Thiocarbonyl acceptors

When planning reactions of thiocarbonyl compounds with electrophilic carbene complexes it should be taken into aceount that thiocarbonyl compounds can undergo uncatalyzed 1,3-dipolar cycloaddition with acceptor-substituted diazomethanes to yield 1,3,4-thiadiazoles. These can either be stable or eliminate nitrogen to yield thiiranes or other products similar to those resulting from thiocarbonyl ylides [1338]. 

Table 4.22. Reactions of thiocarbonyl compounds with acceptor-substituted carbene complexes.

Because of its chemical inertness, no direct way of curing poly(thiocarbonyl fluoride) has been found. However, creep has been reduced and strength at elevated temperatures improved by milling into the polymer a free-radical generator, such as dicumyl peroxide or azobisisobutyronitrile, and a free-radical acceptor, such as N,N -m-phenylenebismaleimide or triacryloylhexahydro-s-triazine, and curing with heat and pressure (65). A better method is to mill in divinylbenzene and a small amount of benzoyl peroxide and cure with heat and pressure (66). The divinylbenzene forms a crosslinked matrix that mechanically traps poly(thio-carbonyl fluoride) molecules. Since the elastomer is in effect filled with poly(di-vinyl benzene), the final composition is less resilient than untreated poly(thio-carbonyl fluoride). 

In a UPS study of thiocarbonyl complexes such as M(CO)sCS, M = Cr, W (202a) it was found that the ionizations associated with the o and it levels of coordinated CS are clearly separated from the other ionizations. It was concluded that CS is a better n acceptor than CO. 

In summary, the insertion of 0=C=S into the Si-N bond of reactive 1 proceeds to give only one apparently rather stable primary product which owes its stability to the matching of thiocarbonyl n-acceptor and dihydropyrazine 7t-donor capacities. 

The positions of the n,7t transitions in a series of thiocarbonyl compounds are shown in Figure 6.16.1258 The shifts between related compounds follow the trends expected from perturbation theory inductive acceptors lower the energy of the np orbital on sulfur and mesomeric interactions raise the energy of the n orbital. 

Martinez-Mayorga, K., Juaristi, E., Cuevas, G. (2004). Manifestation of Stereoelectronic Effects on the Calculated Carbon-Hydrogen Bond Lengths and One-Bond 17C-H NMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (C = 0), Thiocarbonyl (C = S) and Methylidene (C = CH2) Groups toward C-H Donor Bonds. Journal of Organic Chemistry, 69, 7266-7276. 

Carbon monosulfide is not a stable molecule, but carbon monosulfide complexes (more commonly called thiocarbonyl complexes) are known for most of the transition metals. Because of the mstability of free CS, most CS complexes have been prepared by generation of CS from CS, Cl CS, or EtOC(S)Cl in the coordination sphere of the metal, as shown in Equations 2.5, 2.6, and 2.7. Thiocarbonyl ligands vibrate between 1160 and 1410 cm" free CS (in a matrix) vibrates at 1274 cm". A detailed analysis of force constants of mixed carbonyl and thiocarbonyl complexes indicates that the CS ligand can be a weaker or stronger TT-acceptor tiian CO. Seleno- and tellurocarbonyl complexes are also known. ... 

None of the examples presented above support the idea that the carbonyl can be a four-electTOTi draior, as Iji orbitals of CO are generally very low in energy and will be poor donors. The final example here discusses if any possibilities exist that a carbonyl or similar ligand may act as a four-electron donor and the best candidate is the Crabtree s type I complex. For comparison, the thiocarbonyl-bridged species is introduced here, as the thiocarbonyl is both a better Jt-donor and a better ji-acceptor hopefully, this difference may magnify the ligand-metal jt-donor interacticm (Fig. 25). 

Therefore, rr-acceptor versus (T-donor properties of CS and CSe change more drastically than those of CO. In thiocarbonyl complexes additional bonding contribution comes from In orbitals. 

Thiocarbonyls CS is not stable above - 160 C in the free state, but a number of complexes are known, such as RhCl(CS)(PPh3) (Eq. 4.16) and Cp(CO)Ru( x -CS)2RuCp(CO), but so far no pure or homoleptic examples M(CS) . They are usually made from CS2 or by conversion of a CO to a CS group. Perhaps because of the lower tendency of the second row elements such as S to form double bonds, the M =C—S form analogous to 4.2 is more important for MCS than MCO the MC bond therefore tends to be short and CS is a better ir acceptor than CO. Perhaps for this reason, CO and not CS tends to be substituted in a mixed carbonyl-thiocarbonyl complex. 

A w-acceptor ligand, on the contrary, is one that has low-lying empty tt orbitals that can accept electron back-donation from occupied metal d orbitals of the appropriate symmetry. The prototypical 7r-acid ligands are unsaturated molecules and ions such as carbonyl (C=0), thiocarbonyl (C=S), nitrosyl (N O" "), cyanide (C=N ), and isonitriles ( C=N-R). 

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