Acylation of N-t-butylhydroxylamine occurs predominantly on oxygen. With acetic anhydride, essentially pure O-acetyl-N-t-butylhydroxylamine is formed. This can be acylated or1 nitrogen. and the O-acetyl-group removed to give a variety of hydroxamic acids RCON(But)OH which can be oxidised to isolable acyl nitroxides RCON(But)O. ( R = alkyl, aryl, alkoxy, or dialkylamino). Other oxygen-protecting groups for N-t-butylhydroxylamine are discussed. The reactions have been extended to include syntheses of a number of acyl nitroxide biradicals. Ethoxycarbonyl t-ButylNitroxide.--Mihen the technique now generally referred t o as spin trapping was in its infancy, one system which particularly intrigued us was the scavenging, by 2-methyl-2-nitrosopropane, of polyhalogenoalkyl radicals2 It seemed that radicals YC&, where X is halogen, would normally give the ' spin adduct ' ( l ) , but that this could be readily transformed into an acyl nitroxide (2).t When k' was also halogen, halogenocarbonyl nitroxides were formed, and these readily acylated alcohols (ROH) or amines YC-0 But -N -0 I YCX, I BU~-N-O-(R,NH) to give alkoxycarbonyl (2; Y = RO) or dialkylaminocarbonyl (2 ; Y = R,N) nitroxides. Both classes of radical, (1) and (2), were characterised by their e.s.r. spectra, but it was noticed that ethoxycarbonyl t Although the detailed mechanism for the conversion of (1) into (2) remains o b s c ~r e , ~? ~we have subsequently noticed that t-butyl trichloromethyl nitroxide can be generated by shaking a chloroform solution of 2-methyl-2-nitrosopropane with nickel peroxide. When the oxidant has settled out, the solution of radical remains substantially unchanged (e.s.r.) for a long period (hours). This is consistent with the requirement 2 for a reactive radical t o remove chlorine from the trichloromethyl nitroxide as a key step in acyl nitroxide formation, which, in other systems involving t-butyl trichloromethyl nitroxide, is rapid.

A series of coloured compounds of general formula CsPb,-,Sn,X, (X, = Br, or Br,CI) has been prepared. X-Ray and Mossbauer data show that the compounds have highly symmetrical tin-lead environments. Their electronic resistivity and optical-reflectance spectra are interpretable in terms of the partial filling of electronic bands produced by the interaction of orbitals of the bromine atoms, As x increases the band population increases, consistent with the lower ionisation energies of the tin 5s electrons compared to those of the lead 6s electrons. Replacement of Cs+ by Naf, K+, Rbf, or [NH,] + shows that these singly charged ions have no effect on band formation. Replacement of Br, by Br,CI raises the energy of the band due to the inclusion of the higher-energy chlorine orbitals.

On ultraviolet irradiation of the cyclopent-Zenyl methyl1 ketones la-c at -5 4 " s t I 139", photo-CIDNP. effects of the starting ketones, the 1,3-acetyl shifted isomers (2), and radical disproportionation and corn bination products (4-7) were observed. These effects show a unique dependence (of the polarization phase on temperature which is a novel feature in photo-CIDNP. studies. The results of the investigation, which also included experiments using triplet quenchers, triplet sensitizers and radical scavengers, are ratilonalized in terms of Schemes 2 and 3.a-Cleavage is a major excited-state reaction of 1 a-c on direct irradiation.Temperature-activated a-cleavage (k:(t)) to the radical pair R . . R" and intersystem crossing (&) to the T, state are among the competing s, deactivation processes. The T, state in turn cleaves (kz2) to R.-R'3. A 'low-temperature range' with k& k,S ( t ) and a 'high-temperature range' with k,S ( t ) % kisc exhibiting preferential reactivity from the T, and S 1 states, respectively, can be defined for all three p , y-unsaturated ketones la-c.Introduction. -Most 8, y-unsaturated ketones predominantly undergo two characteristic photoreactions: an allylic 1,3-acyl shift on direct irradiation, and a triplet sensitized oxadi-n-methane rearrangement (Scheme These results clearly showed from the beginning that at least two excited states are reactive. It has been universally accepted that the oxadi-n-methane rearrangement occurs from the lowest-lying triplet (TJ, and that this state is predominantly 7z, n+& in nature3). The I) 2, 3, Present address: Wissenschaftliches Hauptlaboratorium WHZ, BASF AG, D-6700 Ludwigshafen. See 111 for comprehensive reviews of P,y-unsaturated ketone photochemistry, and 121 for a progress report on cyclopent-2-enyl methyl ketones. For experimental identifications of Tl(rc,rc*) as the oxadi-rc-methane-reactive state, see 13-51, 0018-019X/79/6/ 1952-14$01.00/0 0 1979 Schweizerische Chemische Gesellschaft 5b. .Y = H b , The denotations (a)-(e) refer to the signal assignments in Figure I . Cf: Figure I A . d)CJ Figure 1B. e, A=enhanced absorption, E=emission. Negligible CIDNP. effect. g) in CD30D at -54".

The reactions of benzoyl t-butyl nitroxide with a range of hydrocarbon substrates are described. Hydrogen abstraction (leading t o substitution) is observed with alkenes having reactive allylic hydrogens, and with aralkanes. Other alkenes give addition products. With alkanes a radical self-reaction competes with hydrogen abstraction, although good yields of substitution product may sometimes be obtained using the more reactive 3,5-dinitrobenzoyl t-butyl nitroxide.Kinetic data are reported for reaction of the benzoyl nitroxide with cumene [equation (3)], and relative reactivity data are recorded for other alkylbenzenes; using a series of substituted toluenes, correlation with D+ gives a p-value of -0.9 (at 90 "C).Reaction of the benzoyl nitroxide with a series of aldehydes gives O-acyl-N-benzoyl-N-t-butylhydroxylamines.

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