Transition Metal Template-Controlled Reactions

Rigby, J.H. and Henshilwood, J.A. (1991) Transition metal template controlled cydoaddition reactions. An efficient chromium(0)-mediated [6it + 2it] cydoaddition. Journal of the American Chemical Society, 113, 5122—5123. 

Some radical reactions occur under the control of transition metal templates. The first example of asymmetric creation of an asymmetric carbon with a halogen atom is shown by the a DIOP-Rh(I) complex-catalyzed addition of bromotrichloromethane to styrene, which occurs with 32% enantioselectivity (Scheme 99) (233). Ru(II) complexes with DIOP or BINAP ligands promote addition of arenesulfonyl chlorides to afford the products in 25-40% ee (234). A reaction mechanism involving radical redox transfer chain process has been proposed. 

The earliest syntheses of rotaxanes were largely based on the statistical or directed methods.2,3 Statistical methods require very precise reaction conditions, and directed methods involve numerous chemicals steps. However, the use of templates allows high control of these synthetic methods resulting in efficient and precise assemblies of rotaxanes that incorporate a wide range of chemical functionalities. Two types of interactions occur in synthetic template methods (1) purely organic and (2) transition-metal-templated. In this latter case, the template can easily be removed at the end of the synthesis, whereas in the former, the interactions between the template and the components of the final rotaxane will often be maintained. Selected examples will now illustrate the statistical, the directed, and templated strategies outlined above. The transition-metal-templated route will be developed separately. 

Currently the main interest in template reactions lies in their key role in the controlled synthesis or the self-assembly of a variety of supramole-cular entities (449). One needs a combination of intuition, conjecture, and serendipity (450) a recent example of successfully combining serendipity and rational design is provided by the silver(I)-promoted assembly of one-dimensional stranded chains (451). One also needs an understanding of mechanism in order to optimize the selection and design of building blocks and templates for the generation of yet more sophisticated supramolecular structures references cited in this present review contain at least some kinetic or mechanistic information or speculation. Template routes to interlocked molecular structures have been reviewed (452), while a discussion of switching by transition metal contains a little about the kinetics and mechanisms of this aspect of template... 

Complexes bearing protic NHC ligands are accessible by various synthetic routes such as the deprotonation of azoles followed by reaction with a transition metal complex, the template-controlled cyclization of functionalized isocyanides, and the oxidative addition of different azoles to transition metal complexes. The complexes with simple monodentate NR,NH-NHCs often tend to tautomerize to give the N-bound azoles. This type of tautomerization is prevented in complexes with donor-functionalized NR,NH-NHCs. Recent smdies demonstrate that complexes with protic NHCs obtained from C2-H azoles are formed by an oxidative addition/reductive elimination reaction sequence. The N—H group in complexes with protic NR,NH-NHCs can serve as a hydrogen bond donor and thus as a molecular recognition unit and may enable various types of bifunctional catalysis. Recent smdies indicate that even biomolecules such as caffeine can be C8-metallated. It... 

As wc know, carbon nanofibers have been widely used in various areas, such as in electronic components, as polymer additives, for gas storage, and as catalyst support [94]. Via the decomposition of hydrocarbons or carbon monoxide over Ni, Co or Fe catalysts, a large quantity of carbon fibers can be produced. Moreover, these carbon fibers with different sizes and shapes, such as straight, bent, thin and helical, etc., could be synthesized by controlling the reaction conditions [95-99]. With carbon nanofibers as template, Ueda and co-workers prepared a number of single and binary transition metal oxide nanofibers and/or nanotubes, including LaMnOs and LaFeOs nanofibers and nanotubes [99-102]. 

The template should be similar to the actual substrate to achieve selective recognition of the substrate. Imprinting a metal complex that resembles the transition state of the catalytic transformation can lead to stereo control of the reaction process. The phosphinato complex 7 was chosen as a template to mimic the transition state of the hydride transfer (Scheme 2). Phosphate analogs are good candidates for... 

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