Halide studies

Pentadienoic acid reacts well with aromatic halides to give 5-aryl-pentadienoic acids. The only vinylic halide studied so far is E-2-bromostyrene. This halide gave a 57% yield of E,E,E-7-phenylheptatrienoic acid.N-(2,4-Pentadienyl)piperidine is very useful... 

A special role in the chemistry of nickel and cobalt alkoxide derivatives is played by the poorly soluble methoxide halids studied in detail by the group of Winter [964, 865]. According to the diffuse reflectance spectra, the coordina-... 

Binary compounds formed between metals and group 6 or group 7 elements usually occur in the form of ionic crystals rather than as isolated molecules. The most typical example is, of course, given by the alkali halides, studied by Lowdin in his classic treatise from 1948 [1], Another important class of ionic crystals, with somewhat different properties, are the metal oxides, which play a central role in many contexts in chemistry and physics. To mention only one example, their catalytic properties have long been recognized and subject to extensive study, and have given rise to numerous applications of enormous practical importance. 

The most simple a-aryl substituted vinyl halide studied, a-bromostyrene, flash photolytically yields the enol of acetophenone in aqueous solution, which has been used to measure rates of ketonization302,303. With a,/2-diarylvinyl halides, loss of the /2 (vinylic) proton from the intermediate vinyl cation is competitive with nucleophilic trapping Irradiation of compounds such as 96 in methanol gives about equal amounts of tolan and... 

Table 8.14. Alkali halides studied by electric resonance methods...

Our earliest alkali halide studies were performed with cesium halides, since the prevailing evidence (Table I) indicated that the vapor would be predominantly monomeric, and hence simplest to interpret. The apparatus employed for these studies (Figure 1) consisted of a cylindrical mirror electron energy analyzer, a non-inductively wound oven for generating the cesium halide vapor, and a helium resonance lamp. The spectra we obtained ( ) for the cesium halides are displayed in Figure 2. They reveal a clearly resolved doublet for Csl, a partially resolved doublet for CsBr, and broad single peaks for CsCl and CsF. We shall briefly reproduce here the arguments we used to interpret these spectra. 

Although alcohols, ethers, and epoxides share many characteristics, each functional group has its own distinct reactivity, making each unique and different from the alkyl halides studied in Chapters 7 and 8. Appreciate the similarities but pay attention to the differences. 

The simplest examples of this type of defect are provided by the alkali halides, studied extensively by Pohl and his colleagues (2). Sodium chloride heated in sodium vapour assimilates sodium atoms, which occupy normal cation sites as TSIa+ ions, while the extra electrons are trapped in the neighbourhood of the newly-created vacant anion sites. Vacant anion sites act as centres of effective positive charge in the crystal and produce a Coulomb-like potential field capable of binding electrons. This will be discussed in more detail in 2.2. It is difficult... 

From all the halides studied it was possible to separate metals, often in good yields. 

An increase in the CH3COCl / 4-methylacetophenone ratio, 5 1 instead of 1 1 does not change the degree of complexed ketone (Table 6) which shows that the interaction between ketone and Lewis acid is much greater than the interaction between the acid chloride and the Lewis acid in the case of the metal halides studied. 

Assuming a pK value of 2 to be the threshold for the carbonate-ion stability in molten salts in a C02-free atmosphere, it may be concluded that in halide melts consisting, even partially, of lithium salts and more acidic multivalent cations (Ca2+, Mg2+, Ln3+, etc.) carbonate ion is unstable and undergoes complete decomposition with the formation of the equivalent quantity of oxide ions in the melt. As for the other alkali metal halides studied, similar behaviour of CO2-ions can be expected only for CsCl, CsBr, and KBr melts at temperatures exceeding 800 °C. Of course, instability of carbonate ion in the melts does not mean an automatic disappearance of the oxygen admixture from the melt, since oxide ions arise instead of CO2- owing to their complete breakdown. This means that only for the melts characterized by a low pK value of the equilibrium (1.2.3), the carbohalogenation method of purification is the most suitable. 

Essentially all the simple geminal di- or poly halides studied have been derivatives of methane. Reaction of diiodomethane (25,103) with triethyl phosphite proceeds normally, to furnish tetraethyl methanedi-phosphonate and iodomethanephosphonate. However, the reaction between carbon tetrachloride and trialkyl phosphite, first investigated by Kamai and Egorova (156,181), is catalyzed by peroxides or ultraviolet light, and accordingly it has been formulated by Kamai and Kharrasova (157) as a radical-chain process. 

Few examples of this mechanism have been clearly demonstrated because of tfie difficulty in establishing that this path occurs from experimental data. The most well-established examples are reactions of nickel complexes with aryl halides Studied by Tsou and Kochi. The rate of the reaction of Ni(PEt3)jWith aryl halides was shown to be first order in nickel and in ArX and retarded by added PEtj, Ortho-methyl substituents had little effect on the rate. Because of the lack of steric effect, electron transfer was proposed to occur after formation of a TT-complex between Ni(PEt3)j and ArX, rather than by direct insertion of the metal into the carbon-halogen bond by a three-centered mechanism. Moreover, the products of the reaction included the Ni(I) species L3NiX and arene. Tliese products are likely to result from the pathway in Scheme 7.4, involving electron transfer from Ni(0) to the aryl halide and escape of the aryl radical from the solvent cage. Other studies of oxidative additions of aryl halides and sulfonates to Ni(0) complexes have been reported. " ... 

Finally, Cohn " examined systematically the reactions of (CH3)2NPF2 with a series of halides taken from the first row of transition elements The metal halides studied were TiCU, FeCl3, NiBr2, CoBr2, CUCI2, CuCl, and GaCl3 as well as NaCl. No reaction was observed with NaCl. Four types of reactions were identified when (CH3)2NPF2 was mixed with metal halide systems. These are ... 

Ackermann has recently described the direct alkylation of heteroarenes (ben-zoxazoles and benzthiazoles) with unactivated primary alkyl halides, using a user-friendly, nitrogen and phosphorus ligand-free catalytic system derived from inexpensive [(diglyme)NiBr2]. Cul was found to be required as a cocatalyst and Nal was beneficial when using alkyl chlorides and bromides. However, the authors noted that the one secondary alkyl halide studied, 2-bromohexane, did not couple in an acceptable yield (Scheme 5.30). ... 

Our model halide studies showed that reactive halides include allylic, tertiary, and benzylic chlorides, bromides, and iodides. Solvible silver salts with anions such as SO3CF3 , BF4 , PFj , AsFj , SbFj, and C104 are most suitable but LiPF and NaCl04 can also... 

This chapter describes the photochemical C-X bond fission in alkene systems that have halogen atoms bonded to the carbon-carbon double bond. The substitution of halogen atoms in alkenes results in a red shift of the n,n absorption band because of the interaction between the lone pair on the halogen atom and the 7t-orbitals. > However, the UV absorption of aliphatic alkenyl halides, except for the iodides, lies in a region of short wavelength less than 254 nm (Table 11.1). Therefore, the majority of the alkenyl halides studied in these photolyses are the iodides. 

---