Halogen donor solvents

SASJ O-Acyl Thlohydroxamates and Halogen Donor Solvents... 

The photolytic or thermal decomposition of 0-acyl thiohydroxamates in halogen donor solvents such as tetrachloromethane or bromotrichloromethane constitutes the most wide-ranging and generally iplic-... 

The last, but certainly not the least, is the Barton modification to the Hunsdiecker reaction.24-26 It involves decomposition of thiohydroxamate esters in halogen donor solvents such as CCU, BrCCh, CHI3, or CH2I2 promoted by a source of radical initiation, which could be radical initiator (e.g., 18—>20),24 thermal (e.g., 21—>22),25 or photolytic26 conditions. The Barton modification is highly compatible with most functional groups. For example, under photolytic conditions, acid 23 was converted to acid chloride 24, which, without isolation, was treated with the sodium salt of Z/-hydroxypyridine-2-thione (19) with bromotrichloromethane as solvent to give alkyl bromide 25 in 90% yield.26... 

Substitution of the halogen X by another electronegative group affords compounds that are stable with respect to intermolecular ligand transfer these compounds do not require donor solvents for stabilization. In particular, ligands with a nitrogen atom attached to the metal fulfill at least one of the... 

Note The polymeric nature of these silver(I) bis-carbene complexes limits their solubility in most organic solvents including the standard CH Cf. Strong donor solvents like DMSO overcome this problem, probably by breaking the silver-halogen bridges. 

Examples of charge-transfer absorptions in solutions of halogens have been described in Chapter 6. In these cases, a strong interaction between a donor solvent and a halogen molecule, X2, leads to the formation of a complex in which an excited state (primarily of X2 character) can accept electrons from a HOMO (primarily of solvent character) on absorption of light of suitable energy ... 

The absorption band, known as a charge-transfer band, can be very intense it is responsible for the vivid colors of some of the halogens in donor solvents. 

However, some isomerization frequently is observed. Although most vinyllithiums are configurationally stable, the a-aryl vinyllithiums (prepared from metal-halogen exchange, see 5.5.2.3.3) equilibrate in the presence of donor solvents, such as THF. Vinyllithiums are stable in ether, in which most preparations are performed (see Table 3). 

A third way of synthesizing six-membered rings is by intramolecular salt elimination from metallated halogen-substituted trisiloxanols. Lithium 1,1,5,5-tetra-tert-butyl-l-fluoro-5-hydroxy-3,3-dimethyl-trisiloxane is stable in n-hexane. Only by weakening the Li-0 contact by using a donor solvent such as THF, can the elimination of LiF with concomitant formation of a six-membered ring be achieved (Eq. 5). ... 

Alkaloids are often rather unstable, e.g., N-oxidation is quite common. In addition to by heat and light, the stability is influenced by solvents (for a review see Baerheim Svendsen and Verpoorte 1983). Halogen-containing solvents are widely used in alkaloid research, and chloroform in particular is one of the most suitable solvents, because of its relatively strong proton donor character. However, these solvents are very active in terms of artifact formation. In chloroform (N-)oxidations occur readily. Also peroxides in ethers may rapidly cause N-oxidations. With dichloromethane, quaternary N-dichlorometho compounds may be formed (Phillipson and Bisset 1972). Similar compounds are formed with minor impurities present in chloroform. Moreover, in chloroform, phosgene is formed, which reacts with the stabilizer ethanol, yielding ethyl chloroformate. This compound may react with secondary amines, causing the formation of ethylcarbamates (Siek et al. 1977). 

Recently, blocked polymeric isocyanates useful in the preparation of polyurethanes, have been prepared by direct carbonylation of a nitro-substituted polymer based on styrene with carbon monoxide in the presence of a catalyst at 60-200 "C and a pressure from atmospheric to 2000 psi in a hydrogen-donor solvent [64]. Catalysts such as PdL2X2 (X = halogen, L = heterocyclic ligand, e.g. pyridine) in the presence of [NEt4][Cl] have been used. However, from the abstract it is not clear if polynuclear ruthenium, iron, and platinum carbonyls are... 

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