Sulfur, carbene complexes

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Further improvements in activity of the ruthenium carbene complexes were achieved by incorporation of methyl groups in 3,4-position of imidazol-2-ylidene moiety. Introduction of sulfur in the trara-position to the N-heterocyclic carbene leads to increased stability of the resulting ruthenium complexes. The synthesis and the first applications of these new rathenium complexes are described herein. 

Analytical thin layer chromatography (TLC) was conducted on 10 x 2.5-cm precoated glass plates (silica gel GF, 0.25-mm thickness, Analtech), eluted with 10% ethyl acetate in hexane, and visualized with both UV (254 nm) and aqueous 50% sulfuric add spray/heating. The carbene complex moves as an orange spot on TLC the reaction is complete when this spot is no longer visible. 

Af-Heterocyclic carbene complexes of sulfur or, more simply, imidaz-ole-2-thiones, have been known for many years (51, 52) and have... 

Several other observations suggest that nucleophilic carbene complexes, similarly to, e.g., sulfur ylides, can cyclopropanate acceptor-substituted olefins by an addition-elimination mechanism. If, e.g., acceptor-substituted olefins are added to a mixture of a simple alkene and the metathesis catalyst PhWCl3/AlCl3, the metathesis reaction is quenched and small amounts of acceptor-substituted cyclopropanes can be isolated [34]. 

Treatment of Fischer-type carbene complexes with different oxidants can lead to the formation of carbonyl compounds [150,253]. Treatment with sulfur leads to the formation of complexed thiocarbonyl compounds [141]. Conversion of the carbene carbon atom into a methylene or acetal group can be achieved by treatment with reducing agents. Treatment of vinylcarbene complexes with diborane can also lead to demetallation and formation of diols [278]. The conversion of heteroatom-substituted carbene complexes to non-heteroatom-substituted carbene complexes... 

Closely related with the synthesis of ylides from carbenes is the use of ylides as carbene transfer reagents (CTR), that is processes in which the ylide is cleaved homolytically, liberating the nucleophile and the carbene, which could remain both coordinated to the metal or not (Scheme 10). Diphosphirane (34) can be obtained from the diphosphene by reaction with sulfur ylide Me2S(0)=CH2, which behave as a carrier of the CH2 unit [95]. Recent work of Milstein et al. shows that sulfur ylides decompose in the presence of Rh derivatives with vacant coordination sites affording Rh(l)-carbene complexes [96, 97]. Complexes (35-37) can be obtained from... 

It has been demonstrated35 that enolate trapping by the electrophilic iron-carbene complex 1 provides 2. Alkylation at sulfur followed by a-elimination again generates a carbene, which then inserts stoichiometrically 1,5 into the C —H bond to give the octahydroindenone 3. Several other applications of this cyclization have been published 3fi. 

Formation of Sulfur Ylide from Metal Carbene Complexes and Subsequent Reactions 164... 

Transfer of a metal carbene moiety from a metal carbene complex to a heteroatom other than oxygen, sulfur, and nitrogen is possible. One such example is the report by Uemura and co-workers, who disclosed catalytic asymmetric reaction of ethyl diazoacetate with ( )-cinnamyl phenyl selenide 193 (Equation (30))." The reaction afforded 194 as a diastereomeric mixture (58 42). Using Rh2(5[Pg.173]

The complete series, thio-, seleno-, and telluroformaldehyde complexes of osmium [OsCl(NO)(r72-E=CH2)(PPh3)2] (E = S, Se, Te), was obtained by reaction of the carbene complex [OsCl(NO)(=CH2)(PPh3)2] with elemental sulfur, selenium, and tellurium, respectively.53... 

These compounds are nucleophihc at the noncoordinated sulfur, and undergo alkylation there with iodomethane or benzyl bromide to give initially (128) (which may be isolated as the PFe salt) and ultimately (129) (equation 28). They also react with electron-deficient alkynes, possibly by a dipolar mechanism, to afford cyclized iron carbene complex (130). Finally, compounds (129) are subject to attack by some nucleophiles hydride attack occurs initially at the metal, but ultimately gives ) -dithioester complexes, such as (131). ... 

Displacement of a good leaving group from silicon in a noncoordinating solvent (equation 5) allows solvent-free complexes (1) to be prepared when the substituents on silicon are bound via sulfur. As would be expected by analogy with carbene complexes, the Si NMR chemical shift for these complexes is at very low held (264.4 and 268.7 ppm, respectively, for R = S-/ -tolyl and SEt, respectively) and they also react readily with donor molecules such as MeCN to give four-coordinate, donor stabilized silylene species. (See Section 6 below for further details of transition metal silyl complexes.)... 

N-heterocyclic carbene adducts of sulfur have been known for many years as a result of their industrial applications in catalysis. These complexes have been described in various reviews " and this section will be confined to carbene complexes of the chalcogenides reported since 1993. 

The sulfur ylid Me2S(0)CH2 is a very weak base, and, when coordinated to nickel in the complex [Ni( / -C2H4)2CH2S(0)Me2] (91), spontaneous decomposition to ethane, cyclopropane, and methane occurs (93). In another reaction, compound 92 rearranges under UV irradiation to yield, with insertion of iron into a phenyl-carbon bond, a cyclic carbene complex, 93 94). 

Addition of phenylethynethiolate or -selenate to the carbene complex 185 (M = Cr or W, R = Me or Et, E = S or Se) affords an anionic adduct (186) which, unexpectedly, contains a reactive thioketene function. Electrophiles can attack either at the sulfur or at the carbon a thereto to produce coordinated carbenes or thio- and seleno aldehydes and esters [H4J45). 

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