Stereochemistry of 1,2-insertion

Biaryl bisphosphine reported by Consiglio and cataiyst reported by Sen for the copolymerization of carbon monoxide and propene to form syndiotactic CO/propylene copolymer. 

Nozaki has shown that the less symmetric BINAPHOS ligand (Equation 17.70) containing one phosphine and one phosphite donor also gives rise to highly isotactic copolymers from carbon monoxide and propylene. - NMR spectroscopic studies on is polymerization process have shown that the acyl group in one intermediate prefers to be located trans to the phosphine donor and the alkyl group of the other intermediate prefers to be located trans to the phosphite donor. Thus, olefin coordinates to the site trans to tiie phosphite prior to insertion, and CO coordinates trans to the phosphine donor prior to insertion. 

Ketal structure of the copolymer of CO and propene formed In the absence of protic component. 

Pauson-Khand Reactions (Written with Dr. Qiiong Shen) 

Chapter 17 closes with a brief presentation of the Pauson-Khand reaction. The Pauson-Khand reaction (PKR) is a formal [2+2+1] cycloaddition reaction involving an alkyne, an alkene, and carbon monoxide to form a cyclopentanone shown generically in Equation 17.71. The Pauson-Khand reaction was initially reported as a stoichiometric reaction mediated by cobalt carbonyl, but it has been translated into a catalytic process in recent years. Most recently, it has developed into an enantioselective catalytic process. Complexes of Ti, Mo, W, Fe, Co, Ni, Ru, Rh, Ir, and Pd have all been shown to catalyze this reaction. 

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The migratory insertion reactions of Pd(acyl) (alkyl acrylate) and Pd(acyl) (vinyl acetate) have been investigated by several authors and valuable mechanistic information on this important elementary step of the chain growth process has been obtained by NMR spectroscopy [li, jj. Of particular relevance are recent NMR studies by Braunstein dealing with the stereochemistry of insertion of methyl acrylate into Pd-acyl bonds derived from the reaction of CO into Pd-Me or... 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

Alkynes also insert into M—H bonds. These are of interest in that the stereochemistry of insertion is often apparent in the metal vinyl complex formed. First row metals have been observed to give trans products in which case a radical mechanism may be operative134, but second and third row metals tend to give cis products as expected from a concerted insertion. 

Flood has recently published an excellent contribution which includes a review of the stereochemistry of insertion reactions of CO and S02 into metal-carbon sigma bonds. 

On the basis of what we have discussed above, as far as enantioface selectivity is concerned, C2-sym-metric chiral metallocenes should not sense on what side, and how often, a monomer approaches the metal center. Indeed, the site control mechanism of these catalysts requires that the stereochemistry of insertion is independent from the previous insertion. The loss of stereospecificity with the decrease in monomer concentration has been accounted for, from a kinetic standpoint, with an equilibrium between active sites having a coordinated monomer (C M) and sites without coordinated monomer Q (Scheme 31). 

In an effort to evaluate if such an effect might have been due to a steric isotope cffect rather than an isotope effect due to an agostic interaction, Bercaw investigated the insertion of 2-butyne-l,l,l-d3, CD3C CHj, into (Cp -dis)2Sc-CH3. Presumably, if steric interactions between the incoming alkync and the SC-CH3 dictated the stereochemistry of insertion, then the corresponding alkyne insertion product, (Cp -d]5)2Sc-C(CH3=C(CH3)-(CD3)), would be formed preferentially to (Cp -di5)2Sc-(C(CD3)=C(CH3)2). The experiment revealed no measurable steric deuterium isotope effect, and the authors concluded that the isotope effects attributed to the agostic interactions were unlikely to be nascent effects from steric interactions. 

The reaction of norbornene with A -methylallylnickel bromide or iodide gives linear polymers of the alkene via cw-insertion. Evidence for the stereochemistry of insertion is provided by the reaction of norbornene with h -methylallylnickel chloride, which gives the alkene complex (6). Various types of reaction of (6) then result in cis-insertion of the norbornene into the nickel-allyl bond. Thus treatment of (6) with sodium acetate gives (7), which is shown by A"-ray studies to be formed by an ejcu-c/s-insertion of the alkene. A similar cw-insertion occurs with the corresponding palladium complex. ... 

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

Replacement of halides with deuterium gas in the presence of a surface catalyst is a less useful reaction, due mainly to the poor isotopic purity of the products. This reaction has been used, however, for the insertion of a deuterium atom at C-7 in various esters of 3j -hydroxy-A -steroids, since it gives less side products resulting from double bond migration. Thus, treatment of the 7a- or 7j5-bromo derivatives (206) with deuterium gas in the presence of 5% palladium-on-calcium carbonate, or Raney nickel catalyst, followed by alkaline hydrolysis, gives the corresponding 3j3-hydroxy-7( -di derivatives (207), the isotope content of which varies from 0.64 to 1.18 atoms of deuterium per mole. The isotope composition and the stereochemistry of the deuterium have not been rigorously established. 

The first step in the reaction is adsorption of Pronto the catalyst surface. Complexation between catalyst and alkene then occurs as a vacant orbital on the metal interacts with the filled alkene tt orbital. In the final steps, hydrogen is inserted into the double bond and the saturated product diffuses away from the catalyst (Figure 7.7). The stereochemistry of hydrogenation is syn because both hydrogens add to the double bond from the same catalyst surface. 

Another aspect of stereochemistry of the CO insertion which has received attention concerns the actual process of formation of the acyl moiety from the coordinated CO and R. Three possible pathways may be envisaged. First, the alkyl moves from the metal onto an adjacent CO. This is known as the alkyl migration mechanism. Second, a coordinated CO moves to insert into the M—R bond—a CO insertion mechanism. Third, both CO and R move in a cooperative manner. These three pathways are represented schematically in Eq. (46). 

A complete description of stereochemistry of the carbon monoxide insertion and decarbonylation requires knowledge of configurational changes at the metal and a-carbon. Calderazzo and Noack (54) showed that the optical activity of the equilibrium mixture... 

Irradiation of 1-azidophosphetan-l-oxide (112) in methanol leads to the phosphonamide esters (113) and (114), although the stereochemistry of these products is not yet fully settled. Their formation is reasonably consistent with the intervention of a nitrene intermediate which inserts into the P—C and C—H bonds. 

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 pyranofurooxazoline 109 can be prepared by a nitrene insertion reaction of the corresponding furan 110 upon treatment with ethyl azidoformate at — 50 °C under photolysis conditions. Compound 109 is moisture sensitive, and upon treatment with wet acidic THF was converted quantitatively to the more polar furanopyran 111. The structure and stereochemistry of 109 were proved unambiguously by X-ray diffraction, showing that the nitrene inserted anti to the bridgehead methyl group <1999JOC736> (Scheme 30). 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

The stereochemistry of the insertion by (phenylthio)carbene to the a C-H bond of trans- and c/s-4-terr-butylcyclohexyloxides 16 was investigated19 to find that the reaction proceeds stereospecifically giving trans and cw-4-rert-butyl-l-methylcyclohexanol 19, respectively, after desulfurization of the primary insertion products 17 (Scheme 9). 

A more complex cumulenyl carbenoid 80 may be generated in situ from 1,4-dihalobut-2-ynes and two equivalents of base (Scheme 3.21). Insertion into organozirconocene chlorides gives allenyl zirconium species 81, which are regioselectively protonated to afford enyne products 82 [38], The stereochemistry of the alkene in 82 stems from the initial elimination of hydrogen chloride to form 80. 

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