Polymerization, and metathesis

Hydrazido and organohydrazido ligands are important intermediates in the reduction or utihzation of N2 in biological and chemical systems, and their molybdenum complexes are potential catalysts for aUcene polymerization and metathesis reactions. 

Equation 7.39 describes a transformation with first the C-H bond of cyclopropane adding to the Rh complex, followed by RE of H2, and then rearrangement to give the rhodacyclobutane.82 Metallacyclobutanes are thought to be intermediates in some alkene polymerization and metathesis reactions these compounds will appear again in Chapter 11. 

This chapter will briefly review the fundamental organometallic reactions that play a key role in almost all metal-catalyzed processes. It will then apply these reaction steps to explain currently accepted mechanisms for some major catalytic cycles hydrogenation, hydroformylation, methanol carbonylation, Pd-catalyzed coupling reactions, and alkene polymerization and metathesis. Each of these catalytic reactions is covered in considerably more detail in later chapters, so the discussion here will be limited to relating and using the various fundamental reactions to build up and describe multistep catalytic cycles. 

The saturated carbon C7 is only 2.293(7) A from Ti (and C2 is 2.579(7) A away). Remarkably C7 is closer to Ti than any of the bound olefinic carbons C4 and C5 (2.30-2.34 A) and Cp carbons, clearly indicative of metal C-C bond interaction. Similar interactions have been found in electron-deficient metallocyclobutane complexes, Cp 2M(CH2)2CHR (M = Ti and Zr but not 18e Mo and W analogues), which may have important implications in olefin polymerizations and metathesis. There is precedence for such interactions in an agostic C-Si interaction with Ti, and other C-X interactions are known. 

At this time ring-opening metathesis polymerization and metathesis of acyclic olefins -originally considered as olefin disproportionation [7] - were regarded as two different reaetions. Calderon recognized in 1972 that these both are two sides of the same coin and introduced the term olefin metathesis for this reaction type [8-11]. 

As mentioned above, Calderon recognized in 1972 that metathesis polymerization and metathesis of acyclic olefins are two aspects of the same reaction [10]. As early as 1968 he had identified the double bonds as the reactive centers in the metathesis of acyclic olefins. Apart from the educts the metathesis reaction of dg-2-butene with 2-butene yielded only d4-2-butene, so he could exclude the cleavage of any single bond [39,40]. Dali Asta and Motroni drew an analogous conclusion for ROMP by copolymerization of 1- C-cyclopentene and cyclooctene (3). After ozonolytic degradation of the polymers the complete radioactivity was found in the C5-fraction, showing the exclusive cleavage of the double bonds (pathway (3b)) [41,42]. 

Complexes with /i -methylidenes are also known. Complex 2.41 in which methylidene and chloride bridges are present between titanium and aluminum is known as Tebbe s complex. Note that had we not known the structure, Tebbe s complex might have been formulated as Cp TiCCH ) plus Me AlCl. As will be seen, Tebbe s complex has relevance in alkene polymerization and metathesis reactions. Complex 2.42 is a unique example of a monometallic complex with a single carbon atom as one of the ligands. From the earlier examples it is clear that representative organometallic complexes with M-CR (n = 0-3) have all been isolated and fully characterized. 

Cyclic Polyolefins (GPO) and Gycloolefin Copolymers (GOG). Japanese and European companies are developing amorphous cycHc polyolefins as substrate materials for optical data storage (213—217). The materials are based on dicyclopentadiene and/or tetracyclododecene (10), where R = H, alkyl, or COOCH. Products are formed by Ziegler-Natta polymerization with addition of ethylene or propylene (11) or so-called metathesis polymerization and hydrogenation (12), (101,216). These products may stiU contain about 10% of the dicycHc stmcture (216). 

K. J. Ivin, J. C. Mol, Olefin Metathesis and Metathesis Polymerization, Academic Press, London, 1997. 

Olefin metathesis, an expression coined by Calderon in 1967,1 has been accurately described in Ivin and Mol s seminal text Olefin Metathesis and Metathesis Polymerization as the (apparent) interchange of carbon atoms between a pair of double bonds (ref. 2, p. 1). This remarkable conversion can be divided into three types of reactions, as illustrated in Fig. 8.1. These reactions have been used extensively in the synthesis of a broad range of both macromolecules and small molecules3 this chapter focuses on acyclic diene metathesis (ADMET) polymerization as a versatile route for the production of a wide range of functionalized polymers. 

Lu H, Wang J, Lin Y, Oieng J (2009) One-pot synthesis of brush-like polymers via integrated ring-opening metathesis polymerization and polymerization of amino acid N-carboxyanhy-drides. J Am Oiem Soc 131 13582-13583... 

Brzezinska KR, Deming TJ (2001) Synthesis of ABA triblock copolymers via acyclic diene metathesis polymerization and living polymerization of alpha-amino acid-N-carboxyanhy-drides. Macromolecules 34 4348 354... 

Quirk RP (ed) (1988) Transition metal catalyzed polymerization Ziegler-Natta and metathesis polymerizations. Cambridge University Press, Cambridge... 

The earliest reported ring-opening polymerizations of functionalized norbornenes were carried out in protic solvents (alcohol, water) using iridium, ruthenium, or osmium salts. Thus, norbornenes substituted with ester (93-95), hydroxy (95), chlorine (96), alkoxy (97), and imide (93) groups have been polymerized via metathesis using noble metal catalysts. 

Ivin KJ, Mol JC (1997) Olefin metathesis and metathesis polymerization. Academic Press, San Diego... 

Ivin, K. I Mol, J. C. Olefin Metatheis and Metathesis Polymerization, Academic Press, 1997, San Diego USA, London UK. Special issue on metathesis Adv. Chem. Cat. 2002, 344, issue 6/7. 

Multiphase homogeneous catalysis (continued) hydroformylation, 42 483-487, 498 hydrogenations, 42 488-491 metal salts as catalysis, 42 482-487 neutral ligands, 42 481 82 organic reactions, 42 495 0X0 synthesis, 42 483-487 ring-opening metathesis polymerization and isomerization, 42 492-494 telomerizations, 42 491-492 diols as catalyst phase, 42 496 fluorinated compounds as catalyst phase, 42 497... 

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