Group 16 VIA

The Group VIA elements are oxygen, sulfur, selenium, tellurium, and polonium. They have a valence shell configuration of ns np. Oxygen (Oj) is a diatomic gas, and it also exists in an allotropic form called ozone (O3). Sulfur forms acidic oxides (e.g., SO, SO3). 

The elements of the Group VIIIA, otherwise known as noble gases are extremely unreactive. They are found as non-combined forms in nature. Because of this, they are called inert gases. They have an outer configuration of ns iy/.  

Other reactions involving compounds with [CpM(CO)3] moieties include the reaction of [CpMo(CO)3Na] with [MeN=C(Cl)Ph] in the presence of [NH4PF8] to afford (13), with some of (14) [thought to be a hydride addition product of (13)] as by-product, the reaction of CpMo(CO)3Cl with lithiated amidines to afford products of general formula (15), and the synthesis of [CpMCH(NR)a- 

The reactions of primary amines with the carbonyl functions of [CpW(CO)b-CHaCOCHg] and [CpW(CO)3COCH3] have been examined, and studies on the reduction of cycloheptatrienyltungsten tricarbonyl have resulted in the synthesis of [(C7H9)W(CO)8Me].  

Downs and A. Wojcicki, Inorg. Chim. Acta, 1978, 27, 91 see also D. A. Ross and A. Wojcicki, Inorg. Chim. Acta, 1978, 28, 59. 

Stereospecific phenyl addition at the azomethine carbon atom has been proposed to occur on reaction of (17) with phenyl-lithium. Reaction of (17) with methyl-lithium has now been found to cause contraction of the five-membered chelate system of the complex [equation (11)]. An A -ray crystal structure 

The compound [CpCr(CO)2]2 has previously been reported to be inert to acetylenes, but now reaction with [PhC CH] in refluxing toluene has been shown to give (19), the crystal structure of which has been determined. The 

Sulfur, which occurs as the free element in numerous natural deposits, has been known since prehistoric times it is the brimstone of the Bible. It was of considerable interest to the alchemists and, following the development of gunpowder (a mixture of sulfur, KNO3, and powdered charcoal) in the 13th century, to military leaders as well. 

Element Ionization Energy (kJ mol Electron Affinity (kJ mol ) Meliing Point CO Boiling Point (°C) Electro- negativity 

Ozone absorbs ultraviolet radiation below 320 nm. It thus forms an indispensable shield in the upper atmosphere, protecting the Earth s surface from most of the potentially hazardous effects of such high-energy electromagnetic radiation. There is now increasing concern because atmospheric pollutants are depleting the ozone layer worldwide, with the most serious depletion over Antarctica as a result of seasonal variations in high-altitude air circulation. In the upper atmosphere, ozone is formed from O2  

Absorption of ultraviolet radiation by O3 causes it to decompose to O2. In the upper atmosphere, therefore, a steady-state concentration of ozone is achieved, a concentration ordinarily sufficient to provide significant ultraviolet protection of the Earth s surface. However, pollutants in the upper atmosphere such as nitrogen oxides (some of which occur in trace amounts naturally) from high-flying aircraft and chlorine atoms from photolytic decomposition of chlorofluorocarbons (from aerosols, refrigerants, and other sources) catalyze the decomposition of ozone. The overall processes governing the concentration of ozone in the atmosphere are extremely complex. The following reactions can be studied in the laboratory and are examples of the processes believed to be involved in the atmosphere  

Ozone is a more potent oxidizing agent than O2 in acidic solution, it is exceeded only by fluorine among the elements as an oxidizing agent. 

The tungsten carbene complex [W(0)(CHCMe3)(PEt3)2Cl2] (3) has been obtained by an indirect route [equation (12)] analogous benzylidene, ethyli-dene, propylidene, and methylene complexes were prepared by treatment with 

Fischer-type carbenes have been found to react with [Pt(cod)2] to form metal-metal bonded species with bridging carbene ligands [equation (13)]. Similar 

Reaction of [MeOC(Me)W(CO)5] with MeLi gave the carbene-bridged bi-nuclear complex [ (OC)4W aHCCH=CMea], the crystal structure of which has been determined. The authors propose a mechanism for the unexpected formation of this product. The binuclear complex rearranges in solution at room temperature to afford [CHCHCMeafWaCCO) ], (5), a species in which the double bond is co-ordinated to one of the metal centres and whose structure has been verified crystallographically. The reactions of (5) with phosphines and 

Porter, F. G. A. Stone, A. A. Wilson, and P. Woodward, J. Chem. Soc., Chem. Common., 

The fluxional -vinyl cation [Mo2(CO)4(a -CHCH2 Cp2]+ obtained by protonation of [Mo2(CO)4(yU-CHCH)(Cp)2] with H[Bp4], is attacked at molybdenum by a variety of nucleophiles the crystal structure of [Mo2 OCOCF3 -(CO)4(/ -CHCH2)Cp2] was determined. 

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An sp sp- single bond where each of the central atoms is in Group VIA (for example, hydrogen peroxide) has a two fold barrier with optirn iitn torsional an glc of 90 degrees, as described by V2=-2,0 kcal/tnol. 

An sp -fip2 or sp -sp - single bond where the sp atom is in Group VIA (for exam pie. the C-0 bond of vinyl alcoholjhasa two fold barrier with an optimal plan ar con figuration described bv V2=+2.0 kcal/inol. 

The telomer 145 of nitroethane was used for the synthesis of recifeiolide (148)[121], The nitro group was converted into a hydroxy group via the ketone and the terminal double bond was converted into iodide to give 146. The ester 147 of phenythioacetic acid was prepared and its intramolecular alkylation afforded the 12-membered lactone, which was converted into recifeiolide (148),... 

Fig. 4. Synthesis of estrone (20) through elimination of unactivated 19-methyl group via intramolecular functionalization of C-19. -TsOH...

Protonolysis. Simple trialkylboranes are resistant to protonolysis by alcohols, water, aqueous bases, and mineral acids. In contrast, carboxyUc acids react readily with trialkylboranes, removing the first alkyl group at room temperature and the third one at elevated temperatures. Acetic and propionic acids are most often used. The reaction proceeds with retention of configuration of the alkyl group via a cycHc, six-membered transition state (206). 

It is generally accepted that transamidation is not a concerted reaction, but occurs through the attack of a free end on the amide group via aminolysis (eg, eq. 4) or acidolysis (eg, eq. 3) (65). Besides those ends always present, new ends are formed by degradation processes, especially hydrolysis (eq. 5), through which the amide groups are in dynamic equiUbrium with the acid and amine ends. 

Teichoic acids (16) are bacterial polymers in which alditols, glycerol, or ribitol are joined through the primary hydroxyl groups via phosphate diester linkages. 

SANDMEYER GATTERMANN AramaUcsubstitution Substitution of an annine group via the diazonkim sait by nudeophiies such as Ci, Br, i, CN R-S, HO, soma via cuprous salt catalysis. 

The occurrence of nucleophilic participation is also indicated by a rate enhancement relative to the rate of solvolysis of n-butyl p-bromobenzenesulfonate. The solvolysis rates of a series of cu-mefhoxyall l p-bromobenzenesulfontes have been determined. A maximum rate is again observed where participation of a methoxy group via a live-membered ring is possible (see Table 5.20). 

Treatment of the dihydroxyacetone side-chain with acid (usually hydrochloric) can result in removal of the 17a-hydroxyl group via the following pathway ... 

Fnedhclm Aubke and Darryl D DesMarteau, Halogen Derivatives of Group VIA Oxyacids... 

FIGURE 11.15 Formadoii of ADP and ATP by the successive addition of phosphate groups via phosphoric anhydride linkages. Note the removal of equivalents of H9O in these dehydration synthesis reactions. 

Triazanaphthalenes (446) substituted with a single leaving group have been little studied. 4-Aminopyrido[3,2-d]pyrimidine and its 6-methyl derivative have been hydrolyzed with 5N acid (100°, 30 min) and lOA alkali (95°, 3 hr, 10% yield).Attempted replacement of the 4-oxo group (via acyloxy intermediates) with phosphorus oxychloride or pentasulfide failed,in contrast to the successful replacement in the more activated 4-oxo-l,3,8-triaza analog discussed below. Similarly, the 2,4-dioxo derivative could not be thionated with the pentasulfide, and its reaction with the oxychloride was less facile than that of the 2,4-dioxo-l,3,8-triaza compound. 

The reaction of hexa-2,4-diyn-l-al (64) with mercaptoacetaldehyde leads to 2-formyl-5-(prop-l-ynyl)thiophene (65). The addition direction is governed by the aldehyde group via intramolecular aldol condensation in the intermediate (77HOU947). 

The metal-vapor technique was applied to cobalt atoms and r-BuC = P (01JOM(635)212). The mixture of products that resulted includes the mixed-ligand sandwiches 170 and 171. Further interaction of complex 170 with [W(C0)5(THF)] leads to the coordination of the W(CO)5-group via the phosphorus heteroatom of the four-membered ring to yield 172. 

Tiazofurine (142) is an antimetabolite with antineoplastic activity. It preferentially affects leukemic lymphocytes over normal cells due to selective activation by formation of its adenine dinucleotide by transformed cells. Of the syntheses available, one starts by conversion of iniidate 138 to methyl 2,5-anhydroallonothioate (139). Next, condensation with ethyl 2-amino-2-cyanoac-etate leads to the thioamide which undergoes thiol addition to the nitrile function to produce the amminothiazolecarboxyester system of 140 directly. Sodium nitrite in aqueous hypophosphorus acid eliminates the superfluous amino group via the diazonium transformation to give 141. This synthesis of tiazofurine (142) concludes by ester amide exchange in methanolic ammonia [48]. 

The NH2 groups can be diazotized and reduced in the presence of thiosulphates and different metal ions. The effect of some metal ions, namely Fe ", Sn, Cu +, and Co on the graft yield of cotton modified with aryl diazonium groups via its reaction with 2,4-dichloro-6-(p-nitroaniline)-5-triazine in the presence of alkali and followed by reduction of nitro group was studied [4]. 

Graft Copolymerization of Vinyl Monomers Onto Macromolecules Having Active Pendant Group via Ceric Ion Redox or Photo-Induced Charge-Transfer Initiation... 

Amino groups can be exchanged for sulfo or halo groups via diazotization and application of the Sandmeyer procedure. An example is the preparation of zinc(II) 2-chloro-9,16,23-trwer/-butylphthalocyaninc (6).3,9... 

Of particular interest are reactions between a-hydroxy ketones and allyl(trifluoro)silanes which proceed with participation of the hydroxy group via a pentacoordinated allylsilane, with intramolecular delivery of the allyl group to the ketone. Excellent stereoselectivity is obtained at up to three contiguous stereogenic centers66. 

Formation of C — C Bonds by Addition to Imino Groups via Strecker and Ugi Reactions... 

Group VIA Mo various derivatives W Various alcohols and acids 69... 

Scheme 13 Possible mode of formation of the cyclopentadiene 61 isomeric with 60a by 1,2-migration of the dimethylamino group via a bridged zwitterionic intermediate 62 [44]...

J ,3J ,4J ,5J )-2,5-bis(benzyloxy)-3,4-dihydroxy-Nd -bis (lS)-2-methyl-l-[(methylamino)carbonyl]propyl hexanediamide is a C2-symmetric HIV-1 protease inhibitor [29]. Derivatization in the para positions of the benzyl-oxy groups via microwave-assisted Stille reaction on the corresponding di-brominated inhibitor smoothly yielded the desired heteroarylated derivatives (Scheme 10). Interestingly, the 1,3-thiazole derivative showed a higher antiviral activity on the wild type virus than the lead compound. The activity remained at the same level in the presence of seriun. Unfortimately, a low activity was observed on mutants. 

Structure. The structure of the refractory-metal carbides increases in complexity with increasing group number. Thus the carbides of Group IVa are characterized by a single cubic monocarbide. In those of Group Va, a M2C phase exists as well as the monocarbide. The carbides of Group Via are far more complex and have several compositions. 

Borides of Group Via. As with the borides of Group Va, the incorporation of free metal in the Group Via borides is difficult to avoid. Both tungsten and molybdenum borides are obtained at high temperature by the hydrogen reduction of the mixed bromides.Bonding appears a more effective method to form these borides in thin layers (see Sec. 2.2 above). 

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