Reactions of Unsaturated Organic Molecules

The photochemical oxidation reactions of unsaturated organic molecules lead to the formation of peroxides which are characterized by the weak bond O-O. In terms of bond energies this is one of the weakest covalent bonds weak enough to be split in thermal processes. When a cyclic peroxide dissociates into carbonyl compounds, one of these may be formed in an electronically excited state (Figure 4.83). The energy difference results from the loss of the weak 0-0 bond and the formation of two C=0 tt bonds over 60kcalmol-1 is then available as free energy. There are also reasons of... 

Reaction of Unsaturated Organic Molecules 10.2.3.3.1. Reactions with Magnesium. 

The catalytic uses of colloidal metals which will be described are found in a relatively small but important group of chemical reactions. These include hydrogenation, hydrosilylation, and hydration reactions of unsaturated organic molecules, as well as redox and other electron transfer processes such as the photochemical splitting of water and photocatalytic hydrogenations. 

With Unsaturated Compounds. The reaction of unsaturated organic compounds with carbon monoxide and molecules containing an active hydrogen atom leads to a variety of interesting organic products. The hydroformylation reaction is the most important member of this class of reactions. When the hydroformylation reaction of ethylene takes place in an aqueous medium, diethyl ketone [96-22-0] is obtained as the principal product instead of propionaldehyde [123-38-6] (59). Ethylene, carbon monoxide, and water also yield propionic acid [79-09-4] under mild conditions (448—468 K and 3—7 MPa or 30—70 atm) using cobalt or rhodium catalysts containing bromide or iodide (60,61). 

Reactions on the surface are interesting. The adsorptions of unsaturated organic molecules on the surface provide a means for fabricating well-ordered monolayer films. Thin film organic layers can be used for diverse applications such as chemical and biological sensors, computer displays, and molecular electronics. 

This interaction with a free electron pair will have the consequence that in various cases of unsaturated organic molecules certain configurations, which contribute little to the stationary state of the free molecule, are of much greater importance in the complex formed, as a result of which many reactions can proceed under the catalytic action of these sextet substances ( 36). With the proton, thus in OH3+ and also in HF, the interaction with the organic molecules is entirely electrostatic... 

Hydrosilylation of unsaturated organic molecules is an attractive organic reaction. Asymmetric hydrosilylation of prochiral ketones or imines provides effective routes to optically active secondary alcohols or chiral amines (Scheme 756). These asymmetric processes can be catalyzed by titanium derivatives. The ( A ebthi difluoro titanium complex has been synthesized from the corresponding chloro compound.1659 This compound results in a very active system for the highly enantioselective hydrosilylation of acyclic and cyclic imines and asymmetric hydrosilylation reactions of ketones including aromatic ketones.1661,1666,1926-1929 An analogous l,l -binaphth-2,2 -diolato complex catalyzes the enantioselective hydrosilylation of ketones.1... 

A variety of catalytic bis-silylation reactions, i.e., addition of Si-Si bonds across multiple bonds, have been reported. Generally the reaction mechanism can be presented as follows (1) formation of bis(organosilyl) transition-metal complexes through activation of Si-Si bonds, (2) insertion of unsaturated organic molecules into the silicon-transition-metal bonds, and (3) reductive elimination of the silicon-element (mostly carbon) bonds giving bis-silylation products. The final step regenerates the active low-valent transition-metal complexes. Not only appropriate choice of transition metal, but also choice of suitable ligand on the transition metal is crucially important for the success of the bis-silylation reaction. In addition, substituents on the silicon atoms of disilane are also of importance. 

The insertion of unsaturated organic molecules such as olefins, acetylenes, CO and RNC into rnetal-carbon bonds is a characteristic and important reaction in organometallic chemistry. The reaction is also termed carbometallation of the unsaturated molecule (i.e., multiply bonded reagents) ... 

Of the series of heavier Group 14 double-bonded species, the silyl derivatives are somewhat unique in retaining their double-bond character while in solution. As such, they are subject to a plethora of cycloaddition reactions with unsaturated organic molecules, forming both [2-1-2] and [2-1-3] cycloadducts with much enhanced reactivity over their carbon analogs (Scheme 14.3) [65, 80]. 

It is important to recognize that exciplex formation is a physical phenomenon and not a chemical one. That is, bonding between the molecules only lasts as long as the excited state lifetime of the molecule, and the exciplex bond dissociates upon radiative or non-radiative de-excitation. Nevertheless, there are cases in which exciplex formation leads to photochemical reactions. For example, exciplexes are believed to be the reactive intermediates in the photochemical pathway of the Diels- Alder cycloaddition reactions of unsaturated organic compounds. Since photoluminescence is a photophysical process, the focus here will be on the photophysical aspect of exciplex formation instead of the photochemical reactions that result from exciplex formation. 

The aim of this chapter is to examine the application of well-defined N-heterocyclic carbene (NHC) complexes, as well as the systems prepared in situ which involve free NHCs or the precursor salt, for the reduction of unsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reactions are based on electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types according to the hydride source used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

The donor-stahilized hydrosilanimine is potentially useful as a hydrosilylation reagent. In addition, the existence of the Si=N double bond allows the cycloaddition reactions with unsaturated organic molecules for the synthesis of silicon heterocycles. 

In spite of very common mentioning of insertion process in a variety of modern studies and in spite of widespread application of addition reactions in modern organic chemistry, molecular picture of this fascinating transformation remains unexplored. The aim of this chapter is to point on understanding at molecular level of insertion (migratory insertion) of unsaturated organic molecules into metal-heteroatom bond. 

A fundamental solution to the problem is to develop novel synthetic processes that are clean by initial design. Eor carbon-heteroatom bond formation it is the hydrofunctionalization process that opens the possibility for environmentally friendly chemical transformations. Hydrofunctionalization of unsaturated organic molecules via direct addition of H-X to multiple bonds is an atom-economical addition reaction which does not produce wastes. In view of the need of green and sustainable chemical procedures, the role of the metal catalysis is crucial to control the reaction, in particular, regio-, stereo-, and enantioselectivity. 

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