orthomanganation reaction  

Both complexes are octahedral, 18-electron Mn(I) species. The formation of a seven-coordinate Mn(III) intermediate by oxidative addition is unlikely and the mechanism is probably a concerted process in which no Mn-H bond is ever formed. 

These too have both synthetic and catalytic significance. Here we are dealing with reactions in which a molecule - such as CO - is inserted into the M-L bond in a coordination/organometallic compound, e.g.  

The term is also apt for reactions of Main Group molecules which may not strictly qualify for admission to the realm of coordination chemistry an example discovered in 1869 is  

The term insertion is usually restricted to reactions in which the central atom undergoes no change in oxidation state in typical oxidative addition reactions, ML, is inserted into X—Y, with oxidation of M. 

A coupling reaction of great interest is McMurry s titanium-mediated synthesis of alkenes from two ketones (Eq. 14.66). This involves a reduced form of titanium, perhaps Ti(0), which may give the sequence of reactions shown in Eq. 14.67. These ideas are supported by the fact that 1,2-diols are also reduced to the alkene. Whatever the mechanism, the reaction shows the strongly oxophilic character of this early metal. A large number of reactions of this type are known with Sml2  

Diels-Alder reactions between a, -unsaturated carbonyl compounds and 

Leighton has applied Rh/PR3-catalyzed hydroformylation (Section 9.3) to a polyol synthesis (. 14.71) as a useful alternative to the aldol strategy (Eq. 14.70). 

In the cyclic enol acetal of Eq. 14.72, this has the great advantage of being highly diastereoselective, producing the 1,3-sy/i-diol derivative. The bulky phosphine, P(o-r-BuC6H40)3 proved the most effective. 

Silylformylation, in which the H2 component of the H2/CO mixture of hydroformylation is replaced by a silane to give net addition of RsSi— and —CHO across an unsaturated bond, is normally effective for alkynes but not alkenes. Leighton and co-workers have used a chelation strategy (Eq. 14.73) to produce products that can be further functionalized by oxidation of the C—Si bond. 

Nearby C=C bonds will insert into the product acyls, a reaction that has been used in a synthesis of aphidicolin (Eq. 14.76 ). We will see further examples of CO insertion reactions of metal alkyls in the next section. 

As we saw in Section 8.3, the binding of a polyene or poiyenyl ligand to a metal can suppress the reactivity toward electrophiles usually seen for the free polyene, and encourages attack by nucleophiles instead. This reversal of the normal reactivity pattern (umpolung) has been very widely used in organic synthesis. 

Cydopentadlenyl Iron Reagent In the case of simple alkenes, the best-studied system is Rosenblum s Fp reagent (see also Section 14.3), (CpFe(CO)2(alkene)). Thanks to its positive ionic charge, it activates even simple alkenes for nucleophilic attack. The sequence shown in Eq. 14.77 illustrates how the alkene complex may be synthesized from a p-alkoxy alkyl 

The palladium selectively attacks an allylic acetate with inversion, even in the presence of other reactive groups, such as a C—Hal bond nucleophilic attack then occurs exclusively at the allyl group, showing the strongly activating effect of the metal (Eq. 14.80)  

The nucleophile usually attacks the exo face of the allyl group (the one opposite the metal), and at the least hindered terminus of the allyl group (although this preference can be partially reversed by addition of ligands). The stereochemical consequences of this sequence have been used to define the relative stereochemistries of two chiral centers five carbons apart in an 

Dimerization of carbenes almost never occurs, at least not directly. The reason for the non-occurrence of the reaction is simply that the carbene concentration is so low that these species cannot find each other. Indirect dimer formation is possible in some special cases, for example, under the conditions of fiash photolysis or in a warming or inert matrix. 

As discussed in Section I, the reaction of ally lie halides with nickel carbonyl at atmospheric pressure leads to coupling products or in some cases, in hydroxylic solvents, to substitutive hydrogenation (34). Under 

The principal investigators in this field (Chiusoli and co-workers) have recently reviewed their contribution (39), and we will here only outline the course of reaction and discuss the most recent developments. 

Two groups of workers (40, 41) have demonstrated that the reaction proceeds through the formation of a 77--allylnickel intermediate which absorbs CO to form a nickel acyl complex. This then liberates a molecule of acyl halide which is hydrolyzed by the solvent. The presence of the intermediate nickel acyl complex in solution has been demonstrated 

2 The complexity of the reactions described in this section does not allow meaningful balanced equations to be written. In the majority of cases the equations are quoted directly from the original publication. 

The formation of the pentacoordinate species (XX) is also supported by the reaction of the triphenylphosphine derivative (XXI) with CO in methanol to give, besides allyl methyl ether, methyl butenoate (14). 

Migration of a coordinated nucleophile to an alkene or acetylene bonded to the central metal atom leads to the formation of alkyl or alkenyl complexes, respectively. The addition of an external nucleophile (not bonded to the metal) to coordinated alkenes occurs rarely (Chapter 13). Most commonly, the migrating ligand is H. However, many alkyl, halogen, etc., migrations are known. 

Reactions of rhodium hydrido complexes with fluoroolefins give migration 

Reactions of diazomethane with transition metal complexes also furnish derivatives possessing M —C r bonds  

Dichlorocarbene, which is formed by thermal decomposition of CCl3COONa, reacts with the tungsten hydrido complex to give the dichloromethyl compound  

In the case of cyclic compounds, hydrogen migration may be accompanied by ring opening  

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Insertion reaction of a vinyl carbene (terminal acetylenes)... 

The reaction of Grignard reagents with a carbonyl group can be understood as an insertion reaction of an unsaturated C=0 bond of the carbonyl group into... 

The main example of a category I indole synthesis is the Hemetsberger procedure for preparation of indole-2-carboxylate esters from ot-azidocinna-mates[l]. The procedure involves condensation of an aromatic aldehyde with an azidoacetate ester, followed by thermolysis of the resulting a-azidocinna-mate. The conditions used for the base-catalysed condensation are critical since the azidoacetate enolate can decompose by elimination of nitrogen. Conditions developed by Moody usually give good yields[2]. This involves slow addition of the aldehyde and 3-5 equiv. of the azide to a cold solution of sodium ethoxide. While the thermolysis might be viewed as a nitrene insertion reaction, it has been demonstrated that azirine intermediates can be isolated at intermediate temperatures[3]. 

Single-Carbon Insertion Reactions. Carbonylation, cyanidation, and "DCME," and related reactions are convenient general processes developed to bring about the transfer of organic groups from boron to a siagle-carbon atom. 

Insertion Reactions. Isocyanates also may undergo iasertion reactions with C—H bonds. Acidic compounds, such as 1,3-dicarbonyl compounds (6), react readily at room temperature to form carboxyamides. At higher temperatures carboxyamides frequentiy undergo secondary reactions leading to cyclized products (33,34). 

Naphthaleneacetic acid has also been prepared by the carbonyl-insertion reaction of 1-chloromethylnaphthalene cataly2ed by carbonyl cobalt cation (90,91). Carboxylation of 1-chloromethylnaphthalene in the presence of the catalyst Pd[P(CgH )2]2Cl2 under phase-transfer conditions gave 1-naphthaleneacetic acid in 78% yield (92). 

Organometallic Compounds. Mononuclear carbon monoxide complexes of palladium are relatively uncommon because of palladium s high labihty, tendency to be reduced, and competing migratory insertion reactions in the presence of a Pd—C bond (201). A variety of multinuclear compounds... 

Thorium compounds of anionic nitrogen-donating species such as [Th(NR2)4], where R = alkyl or sdyl, are weU-known. The nuclearity is highly dependent on the steric requirements of R. Amides are extremely reactive, readily undergoing protonation to form amines or insertion reactions with CO2, COS, CS2, and CSe2 to form carbamates. Tetravalent thorium thiocyanates have been isolated as hydrated species, eg, Th(NCS)4(H20)4 [17837-16-0] or as complex salts, eg, M4 Th(NCS)g] vvH20, where M = NH, Rb, or Cs. 

The phosphido complex, Th(PPP)4 [143329-04-0], where PPP = P(CH2CH2P(CH2)2)2) has been prepared and fully characterized (35) and represents the first actinide complex containing exclusively metal—phosphoms bonds. The x-ray stmctural analysis indicated 3-3-electron donor phosphides and 1-1-electron phosphide, suggesting that the complex is formally 22-electron. Similar to the amido system, this phosphido compound is also reactive toward insertion reactions, especially with CO, which undergoes a double insertion (35,36). 

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. 

The catalytic cycle (Fig. 5) (20) is well estabUshed, although the details of the conversion of the intermediate CH COI and methanol into the product are not well understood the mechanism is not shown for this part of the cycle, but it probably involves rhodium in a catalytic role. The CH I works as a cocatalyst or promoter because it undergoes an oxidative addition with [Rh(CO)2l2]% and the resulting product has the CO ligand bonded cis to the CH ligand these two ligands are then poised for an insertion reaction. 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

Stannacyclopentane, 1,1-dimethyl-ring opening, 1, 608 Stannacyclopentanes, 1, 605-609 chemical properties, 1, 607-608 insertion reactions... 

Reactions of Organic nuorine Compounds Table 31. Insertion Reactions of Iminoborones [11S ... 

The 2-pentafluoropropenylzinc reagent has been prepared via a novel one-pot dehalogenation-insertion reaction [103] (equation 82)... 

Thermal insertion reactions of dihydndobis(T) - cyclopentadienyl)molybdenum with tnfluoromethylacetylene give exclusively syn addition At —40 °C, hexafluoro-2-butyne inserts to the Mo-H bond to give anti addition [40] equation 30)... 

The thiazyl cation is used for the preparation of other important S-N compounds (Scheme 5.3). For example, the insertion reactions with S4N4... 

Here arc tbc transition stnKtures for the silicon insertion reaction (left) and the 1.2 hstirogen migiation reaaion ... 

Metal-promoted alkyne-insertion reactions afford another good method (see structure 12 for cluster geometry and numbering)... 

If the starting material contains M-H or M-C bonds a further complication can arise due to the possibility of a CO2 insertion reaction. Thus, both [Ru(H)2(N2)(PPh3)3] and [Ru(H)2(PPh3)4] react to give the formate [Ru(H)(OOCH)(PPh3)3], and similar CO2 insertions into M-H are known for M = Co, Fe, Os, Ir, Pt. These normal insertion reactions are consistent with the expected bond polarities M +-H and 0 =C +=0, but occasionally abnormal insertion occurs to give metal carboxylic acids... 

CS2 is rather more reactive than CO2 in forming complexes and in undergoing insertion reactions. The field was opened up by G. Wilkinson and his group in 1966 when they showed that [Pt(PPh3)3] reacts rapidly and... 

Insertion reactions of CS2 arc known for all the elements which undergo CO2 insertion... 

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