Photochemistry of Alkyl Complexes

The photochemistry of this class of compounds was reviewed in the 1979 book Organometallic Photochemistry (9) which reviewed the literature comprehensively through 1977. In the years since that review, a number of significant advances have been made. Indeed, the general conclusions that can now be drawn concerning the photochemistry of alkyl complexes are quite different from those given in that earlier review. Then, few alkylidene and alkylidyne complexes had been studied, but that too has changed. A comprehensive review on this subject is thus timely. 

Irradiation of these thorium complexes gives ThCps (Cp = in essentially quantitative yield, as well as almost equimolar amounts of alkane and alkene [Eq. (1)] 10, 10a). The authors suggested that the 

Photolysis of the related Th(idenyl)3(n-C4Hg) complex gave a 96.5% yield of butane and butene but in vastly different relative proportions 

The photochemistry of the methyl complex ThCpaCHa differed markedly from that of thorium alkyls containing /3 hydrogens [Eq. (3)]. The reaction was appreciably slower and was only 50% complete after 

20 hours 10, 10a), The principal organic product was methane and labeling studies showed the Cp rings and the solvent, in that order, to be the major sources of hydrogen uptake in its production. 

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In principle, the photoreactions of CT s are able to offer a great number of photoinitiator systems for radical polymerization. But, so far, this subject has only received little attention, and the current knowledge relative to the photochemistry of such complexes is poor. In addition to the amine complexes mentioned above, chinoline-bromine [124-127], chinoline-chlorine [128], 2-methylpyridine-chlorine [129], pyridine-bromine [130], IV-vinylpyrrolidone-bromine [131], acridone-bro-mine [132], acridone-chlorine [133], benzophenone-S02 [134], isoquinoline-S02 [135, 136], and 2-methylquinoline-S02 [136] combinations are used for radical polymerization of AN, alkyl methacrylates, acrylic and methacrylic acid, and for... 

Photochemistry of Alkyl, Alkylidene, and Alkylidyne Complexes of the Transition Metals... 

The photochemistry of these complexes has been studied extensively (50-63a), and the nature of the primary photoprocess has been a controversial subject. Both carbon monoxide dissociation [Eq. (64)] (50, 51a,b, 52-56, 59, 60, 61-63a) and metal-alkyl bond homolysis [Eq. (65)] (51, 51c, 57, 58, 60a) have been proposed on the basis of different experimental results. However, more recent work has shown that CO... 

The photochemistry of ir-allylpalladium complexes has been studied to a limited extent. Two basic reactions have been observed. Irradiation at 366 nm of ir-allylpalladium complexes produced 1,5-diene dimers, reportedly via a radical coupling mechanism.334 333 Similar irradiations in the presence of species capable of trapping the presumed allyl radical intermediate, such as BrCCb, BrCH2Ph or allyl bromide, now yield alkylated and halogenated allyls, in addition to 1,5-diene dimer. This reaction fails for simple alkyl or aryl halides due to the instability of the associated radical (equations 130 and 131 ).336... 

The synthesis and photochemistry of related rhodium(lll) bis(alkyl) complexes cA,cA-[Rh(R)2(I)(CO)(dmb)] (R = Me, Tr) have been studied. The complexes showed an IT (tv - r (dmb)) absorption band at ca. 295 nm. There was another band at ca. 370nm for both complexes, which was assigned to an XTCT (p(l)7r (dmb)) transition. Irradiation of both complexes in solution resulted in Rh-R bond homolysis, as evidenced by IR, UV-vis, and spin-trap EPR investigations. The photoreaction was proposed to occur after crossing from the XLCT state to the reactive SBLCT state. 

As the above results indicate, the photochemistry of these uranium-alkyl complexes is not fully resolved. The olefin formation during Marks s siudy of UCP3R in toluene solution suggests that photoinduced /3-hydride elimination occurs to some extent, presumably via a mechanism similar to that of Scheme 1. However, the other data appear to indicate a predominant radical mechanism via photoinduced M—R bond homolysis. 

The photochemistry of the iron complexes in this family has been extensively investigated and four different processes have been proposed by different authors for the primary photochemistry step. These are CO dissociation [Eq. (86)], shift of the Cp ring [Eq. (87)], CO insertion into the metal-alkyl bond [Eq. (88)], and metal-alkyl bond homolysis [Eq. (89)]. 

The photochemistry of these aryl complexes has been briefly investigated by Nesmeyanov and co-workers (98,98a) and is quite similar to that of the analogous alkyl complexes. Irradiation of CpFe(CO)2Ar complexes in the presence of tertiary phosphines at room temperature led to the formation of the corresponding monosubstituted derivatives (98) [Eq. (96)]. Prolonged irradiation in the presence of triphenylphosphite... 

Mitani et al7 have studied the photoaddition of alkyl halides to alkenes using copper(i) complexes as catalysts. A typical example is shown in Scheme 1. The solvent-dependent photochemistry of the alkene (2) has been studied. ... 

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