Photochemical reactions metathesis

Merlic developed a new variation of the thermally induced benzannulation reaction. The dienylcarbene complex 132 was reacted with isonitrile to give an orf/zo-alkoxyaniline derivative 135 [76] (Scheme 56). This annulation product is regiocomplementary to those reported from photochemical reaction of chromium dienyl(amino)carbene complexes. The metathesis of the isocyanide with the dienylcarbene complex 132 generates a chromium-complexed di-enylketenimine intermediate 133 which undergoes electrocyclisation. Final tau-tomerisation and demetalation afford the orf/zo-alkoxyaniline 135. 

Singh, V., Praveena, G.D., Karki, K., and Mobin, S.M. (2007) Cydoaddition of cydohexa-2,4-dienones, ring-closing metathesis, and photochemical reactions a common stereoselective approach to duprezianane, polyquinane and... 

Important advances in the field of C-H bond activation have involved the photochemical reactions of boryl complexes such as Cp W(CO)3(Bcat) (98, cat = 1,2-02-3,5-Me2C6H2). Transient species derived from these complexes efficiently activate the C-H bonds of alkanes and arenes (see Alkane Carbon-Hydrogen Bond Activation), and they can convert hydrocarbon solvents into alkylboronate esters (equation 27). Experimental and theoretical studies have shown that these reactions proceed via a boron-assisted, a-bond metathesis see a-Bond Metathesis) pathway involving back donation of electron density from the tungsten atom to a formally unoccupied p orbital centered on the boryl ligand. ... 

The photochemical reactions of arenecarboxylic acid esters with alkenes has received recent attention by Cantrell. - For example, irradiation of 2,3-dimethyl-2-butene and methyl benzoate gave a mixture of alkoxyoxetane (56), carbonyl-alkene metathesis product (57) and ketone (58), resulting from alkoxy radical allylic hydrogen abstraction and radical recombination. Such alkoxyoxetane photoproducts are... 

Methods for the synthesis of central hydroisoquinolines or pyrrolo-hydroisoquinoHnes of manzamine A can be classified into 11 types of reactions (1) intermolecular Diels-Alder reaction, (2) intramolecular Diels-Alder reaction, (3) photochemical reaction, (4) radical reaction, (5) ionic cyclization, (6) intramolecular Michael reaction, (7) intramolecular Maimich reaction, (8) intramolecular [3 + 2] cycloaddition, (9) intermolecular [3 + 2] cycloaddition, (10) Pauson-Khand reaction, and (11) eneyne metathesis. 

Since that discovery there has been much interest in this photochemical reaction, which produces a catalyst working in mild conditions without an organometallic component [1-23]. The same catalyst induces the metathesis of acyclic alkenes [1-17] and the ringopening metathesis polymerisation (ROMP) of cyclic olefins studied recently by Sundararajan et al. [18, 22, 23], as well as the polymerisation of various substituted acetylenes investigated by Masuda et ai [13, 19]. I was first of all intrigued to know what intermediates are formed in the above photochemical reaction and which one is the true catalyst in metathesis and related reactions. 

However, the exact nature of the catalytically active species formed in the photochemical reaction of W(CO)6 in tetrachlorocarbon remains a matter of speculation. Later, it was observed that the more active and tractable catalyst for the metathesis of alkenes and the polymerisation of alkynes can be photogenerated in reaction of W(CO)6 carried out in alkane solution containing catalytic amounts of Lewis acid such as the halides group four, thirteen and fourteen elements. In such a system even higher catalytic activity is achieved than in tetrachlorocarbon solution (Eq.2) [12, 15, 20]. 

A chromium "nitrene" complex is probably the result of addition of PhN=NPh to the carbene Cr[C(OMe)Me](CO)3 in a photochemical reaction. This is formed by cycloaddition to the carbene followed by metathesis forming the iminoether PhN=C(OMe)Me and the nitrene, inferred by its reaction with acid resulting in PhNH2. 27... 

One of the most typical photochemical reactions, C=C double bond isomerization, may have a synthetic value. Thus, the highly oxygenated cembrane 17-deoxyprovidencin 20 has been built by joining two separately prepared building blocks and further elaboration of the macrocycle 21 via a Z-selective ring closing metathesis. The Z C=C bond isomerizes by irradiation with Pyrex-filtered light that... 

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

Reaction pathways apparently analogous to d and f of Eq. (26) yield a mixture of propylene and cyclopropane. Only when photochemical activation was employed were the major products olefins derived from metathesis-decomposition of the metallocycle. The failure to form metathesis olefins under moderate conditions is significant. It may be that either unimolecular dissociation of the olefin from the complex (in the absence of excess olefin to restabilize the carbene) is energetically unfavored, or the metallocyclobutane structure in the equilibrium given by steps a and b in Eq. (26) is highly stabilized and favored. These results... 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Metathesis of Alkynes. - Alkyne metathesis has been studied far less than has olefin metathesis. Mortreux and co-workers °° demonstrated that the metathesis of alkynes occurs at ambient temperatures with photochemical activation on a Mo(CO)6-3ClPhOH catalyst. The reaction proceeds via a two-step mechanism involving activation of the Mo(CO)6 by the alkyne and subsequent catalysis by the phenol. Heterogenization of the transition metal and... 

Fig.5 shows the dependence of the catalytic activity of Nb20s/PVG photoactivated in the presence of ethene on the niobium content in the catalyst. The activity increases monotonously with increase in the Nb content. On the other hand, Anpo et al-have shown that tetrahedrally coordinated Mo or V ion with double-bonded oxygen ions(see Reactions[1] and [2]), which take part in photochemical process of their oxides, is predominantly formed at a rather low content in porous Vycor glass (16,17). Furthermore, it has been found that photocatalyzed metathesis (13) and photooxidation of carbon monoxide observed for Nb205/PVG show a maximum activity at 1 mg g-PyG" of the Nb content. This difference in the dependence of the activities on the Nb content... 

---