Photochemical reactions, transition-metal

The vinylcyclopropane rearrangement is of synthetic importance, as well as of mechanistic interest—i.e. the concerted vs. the radical mechanism. A reaction temperature of 200 to 400 °C is usually required for the rearrangement however, depending on substrate structure, the required reaction temperature may range from 50 to 600 °C. Photochemical and transition metal catalyzed variants are known that do not require high temperatures. 

During the past few years, increasing numbers of reports have been published on the subject of domino reactions initiated by oxidation or reduction processes. This was in stark contrast to the period before our first comprehensive review of this topic was published in 1993 [1], when the use of this type of transformation was indeed rare. The benefits of employing oxidation or reduction processes in domino sequences are clear, as they offer easy access to reactive functionalities such as nucleophiles (e. g., alcohols and amines) or electrophiles (e. g., aldehydes or ketones), with their ability to participate in further reactions. For that reason, apart from combinations with photochemically induced, transition metal-catalyzed and enzymatically induced processes, all other possible constellations have been embedded in the concept of domino synthesis. 

A variety of silicon-functionalized diazoacetic esters are available by reacting (trifloxysilyl)- or (chlorosilyl)diazoacetic esters with appropriate nucleophiles [1]. Thermally, photochemically, or transition metal induced intramolecular carbene reactions of these novel diazoesters lead to four-, five-, and six-membered silaheterocycles. 

The transformation shown in equation (54) retains many of the features of ordinary photochemical and transition-metal-catalyzed thermal reactions of organic compounds, but displays some unique characteristics as well. In cases where irradiation serves only to accelerate the rate of the expected thermal process, higher chemical yields of product can result, reaction rates are subject to greater control through regulation of light intensity, and thermally sensitive products are isolated more readily since elevated reaction temperatures can be avoided. Alternatively, the function of M may be to facilitate known photochemical reactions of O or perhaps introduce new reaction channels not observed upon irradiation of O alone. A detailed discussion of the mechanisms and synthetic applications of these processes has been presented.177... 

All of these carbenes are reactive intermediates that must be generated from the appropriate precursors in the presence of the alkene (or arene) which is to be cyclopropanated. The following methods of carbene-transfer reactions to C-C double bonds will be discussed path a. from a-halo-a-metal (or alkylmetal) compounds by a-elimination path b. from iodine or sulfur ylides by thermal, photochemical or transition metal catalyzed decomposition ... 

The characterization of electronic excited states has attracted much attention in connection with photochemistry. For example, transition metal complexes are characterized by a variety of absorption spectra in the visible and ultraviolet (UV) regions. The absorption spectra essentially give us information about the electronic excited states corresponding to dipole-allowed transitions due to their high symmetries, while some of the data in crystalline fields indicate the existence of several excited states to which dipole transitions are forbidden in the absence of perturbation. Most photochemical reactions of metal complexes, which are occasionally important as homogeneous photocatalytic reactions, involve both allowed and forbidden excited states. Thus, the systematic understanding of the nature of these excited states is essential in designing photochemical reactions. 

The structural consequences of bonding for carbene complexes are discussed. Photochemical reactions of metal alkyl complexes result in very reactive intermediates that can be trapped and used preparatively. Matrix isolation studies of organometallic intermediates are considered in detail. Transition metals in organic synthesis for the year 1982 are reviewed extensively and a further article concerns a-(1,n)-alkanediy1 complexes. ... 

R. L. Funk and K. P. C. Vollhardt, Thermal, Photochemical and Transition-metal Mediated Routes to Steroids by Intramolecular Diels-Alder Reactions of o-Xylylenes(o-Quinodimethanes) , Chem. Soc. Rev., 1980,9, 41. 

In order to understand the photochemical reactions of metal complexes at the molecular level, it is necessary to know both the number and the energy levels of the spectroscopic states of the complex. The first step in developing a state model is to know the coordination number and structure of the complex about the metal center. For complexes of the lanthanide and actinide ions the coordination number is commonly 8 or 9, but for transition metal complexes a coordination number of 6 is that most frequently observed. 

According to theoretical calculations, there is a low-energy pathway for the direct interaction of diazomethane with carbon monoxide [4j. However, experimental results so far show only evidences for two-step pathways. These involve first the dediazotation of the diazoalkanes in thermal, photochemical, or transition metal mediated reactions [5] resulting in free carbenes (Equation 8.2) or transition metal coordinated carbenes (Equation 8.3), which under proper reaction conditions couple in the second step with carbon monoxide (Equation 8.4 or 8.5) to form ketene products. 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

The photochemistry of transition metal 1,3-diketone chelate complexes has been known for some time [30,31], and their photophysical and photochemical properties and photocatalytic activity in different chemical reactions were reviewed in 1990 by Marciniak and Buono—Core [32]. Further discussion on the photochemistry of meta] chelate will not take place here since this subject is out of the scope of this chapter. 

Solid state photochemical reactions of transition metal coordination compounds. E. L. Simmons and W. W. Wendlandt, Coord. Chem. Rev., 1971, 7.11-27 (88). 

Transition metal complexes as mediators in photochemical and chemiluminescence reactions. V. Balzani and F. Bolleta, Comments Inorg. Chem., 1983, 2, 211-226 (33). 

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

Muller et al. have also examined the enantioselectivity and the stereochemical course of copper-catalyzed intramolecular CH insertions of phenyl-iodonium ylides [34]. The decomposition of diazo compounds in the presence of transition metals leads to typical reactions for metal-carbenoid intermediates, such as cyclopropanations, insertions into X - H bonds, and formation of ylides with heteroatoms that have available lone pairs. Since diazo compounds are potentially explosive, toxic, and carcinogenic, the number of industrial applications is limited. Phenyliodonium ylides are potential substitutes for diazo compounds in metal-carbenoid reactions. Their photochemical, thermal, and transition-metal-catalyzed decompositions exhibit some similarities to those of diazo compounds. 

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

Reaction of diazo compounds with a variety of transition metal compounds leads to evolution of nitrogen and formation of products of the same general type as those formed by thermal and photochemical decomposition of diazoalkanes. These transition... 

The effects of transition metals on the photochemical reduction of C02 to formaldehyde (0.1 %), formaldehyde to methanol (6-8%), and methanol to methane (ca. 10 5%) were examined172 in aqueous solutions, but the yields were very low as shown in parentheses for each reaction. 

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... 

Besides the numerous examples of anionic/anionic processes, anionic/pericydic domino reactions have become increasingly important and present the second largest group of anionically induced sequences. In contrast, there are only a few examples of anionic/radical, anionic/transition metal-mediated, as well as anionic/re-ductive or anionic/oxidative domino reactions. Anionic/photochemically induced and anionic/enzyme-mediated domino sequences have not been found in the literature during the past few decades. It should be noted that, as a consequence of our definition, anionic/cationic domino processes are not listed, as already stated for cationic/anionic domino processes. Thus, these reactions would require an oxidative and reductive step, respectively, which would be discussed under oxidative or reductive processes. 

To date, only one example of a combination of a photochemically induced transformation with a transition metal-catalyzed reaction has been found in the literature. This hv/Pd°-promoted process allows the synthesis of five-membered cyclic y-keto esters 5-119 from 5-iodoalkenes 5-117 in the presence of CO and an alcohol 5-118 as a nucleophile (Scheme 5.24) [41]. The yields are high, and differently substituted iodoalkenes can be employed. 

Allyloxysilyl)diazoacetic esters 6 and the homologous butenyloxy derivative 9 undergo intramolecular cyclopropanation to form 7 and 10, respectively, when decomposed photochemically or by transition metal catalysis. The thermal reaction of 9 produces the same result, whereas the l-oxa-2-sila-3-cyclopentene 8 is formed from 6, presumably via a pyrazoline intermediate. 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

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