Insertion alkene into

The acylpalladium complex formed from acyl halides undergoes intramolecular alkene insertion. 2,5-Hexadienoyl chloride (894) is converted into phenol in its attempted Rosenmund reduction[759]. The reaction is explained by the oxidative addition, intramolecular alkene insertion to generate 895, and / -elimination. Chloroformate will be a useful compound for the preparation of a, /3-unsaturated esters if its oxidative addition and alkene insertion are possible. An intramolecular version is known, namely homoallylic chloroformates are converted into a-methylene-7-butyrolactones in moderate yields[760]. As another example, the homoallylic chloroformamide 896 is converted into the q-methylene- -butyrolactams 897 and 898[761]. An intermolecular version of alkene insertion into acyl chlorides is known only with bridgehead acid chlorides. Adamantanecarbonyl chloride (899) reacts with acrylonitrile to give the unsaturated ketone 900[762],... 

In the first process alkenes insert into the Al-C bonds of monomeric AIR3 at 150° and 100 atm to give long-chain derivatives who.se composition can be clo.sely controlled by the temperature, pressure and contact time ... 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

The phosphine-based platinum(O) catalysts do not catalyze the diboration of alkenes because of the high coordination ability of phosphine over the alkene double bond, but platinum(O) complexes without a phosphine ligand such as Pt(dba)2 [128] and Pt(cod)2 [129] are an excellent catalyst allowing the alkene insertion into the B-Pt bond under mild conditions (Scheme 1-30). The diboration of aliphatic and aromatic terminal alkenes takes place smoothly at 50°C or even at room temperature. The reaction is significantly slow for disubstituted alkenes and cyclic alkenes, but cyclic alkenes having an internal strain afford ds-diboration products in high... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

Alkene insertion into the Pd-H bond with formation of an alkyl intermediate (Eq. (5)). 

In the case of ethylene hydrogenation, the mechanism proposed by Parshall [61] involves the coordination of an alkene molecule through a five-coordinate intermediate (Eq. (13)) the subsequent alkene insertion into the Pt-H bond (Eq. (14)) and intervention of a second molecule of H2 (Eq. (15)) leads to the elimination of ethane and restoration of the catalytic active species [PtH(SnCl3)]2. However, in 1976 Yasumori and coworkers reported a kinetic analysis conducted on the hydrogenation of ethylene catalyzed by the Pt-Sn complex [(Me)4N]3[Pt(SnCl3)5] [70], under much milder conditions than those... 

Figure 6.1. Substrate-catalyst interactions favor a specific mode of alkene insertion into the zirconocene—alkene complex.

The metal hydride bond is stronger than a metal carbon bond and the insertion of carbon monoxide into a metal hydride is thermodynamically most often uphill. Alkene insertion into a metal hydride is thermodynamically allowed and often reversible. 

Two possible scenarios exist which are consistent with this rate equation. Rate determining alkene coordination by 3c and 3t followed by rapid alkene insertion into the Rh-H bond is one possibility. An equally valid alternative explanation for the observed kinetics involves rate determining migratory... 

While the reductive elimination is a major pathway for the deactivation of catalytically active NHC complexes [127, 128], it can also be utilized for selective transformations. Cavell et al. [135] described an interesting combination of oxidative addition and reductive elimination for the preparation of C2-alkylated imida-zohum salts. The in situ generated nickel catalyst [Ni(PPh3)2] oxidatively added the C2-H bond of an imidazolium salt to form a Ni hydrido complex. This complex reacts under alkene insertion into the Ni-H bond followed by reductive elimination of the 2-alkylimidazolium salt 39 (Fig. 14). Treatment of N-alkenyl functionalized azolium salts with [NiL2] (L = carbene or phosphine) resulted in the formation of five- and six-membered ring-fused azolium (type 40) and thiazolium salts [136, 137]. 

The n complex 1 has not been isolated or observed directly, but its involvement is strongly supported by indirect evidence. In the second step the alkene inserts into the cobalt-hydrogen bond to yield an alkylcobalt complex (2), which is transformed via the migratory insertion of CO into a coordinatively unsaturated acylcobalt complex (3). 

Both five-coordinate and four-coordinate pathways have been proposed for these reactions. The associative (five-coordinate) mechanism involves the formation of a trigonal bipyramidal or square pyramidal intermediate, which can revert back to tetracoordination by alkene insertion into the Pt—H bond.151 The dissociative (four-coordinate) mechanism involves initial substitution of a ligand other than hydride by alkene, followed by insertion to form the alkyl product. The ligand which is substituted is usually the anionic ligand, and if this group is trans to hydride an isomerization will need to occur prior to insertion of the coordinated alkene into the Pt—H bond. 

The compound (dppe)PtMe(OMe),256 which is prepared by a metathesis reaction involving NaOMe and (dppe)PtMe(Cl) in a mixed benzene/methanol solvent system (dppe = bis(l,2-di-phenylphosphino)ethane), does not react with ethylene or pentene but does react with activated alkenes such as acrylonitrile, methylacrylate and fluoroalkenes. The reaction involving tetrafluoro-ethylene has been shown to give (dppe)PtMe(CF2CF2OMe), providing the first example of an alkene insertion into an M—OR bond.256 Interestingly, no insertion into the Pt—Me bond was observed. 

The 5-phenyldibenzophosphole (25) complex [RhH(DBP)4] catalyzes the hydrogenation of terminal alkenes, 1,5-hexadiene and some substituted alkenes. With 1-hexene it was far more active than [RhCl(PPh3)3] or [RhH(PPh3)4]. The mechanism is again thought to involve dissociation of DBP followed by alkene insertion into the rhodium hydride bond and subsequent reaction with hydrogen.123... 

Dimethylbutadiene)HfCp (Cl)] (74c) reacts with one molar equivalent of acetylene to yield the unusual product 87. This is probably formed by a conventional butadiene/alkyne coupling at the Group 4 metal center, followed by an intramolecular alkene insertion into the adjacent hafnium to carbon cr-bond. The resulting alkylidene complex (86) then rapidly dimerizes to yield the observed final product (see Scheme 28), that was characterized by X-ray diffraction.96... 

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry. For example, if the alkene insertion reaction continues with further alkene inserting into the M C bond, it can form the basis for catalytic alkene polymerisation. Other catalytic cycles may include oxidative addition and reductive elimination steps. Figure above shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

Besides enyne metathesis [66] (see also the chapter Recent Advances in Alkenes Metathesis in this volume), which generally produces 1-vinylcyclo-alkenes, ruthenium-catalyzed enyne cycloisomerization can proceed by two major pathways via hydrometallation or a ruthenacycle intermediate. The RuClH(CO)(PPh3)3 complex catalyzed the cyclization of 1,5- and 1,6-enynes with an electron-withdrawing group on the alkene to give cyclized 1,3-dienes, dialkylidenecyclopentanes (for n=2), or alkylidenecyclopentenes (for n= 1) [69,70] (Eq. 51). Hydroruthenation of the alkyne can give two vinylruthenium complexes which can undergo intramolecular alkene insertion into the Ru-C bond. 

Ans. (a) PR, and R. (b) Alkene insertion into Rh-H bond followed by reductive elimination of alkane, (c) No back-donation from Ti4+(d°) but back-donation from Pt2+(d8). (d) Facile /3-elimination from C2H5. 

The latter complex undergoes CO loss to generate coordinatively unsaturated 4.28. Conversion of 4.28 to 4.30 is the crucial step that is responsible for the formation of the branched isomer. Obviously this reaction is possible only when propylene is present as one of the reactants, or under reaction conditions where propylene from //-propanol is generated in situ. Conversion of 4.28 to 4.30 is an example of alkene insertion into an M-H bond in a Markovnikov manner (see Section 5.2.2 for a discussion on Markovnikov and anti-Markovnikov insertion). The anti-Markovnikov path leads to the formation of 4.29, which is in equilibrium with 4.24. Complexes 4.25 and 4.26 are analogues of 4.4 with //-butyl and /-butyl groups in the place of methyl. They reductively eliminate the linear and branched acid iodides. In the presence of water the acid iodides are hydrolyzed to give //-butyric and / -butyric acids. 

Figure 5.2 Alkene isomerization by alkene insertion into the M-H bond followed by /3-hydride elimination. Note that the insertion is in a Markovnikov manner. The 13-carbons from which /3-hydride elimination leads to isomerization are marked by asterisks.

A general catalytic cycle proposed for Heck reaction is shown in Fig. 7.17. While all the steps in the catalytic cycle have precedents, the proposed reaction mechanism lacks direct evidence. The basic assumption is that under the reaction conditions, the precatalyst is converted to 7.64, a coordinatively unsaturated species with palladium in the zero oxidation state. Oxidative addition of ArX, followed by alkene coordination, leads to the formation of 7.65 and 7.66, respectively. Alkene insertion into the Pd-C bond followed by /3-H abstraction gives 7.67 and 7.68, respectively. Reductive elimination of HX, facilitated by the presence of base B, regenerates 7.64 and completes the catalytic cycle. The C-C coupled product is formed in the 7.67 to 7.68 conversion step. 

A hypothetical catalytic cycle for asymmetric hydroformylation reaction is shown in Fig. 9.13. The precatalyst Rh(acac)(P-P) reacts with H2 and CO to give the square planar catalytic intermediate 9.47. Alkene addition to 9.47 can lead to the formation of 9.48, 9.49, and 9.50. The steric requirements of the chelating ligand would have to be such that the formation of 9.50 is avoided. This is because alkene insertion into the Rh-H bond in this case would lead to the formation of the linear rather than the branched alkyl. Both 9.48 and 9.49, which differ in the coordination positions of the phosphorus atoms, can give 9.51, which has the desired branched alkyl ligand. 

Ans. Similarity Both prochiral and undergo enantioface selective alkene insertion into M-H bond (hydride attack). Difference The first acts as a chelating ligand all throughout the catalytic cycle, but the second only in 9.32. 

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