The kinetics of the nitration of anisóle and p-iodo-, -bromo-, and p-chloroanisole have been studied. The relative reactivities of the ortho positions of these compounds are 1:0.119:0.077:0.069. In acetic anhydride attack by N02 + para to the methoxyl occurs to the extent of 30.6, 40, 31, and 28% of total attack. Therefore, z'fx, the ipso partial rate factors for the halogens iodine, bromine, and chlorine, are 0.18, 0.07g, and 0.06i. These values represent the directive effect of the halogen on the rate of attack, not ortho, meta, or para to the substituent, but directly at the position bearing the substituent.There exists an enormous body of data concerning directive effects in electrophilic aromatic substitution. Not only are substituents classed as activating or deactivating,ortho-para directing or meta directing, but also quantitative measures of these effects have been determined. The directive effects of a substituent, R, are expressed by the partial rate factors, oR, mR, and pfR, representing the rates of electrophilic attack at a single position ortho, meta, or para to the substituent, relative to the rate of attack at a single position of benzene.However, there exist extremely few data concerning the directive effect of the substituent for attack directly at the ipso (Latin: itself) position, the position bearing (1) Alfred P.
A novel variation of an intramolecular Nicholas reaction is described that provides an efficient method for the synthesis of fused and bridged bicyclic compounds. The mechanism appears to involve an initial regioselective in situ double bond migration of a terminal alkene. The more stable disubstituted alkene then undergoes an intramolecular cyclisation reaction with a dicobalt hexacarbonyl stabilised cation. This transformation generates a second cation, which is subsequently quenched by a halide ion derived from the Lewis acid.Interest in the chemistry of dicobalt octacarbonyl has arisen from its uses in the Pauson-Khand reaction 1 the cyclotrimerisation of alkynes, 2 the Nicholas reaction 3 and other useful applications. 4 Our interest in the chemistry of dicobalt hexacarbonyl clusters stems from their use in the Nicholas reaction which is the reaction of a cobalt stabilised propynyl cation with a nucleophile.In general it is found that for both inter and intramolecular examples of the Nicholas reaction activated alkenyl derivatives are used to quench the dicobalt hexacarbonyl stabilised propynyl cation. Representative examples include O-silylenol ethers, 5 allyl silanes, 6 enamines 7 and aromatic compounds. 8Due to the instability of some of these derivatives, as well as the additional steps required to bring about their formation, we have instigated a series of studies aimed at investigating the synthetic utility of less functionally sensitive moieties and their use in intramolecular cyclisation reactions. To this aim we have recently shown that gem-dimethyl substituted double bonds are sufficiently activated to attack a Nicholas cation and effect a cyclisation reaction. 9Following on from an earlier observation 10 we have now been able to effect a range of cyclisation reactions using a simple pendant alkenyl moieties to quench the Nicholas cation. This communication describes the results obtained from these investigations into the synthesis of both fused and bridged ring carbocycles and serve to compliment the cyclisation reactions that we have previously revealed. 11In order to access both fused ring and bridged ring carbocycles cyclisation precursors were produced such that the propynyl alcohol moiety was either a or b to a pendant alkenyl group. Our initial study for the synthesis of the bicyclo[4:3:0] nonane framework is illustrated in (Scheme 1). The propynyl alcohol 2 was formed, from cyclohexene oxide 1, via a copper catalysed ring opening reaction with 4-pentenylmagnesium bromide. 12 Oxidation of the hydroxyl group and propargylation provided 2 in an overall yield of 42%. Exposure of 2, to dicobalt octacarbonyl, followed by an in situ dehydroxylation reaction led to cyclisation. Decomplexation of the dicobalt hexacarbonyl moiety with CAN provided, compound 3a, using TiCl 4 , or 3b, using TiBr 4 , in good overall yields. Scheme 1In order to gain access to bridged ring carbocyclic compounds cobalt clusters derived from propynyl alcohols, such as compound 4, were synthesised (Scheme 2) with the termina...
The hazards resulting from smoke emission were widely recognized in the early 1970s as a result of several major fire disasters. Since that time considerable interest has been shown in the development of smoke-suppressants. This paper reviews some of the very promising results that have been obtained using molybdenum compounds as flame-retardants and smoke-suppressants in halogenated polymers. They have been shown to be effective in both rigid and flexible grades of PVC. In the rigid grades these compounds can reduce smoke emission by a t least 80%. Reductions in excess of 50% have been observed with typical plasticized formulations, although the magnitude of the effect here depends on both the type and level of plasticizer. Comparisons of the results of different smoke tests suggest that the small-scale tests commonly used do not fully reflect the true extent of the smoke-reductions that are obtainable with these compounds. Significant reductions in smoke-emission have also been obtained with halogenated polyester thermosets and chlorinated elastomer formulations. With the polyesters some of the best results occurred when molybdenum oxide was used in conjunction with aluminium trihydrate. In all cases these compounds proved to have significant flame-retardant properties. Reference has been made in the paper to their mode of activity but the actual mechanistic investigations have not been discussed in detail.Many compounds have been suggested in the literature for use as smoke-sutmressants.2 Thev can be classified as 0308 0501 ,'86/02oO63-07$05.OO 1186 by
Abstract-Oxygen index results for a series of unsaturated polyesters, containing molybdenum oxide and various halogenated compounds, have provided definite evidence for some form of flame retardant synergistic effect between molybdenum and halogen. With the halogenated compounds used, the magnitude of the effect was greater in the presence of bromine but was dependent on the type of compound. When dibromoneopentyl glycol was used as the bromine source, the synergistic effect exhibited by molybdenum oxide was comparable to that shown by antimony oxide. Since molybdenum oxide also acts as a smoke suppressant, it could offer a useful alternative to antimony oxide particularly in the light of probable changes in standards and regulatory control regarding smoke emission. IntrQductionDURING THE last three decades a vast number of flame retardant additives have been suggested and patented for use in various polymer formulations.1 Since polymers burn and degrade by different mechanisms and there are many desirable properties required of a flame retardant,z it is not surprising that a universal flame retardant does not exist. The most commonly used commercial compounds are based on the elementschlorine, bromine, antimony, phosphorus and boron. Using various combinations of these available flame retardants it is possible to exercise reasonable control over the rate of burning and flame spread in polymers. These systems do little, however, to reduce the amount of smoke generated by a polymer if it burns and there is evidence to suggest that flame retardant polymer formulations usually generate a denser smoke than untreated ones. This is particularly disturbing when an analysis of the statistics of fire casualties in the United Kingdom3 and in North America4 shows that the effects of smoke and toxic gases are increasing and now account for almost 50% of all deaths-smoke being one of the main causes of death.The recognition of the smoke and toxicity hazards of burning plastics has resulted in a need for the development of flame retardant formulations which incorporate smoke suppressant properties. Recent reports5-7 have illustrated that certain molybdenum compounds are potential smoke suppressants and that they also behave as flame retardant synergists in the presence of halogencontaining compounds. The original study on molybdenum by Moore and Church597 involved a screening evaluation of the effect of selected molybdenum compounds, mainly molybdenum oxide and ammonium dimolybdate, on the burning characteristics of PVC and unsaturated polyester resins-two plastics which consume large quantities of flame retardants. Since the initial trials were geared to the use of the inorganic molybdenum compounds as replacements for antimony oxide, polymer formulations were prepared containing the molybdenum compounds and/or antimony oxide and evaluated for their flame retardancy and smoke formation using the Michigan Chemical Corporation's Oxygen Index-Smoke Densitometer Unit. In both the PVC and the polyester resin (Atlac 711-O5A, an I.C...
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