ANDREW P. MASTERS and TED S. SORENSEN. Can. J. Chem. 68, 492 (1990). Reactions of pentacarbonyl manganate anion with 4-halocrotonate esters or 2-halocarboxylate esters result in a complex set of inorganic and organic products, usually including the expected dienolate (or enolate) complexes. The reaction variables include the counterion, solvent, and halo group. The mechanism of the reaction has been investigated by conducting a thorough characterization of the reaction products under various conditions and also by carrying out model reactions. One can rationalize most of the non-organometallic products using either a radical or carbanion mechanism, but the latter seems to fit the available data better. Experimental procedures for optimizing the yield of the organometallic dienolate or enolate complexes have been worked out.Key words: pentacarbonyl manganate, metalate nucleophilicity, enolate complex, nucleophilic substitution, 5 5~n NMR spectroscopy.ANDREW P. MASTERS et TED S. SORENSEN. Can. J. Chem. 68, 492 (1990). Les reactions de l'anion pentacarbonyl manganate avec les esters 4-halocrotonates ou 2-halocarboxylates conduisent a la formation d'un ensemble de complexes de produits organiques et inorganiques qui contiennent gtnkralement les complexes dienolates (ou Cnolates) attendus. Les facteurs qui influencent la rCaction comprennent le contre-ion, le solvant et la nature de I'halogbne. Dans le but de determiner le mCcanisme de la rCaction, on a procCdC ii un examen minutieux des produits obtenus au cours de rtactions rCalisCes sous diverses conditions et on a rCalisC diverses reactions modkles. On peut rationaliser la formation de la plupart des produits qui ne sont pas des organomCtalliques par des mtcanismes soit radicalaires ou carbanioniques; toutefois, ces derniers semblent mieux accommoder les donnCes disponibles. On a dCterminC les conditions exptrimentales appropriCes pour obtenir les meilleurs rendements possibles de complexes diCnolates ou Cnolates.Mots clks : pentacarbonyl manganate, nucliophilicitC mttallate, complexe Cnolate, substitution nuclCophile, spectroscopie RMN du "~n .[Traduit par la revue]The metal-centered anion Mn(CO)5-has been extensively used to prepare R-Mn(CO)5 type compounds (R = alkyl, perfluoroalkyl, etc.). However, there are numerous instances where eq. [l] fails, where the yields are inexplicably poor, or where other products such as M~I~( C O )~~ are formed. In fact the present study evolved when we had difficulties in applying eq.[ l ] to the synthesis of a simple ester dienolate complex (R = dienolate).For a background to transition-metal enolate complexes, the recent papers of Heathcock and co-workers ( I ) provide an extensive bibliography. Dienolate complexes of transition metals have even more potential bonding modes, including the straightforward -q I-, -q3-, or -q5-complexes 1,2, or 3. The initial intent of our work was to study the synthetic utility of the -q3-complex, with the expectation that the geometry in these could be controlled via the -q5-complex, as indic...

A. P. MASTERS, M. PARVEZ, and T. S. SORENSEN. Can. J. Chem. 69, 2136 (1991.Cr2(CO)s(NO)2 (1) has been prepared and characterized, and the crystal structure determined, along with that of the related salt Cr2(C0)9NO-monoanion (2) as the CH2C12-solvated PPN+ salt (where PPN+ denotes the (PPh&N+ monocation). Both 1 and 2 possess a metal-metal bonded structure analogous to that of Mn2(CO)Io and the electronically equivalent Cr2(CO)lo2-dianion. Although Cr2(C0)8(N0)2 (1) is slowly formed in poor yield from Cr(C0)4NO-and Cr(C0)4NOBr, it can be prepared in an almost quantitative yield by the one-electron oxidation of the monoanion with trityl hexafluorophosphate. The slowness of the former reaction makes Cr(C0)4NO-useful as a two-electron reductant of organohalide bonds. Triclinic crystals of 1 incorporate two distinct molecules in the ~T -s~a c e group, although the geometries of each are almost identical.Triclinic crystals of monoanion (2) also belong to the PI space group. The Cr--Cr bond length is 3.0Ck3.02 A in 1 and 2.995 A in 2, compared to 2.904 A for the central Mn-Mn bond in Mn2(CO)lo.Key words: chromium nitrosyl, chromium carbonyl, X-ray crystal structure, metalate anion, metal-metal dimer.On a prCparC le Cr2(CO)s(N0)2 (I), on l'a caractiris6 et on a dCterminC sa structure cristalline ainsi que celle du monoanion Cr2(C0)9NO-(2), sous la forme de sel du PPN+ (oh PPN+ = monocation (PPh3) Mots clis : nitrosyle de chrome, carbonyle de chrome, structure cristalline par rayons-X, anion mitalate, dimkre mCtalmCtal.[Traduit par la ridaction] Introduction yields along with slow reaction rates. Nevertheless, sufficient In the reaction of Mn(C0)5-with hindered a-brorno carbony1 compounds, one can get a facile two-electron reduction of the C-Br bond, resulting in Mn(CO)5Br and an enolate (1). However, a side product of this reaction is Mn2(CO)lo, which appears to be formed from Mn(CO)5Br and Mn(CO)5-in a reaction which can be competitive with the enolate-forming reaction.In reactions of the isoelectronic anion Cr(C0)4NO-(2) with a-bromo carbonyl compounds, similar complications have not been observed; however, we have occasionally seen traces of a very mobile light yellow TLC spot when analyzing crude reaction mixtures. Since Mn2(CO)lo behaves very similarly on TLC elution, there was a possibility that traces of the isoelectronic compound Cr2(C0)8(N0)2 (1) were being observed in the former case. We were therefore interested in preparing and characterizing an authentic sample of Cr2(CO)8(NO)2.Several mixed monomeric chromium carbonyl nitrosyl complexes have been prepared under low-temperature photolysis conditions, including a possible Cr(C0)4NO species (3). Dimeric cyclopentadienylchromium carbonyl nitrosyl complexes are also known (4).

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A new in situ low-temperature synthesis of cyclopropanones from cu,cul-dibromo ketones is described. The synthetic procedure is particularly well suited to NMR tube-scale experiments, which then easily allows one to spectroscopically characterize low-temperature solutions of these unstable molecules. Nine systems were studied, seven of which give high yields of the cyclopropanones at -78OC or, in one case, -93°C. ' The major rearrangement pathway of these cyclopropanones leads to an a,P-unsaturated ketone. The mechanism of this reaction has been studied in some detail, including deuterium isotope studies, and the direct observation of enols, silyl enol ethers, and dimers. The rearrangement reaction is postulated to involve the intermediacy of an oxyallyl, the transfer of a hydrogen from the anti-alkyl substituent of this oxyallyl to the oxyallyl oxygen atom, which leads to an enol, and then finally tautomerization of the en01 to the ketone. The thermal stability of the cyclopropanones is directly related to the ease of this rearrangement and, in two cases studied here, the rearrangement takes place rapidly even at -95°C and the cyclopropanones could not be observed. There is a remarkably strong correlation between cyclopropanone structure and reactivity, with the main structural factor being the external angles of the cyclopropanone substituents.C. BLACK, P. LARIO, A.P. MASTERS, T.S. SORENSEN et F. SUN. Can. J. Chem. 71, 1910 (1993. On dCcrit une nouvelle synthkse in situ et a basse temptrature des cyclopropanones a partir d1cu,a'-dibromocttones. La mkthode de synthkse est particuliitrement adaptCe a des etudes dans des tubes de RMN; cette particularit6 permet d'obtenir facilement une caracterisation spectroscopique basse temperature de solutions de ces espkces instables. On a Ctudit neuf systitmes, dont sept qui donne d'excellents rendements en cyclopropanones 2 -78°C ou, dans un cas, 2 -93°C. La voie reactionnelle du rkarrangement principal de ces cyclopropanones conduit a une cCtone a,P-insaturee.On a CtudiC en dCtail le mCcanisme de cette reaction, incluant des ttudes d'effet isotopique du deuterium ainsi que l'observation directe d'enols, d'kthers Cnoliques silylCs et de dimeres. On suggkre que la reaction de rkarrangement implique un oxyallyle comme intermkdiaire, le transfert d'un hydrogitne a partie du substituant alkyle anti de cet oxyallyle vers l'atome d'oxygbne de l'oxyallyle, qui conduit a un Cnol, et finalernent une tautomCrisation de I'Cnol en cktone. La stabilitk thermique des cyclopropanones est directement reliee a la facilite de ce rearrangement et, dans deux cas CtudiCs ici, ce rearrangement se produit tellement rapidement m2me a -95°C que les cyclopropanones n'ont pu &tre observees.I1 existe une forte corrtlation importante entre la structure de la cyclopropanone et la reactivit6, le facteur structural principal etant les angles externes des substituants de la cyclopropanone.[Traduit par la rkdaction]Turro and co-workers pioneered the first preparations (as in situ solution species) of sever...

, and -q5-ester dienolate complexes of Mn(CO),, have been prepared and the crystal structures of the phenyl ester member have been obtained for all three bonding modes. The allyl (anti--q3) member shows some bonding distortions compared to a simple allyl ligand. The structure of the r15-1-oxapentadienyl ligand complex is substantially different from a rhenium analog in that the organic ligand is non-planar with significantly different bond distances. The r15-complex reacts rapidly with triphenylphosphine to generate the syn--q3-complex.Key