Mechanistic and Computational Studies

Both mechanistic and computational studies have been used to explore the catalytic process. A crystal structure of the catalysts is available (Figure 5.7).152 The... 

Diphenylpyrrolidine (77) catalyses the enantioselective cy-chlorination of aldehydes.299 Mechanistic and computational studies suggest that - in contrast to pre- viously proposed mechanisms involving direct formation of the carbon-electrophile bond - iV-chlorination occurs first, followed by a 1,3-sigmatropic shift of chlorine to the enamine carbon. The product iminium ion is then hydrolysed in the ratedetermining step. 

The examples presented in this chapter also illustrate a development of the structure correlation method itself. Initially, it was applied to whatever representatives of a specific fragment happened to be available (Cd). Later, directed searches in the CSD [6] led to sometimes surprising new types of correlations (Sn). More recently, compounds have been synthesized in a planned way and their structures determined, so that a specific structural feature or reactivity problem could be studied (Si, B). We have also seen that the methods and interpretations of structure correlation can be applied to results of quantum-chemical calculations. Combinations of kinetic, mechanistic, and computational studies together with structure correlations, are just beginning to illuminate as yet poorly understood problems of chemical reactivity and selectivity, e.g. the factors differentiating between substitutions proceeding with retention or inversion at Si. 

The [t 4j + t 2j] cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes. This reaction is known as the Diels-Alder (abbreviated D-A in this chapter) reaction. The transition structure for a concerted reaction requires that the diene adopt the s-cis conformation. The diene and substituted alkene (called the dienophile) approach each other in approximately parallel planes. This reaction has been the object of extensive mechanistic and computational study, as well as synthetic application. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, ethene with butadiene, as demonstrated by isotopic labeling. ... 

The preparation of iodonium phenolates 410 was first reported in 1977 via a reaction of phenols 409 with (diacetoxyiodo)benzene followed by treatment with pyridine (Scheme 2.119) [553]. The system of an iodonium phenolate is stabilized by the presence of at least one electron-withdrawing substituent on the aromatic ring. Monosubstituted iodonium phenolates 410 are relatively unstable and easily rearrange to iodo ethers 411 under heating. Such a 1,4 aryl migration is a very common phenomenon for iodonium ylides of types 405-408 according to mechanistic and computational studies it is an intramolecular rearrangement via a concerted mechanism [554,555]. 

Recently, mechanistic and computational studies aiming to establish whether organic electrophiles bearing halides on a sp -hybridized carbon could indeed undergo an oxidative addition leading to paUadium(IV) complexes, and to uncover alternative pathways that could account for the formation of the observed products, have appeared in the literature. In a representative study, Echavarren reported DPT calculations comparing the feasibility of a direct oxidative addition of vinyl halides to pallada(II)cycles analogous to those proposed in Dyker s and Catellani s... 

Dove AP, Gibson VC, Marshall EL, Rzepa HS, White AJP, Williams DJ. 2006. Synthetic, Structural, Mechanistic, and Computational Studies on Single-Site 3-Diketiminate Tin(ll) Initiators for the Polymerization of rac-Lactide. J Am Chem Soc 128 9834-9843. 

Ball ND, Gary JB, Ye Y, Sanford MS. Mechanistic and computational studies of oxidatively-induced aryl-CFs bond-formation at Pd rational design of room temperature aryl trifluoromethylation. J. Am. Chem. Soc. 2011 133 7577-7584. 

Schultz MJ, Adler RS, Zitakiewicz W, Privalov T, Sigman MS (2005) Using mechanistic and computational studies to explain ligand effects in the palladium-catalyzed aerobic oxidation of alcohols. J Am Chem Soc 127 8499-8507... 

The Brookhart laboratory has contributed much of the knowledge of the polymerization mechanism for the late transition metal a-diimine catalysts. The review by Ittel provides a concise summary of the mechanistic understanding as of the year 2000 [26]. Some of the early findings will be reviewed here and additional insights reported afterward will be presented. In addition to the experimental work, many theoretical and computational studies worthy of discussion have also been carried out. These efforts have been most important in providing insight into the mechanistic details of the highly reactive nickel system, which is often difficult to study experimentally. 

It is the goal of this book to present in one place the key features, methods, tools, and techniques of physical inorganic chemistry, to provide examples where this chemistry has produced a major contribution to multidisciplinary efforts, and to point out the possibilities and opportunities for the future. Despite the enormous importance and use of the more standard methods and techniques, those are not included here because books and monographs have already been dedicated specifically to instrumental analysis and laboratory techniques. The 10 chapters in this book cover inorganic and bioinorganic spectroscopy (Solomon and Bell), Mossbauer spectroscopy (Miinck and Martinho), magnetochemical methods (Kogerler), cryoradiolysis (Denisov), absolute chiral structures (Riehl and Kaizaki), flash photolysis and studies of transients (Ferraudi), activation volumes (van Eldik and Hubbard), chemical kinetics (Bakac), heavy atom isotope effects (Roth), and computational studies in mechanistic transition metal chemistry (Harvey). 

We have discussed recent computational and spectroscopic results on the photoinduced hydrogen transfer and proton transfer chemistry in hydrogen-bonded chromophore-solvent clusters. The interplay of electronic spectroscopy of size-selected clusters and computational studies has led to a remarkably detailed and complete mechanistic picture... 

Subsequent kinetic, spectroscopic, and computational studies, however, support a different mechanistic pathway [28, 29]. A simplified version of the mechanism features oxidation of Cu and TEMPOH by Oj to afford a Cu -OH species and TEMPO (Scheme 6.1). The copper hydroxide and nitroxyl radical... 

Abstract The oxidative functionalization of olefins is an important reaction for organic synthesis as well as for the industrial production of bulk chemicals. Various processes have been explored, among them also metal-catalyzed methods using strong oxidants like osmium tetroxide. Especially, the asymmetric dihydroxylation of olefins by osmium(Vlll) complexes has proven to be a valuable reaction for the synthetic chemist. A large number of experimental studies had been conducted, but the mechanisms of the various osmium-catalyzed reactions remained a controversial issue. This changed when density functional theory calculations became available and computational studies helped to unravel the open mechanistic questions. This mini review will focus on recent mechanistic studies on osmium-mediated oxidation reactions of alkenes. 

The first suggestion that ligand loss was important in reductive eliminatimi reactions from Pt(lV) complexes dates back more than 40 years. In 1969, Ettore observed that added iodide inhibited the reductive elimination of Phi from L2PtPh2l2 in methanol and suggested iodide loss to form a six-coordinate solvento intermediate [13]. Several years later, a five-coordinate intermediate was proposed by Puddephatt in studies of C-C reductive eliminations from Pt(IV) complexes (see Sect. 2.1.1) [15]. Since then, the involvement of five-coordinate intermediates has been supported so consistently in both experimental (e.g., [10,13-42]) and computational studies (e.g., [29, 30, 43 8]) of alkyl C-C, C-H, and C-X reductive elimination that it is now accepted as the norm in mechanistic schemes for reductive elimination from Pt(IV). 

Abstract The Shilov system, a mixture of di- and tetravalent chloroplatinate salts in aqueous solution, provided the first indication of the potential of organotransition metal complexes for activating and functionalizing alkanes under mild conditions the participation of higher-valent species plays a crucial role. In this chapter, we discuss the experimental and computational studies that have led to detailed mechanistic understanding of C-H activation and functionalization by both the original Shilov system and the many subsequent modifications that have been developed, and assess the prospects for practical, selective catalytic oxidation of alkanes using this chemistry. 

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