Chromium photolysis

Photolysis of several arene chromium tricarbonyls (Ar Cr ( 0)3) (Ar = benzene, toluene, or xylene) and... 

The coordinated silylenes in both the iron and the chromium compounds can be photolytically activated Photolysis of the complexes in the presence of triphenylphosphine gives the trans-silylene-phosphine complex, which in a second step is transformed into the trnns-bisphosphine compound by excess phosphine. If the silylenes are not trapped, polysilanes are isolated in almost quantitative... 

Chromium, tris( 1,2-ethanediamine)-pentacyanonickelate isomers, 1, 207 photochemistry reactivity, 1, 398 photolysis, 1. 397 reactions, 1, 27 trichloride... 

Merlic et al. were the first to predict that exposing a dienylcarbene complex 126 to photolysis would lead to an ort/zo-substituted phenolic product 129 [74a]. This photochemical benzannulation reaction, which provides products complementary to the classical para-substituted phenol as benzannulation product, can be applied to (alkoxy- and aminocarbene)pentacarbonyl complexes [74]. A mechanism proposed for this photochemical reaction is shown in Scheme 54. Photo activation promotes CO insertion resulting in the chromium ketene in-... 

Photolysis of chromium alkoxycarbene complexes with aldehydes in the presence of Lewis acids produced /J-lactones [83]. Intermolecular reactions were slow, low-yielding, and nonstereoselective, while intramolecular reactions were more efficient (Eqs. 19 and 20). Subsequent studies showed that amines, particularly DMAP, could also catalyze this process (Table 13) [84], resulting in reasonable yields and diastereoselectivity in intermolecular cases. 

Photolysis of chromium alkoxycarbenes with azoarenes produced 1,2- and 1,3-diazetidinones, along with imidates from formal azo metathesis (Eq. 21) [85, 86]. Elegant mechanistic studies [87-89] indicated the primary photoprocess was trans-to-cis isomerization of the azoarene followed by subsequent thermal reaction with the carbene complex. Because of the low yields and mixtures obtained the process is of little synthetic use. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Activated esters for use in peptide-coupling reactions were produced by photolysis of optically active chromium aminocarbenes with alcohols which are good leaving groups, such as phenol, pentafluorophenol, 2,4,5-trichlorophenol, and N-hydroxysuccinimide (Table 17) [ 109]. Since arylcarbenes bearing the op-... 

Ketenes react with tertiary allylic amines in the presence of Lewis acids to give zwitterionic intermediates which undergo [3,3]-sigmatropic rearrangement [119]. Photolysis of chromium carbene complexes in the presence of tertiary amines results in similar chemistry [120]. Cyclic (Table 21) and strained allylic amines (Eq. 34) work best, while acylic amines are less reactive (Eq. 35). 

The time resolved spectra produced on excimer laser photolysis of Mn2(CO)io are shown in figure 6. Note that as in the case of iron pentacarbonyl and chromium hexacarbonyl photolysis, there is a distinct increase in the amplitude of the lower frequency absorption bands as the photolysis energy increases. By comparison with the frequency of matrix isolated and solution phase Mn(C0)5, the band at -1996 cm l is assigned to the gas phase Mn(C0)5 radical [33]. This... 

The nitrogen complex had already been synthesized in a solid matrix, but its decomposition kinetics and its further photolysis could be studied only in solution. The liquid noble gas technique is superior to the solid matrix technique, especially for the synthesis of multiple substituted chromium carbonyl nitrogen complexes. Their IR spectra were extremely complex in matrices, due to "site splittings" which arise when different molecules are trapped in different matrix environments /18/. 

G.A. Ozin, University of Toronto In our Cr/CO matrix cocondensation experiments (Angew. Chem., Int. Ed. Eng. 1975, 14, 292), we reported evidence for the facile formation of a binuclear chromium carbonyl complex Cr2(CO)i0 or Cr2 (CCOi x which could be described as square pyramidal Cr(CO)5 weakly interacting with either a Cr(CO)5 or Cr(CO)6 moiety in the vacant (sixth) site. As a result, the infrared spectrum of this "weakly-coupled" binuclear species closely resembled that of the mononuclear fragment Cr(CO)5. I would like to ask you, whether or not you have any evidence for the existence of such a binuclear species in your Cr(CO)6 /Xe cryogenic solutions following various photolysis treatments. 

A. W. Adamson, A. Vogler, H. Kunkely, R. Wachter. Photocalorimetry. Enthalpies of Photolysis of trans-Azobenzene, Ferrioxalate and Cobaltioxalate Ions, Chromium Hexacarbonyl, and Dirhenium Decacarbonyl. J. Am. Chem. Soc. 1978, 100, 1298-1300. 

An indirect carbonylation of aldehydes via photolysis of chromium alkoxy-carbenes with aldehydes in the presence of Lewis acids was reported by Hegedus [66]. The formation of /1-lactones was especially efficient when the aldehyde was incorporated into either chain of the carbene ligand resulting in an intramolecular process. 

Experimental Procedure 2.2.1. Photolysis of a Chromium Carbene Complex 2-Benzyl-4-benzyloxy-4-methyl-2,3,4,4a,7,7a-hexahydro-li/-cyclopenta[c]pyri-din-3-one [294]... 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Photolysis of pentacarbonylcarbenechromium complexes produce species that react as if they were ketenes. although no evidence for the generation of free ketenes has been observed. Indeed, photolysis of chromium (alkoxy) carbenes in the presence of a range of simple alkenes produced cyclobutanones 1 in good to very good yield.8,9... 

In their original investigations, Ayscough and Steacie42 stressed the simplicity of the reaction scheme and were unable to identify any product which could arise from the reactions of a trifluoroacetyl radical. Further, the ratio of quantum yields of carbon monoxide and hexa-fluoroethane was always close to unity, a fact which has been confirmed by later workers. Recently, Tucker and Whittle43 have shown that when hexafluoroacetone is photolyzed in the presence of excess bromine, trifluoroacetyl bromide is formed, suggesting that the trifluoroacetyl radical must intervene. The absence of hexafluorodiacetyl in the photolysis products of hexafluoroacetone is explained by the assumption that it is not stable. In fact, however, hexafluorodiacetyl may be prepared by the chromium trioxide oxidation of l,l,l,4,4,4-hexafluoro-2,3-di-... 

The Cotton effects in mixed amino acidate/acetylacetonate complexes [Cr(acac)2L] (L = l-alanine, L-valine or L-phenylalanine) have been studied 774 absolute configurations were assigned by reference to the parent tris(acetylacetonate) complexes. Synthesis was achieved by the photolysis of mixtures of the amino acid and [Cr(acac)3]. The partial photoresolution of both cis- and irans-(l,l,l-trifluoro-2,4-pentanedionato)chromium has been accomplished by irradiation with circularly polarized light (5461 A) in chlorobenzene solution.775 The results indicated that both bond rupture and twist mechanisms were important. A number of other jS-diketonates have also been investigated.776... 

Photolysis of the azido complex CrN3(salen)-2H20 produces1110 the nitrido chromium(V) complex CrN(salen)-H20 and the reaction1111 of [Cr(salen)(H20)2]+ with iodosylbenzene gives the oxo chromium(V) derivative [CrO(salen)]2+ chromium(III) porphyrin complexes behave similarly (Section 35.4.9.1). 

Similarly, photolysis of chromium complex (CO)5Cr=C(OMe)(c-Pr) in the presence of two equivalents of the optically active 3-ethenyl-6(S)-phenyl-2-oxazolidinone under 90 psi CO pressure produced in high regio- and diastereo-selectively the 3-oxazolidine-substitued (2i ,3S)-cyclobutanone, in optical purity of > 97% de (equation 75)148. 

Alkyl and aryl isocyanates (and ketenes) can be used in a [6 + 2] cycloaddition process, promoted by chromium(O) species under photolysis, to give useful bicyclo compounds (equation 169)607. In this process the yields are 20-45%, which is a great improvement over other attempts at metal-mediated [6 + 2] cycloaddition reaction608-610. 

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

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