James E. Galle

Bora-aromatic systems. Part 8. The physical and chemical consequences of cyclic conjugation in boracyclopolyenes. The antiaromatic character of pentaarylboroles

Pentasubstituted boroles or boracyclopentadienes have been prepared by two routes: (1) the interaction of an (E&)-( 1,2,3,4-tetraaryl-1,3-butadien-l,4-ylidene)dilithium in ether solution with an aryl(diha1o)borane to form the borole etherate; and (2) the exchange reaction between a 1,1-dialky1-2,3,4,5-tetraarylstannole in nondonor media to produce the unsolvated borole. The boroles are unexpectedly strong Lewis acids, complexing with amines and even ethers and nitriles, and very prone to oxidation, solvolytic cleavage, and Diels-Alder reactions. The foregoing chemical behavior, taken together with their unusual nuclear magnetic and electronic spectral properties, can be straightforwardly interpreted in terms of the Hiickel antiaromatic character of the 4-*-electron boracyclopentadiene nucleus. By treating such nuclei as perturbed cyclopentadienyl 7r-systems, a qualitative understanding of both the spectral and chemical properties can be attained. From such considerations, it is evident that the conjugation between the tricoordinate borons 2p,-orbital and the four-carbon butadienylidene array is destabilizing and is the source of the high reactivity of boroles. An

Organosilicon compounds with functional groups proximate to silicon: XVII: synthetic and mechanistic aspects of the lithiation of α,β-epoxyalkylsilanes and related α-heterosubstituted epoxides

-hybridized boron atom might be expected to form cyclic conjugated systems with arrays of sp2-hybridized carbons, because both the covalent radius (0.80 A) and the Allred-Rochow electronegativity (2.01) of boron are similar to those of carbon (0.77 A and 2.51).4 In such conjugated rings the boron center might exert a stabilizing influence if the total number of x electrons would conform to the Hiickel 4n + 2 criterion for a r o m a t i ~ i t y . ~ ~ In fact, heterocyclic organoboranes bearing the isoelectronic J3=N unit in place of one or more C=C linkages of a benzenoid ring have been shown to possess chemical and physical properties similar to classic aromatic compounds.5b Boracyclopolyenes not possessing the appropriate number of ?r electrons might be destabilized by conjugation and accordingly display unusual reactivity or antiaromaticityS6 An example of the destabilization of a carbocyclic ring by such antiaromaticity is the instability of the pentaphenylcyclopentadienyl cation (1). This deep-blue ion is highly reactive and has a thermally populated triplet (compound 2). This behavior indicates that the gain in conjugated stabilization attained by placing the four A electrons as pairs in bonding molecular orbitals is not large.

Organic chemistry of subvalent transition metal complexes: XI. Oxidative additions of nickel(0) complexes to carbon-carbon bonds in alkynes: Nickelirenes and nickeloles as catalytic carriers in the oligomerization of alkynes☆

Abstract A series of α-heterosubstituted epoxides, , has been found to undergo lithiation in the temperature range of −75 to −115°C at the CH bond of the epoxide. The substituent Z could be Me3Si, Ph3Si, n-Bu3Sn, Ph3Sn, PhSO2, (OEt)2PO and Ph; the groups R and R′ were H, Ph and n-C6H13; and the lithiating reagents were n-butyllithium, t-butyllithium and lithium diisopropylamide in donor media of THF or TMEDA. The lithiation occurs with retention of configuration and the resulting lithio-epoxide is unstable above 0°C, decomposing in a carbenoid manner. The lithiation is facile except for compounds where Z and R (an alkyl or aryl) are cis-oriented; where Z = R3Sn, lithiation occurs by tin-lithium, rather than hydroen-lithium, exchange. The lithio-epoxides thereby generated can be quenched with various reagents to yield epoxides where the epoxide H has been replaced by D, Me3Sn, R, RCO and COOH. The utility of this procedure in organic synthesis is emphasized. Finally, the possible explanations for the acidity of such α-heterosubstituted epoxides and for the relative stability of the derived lithio-epoxides are considered and assessed.

Electron-transfer processes in the alkylation of α, β-unsaturated sulfones by organometallic reagents

The formation of 2,3,4,5-tetraphenylnickelole-bis(triphenylphosphine) (IIIa) and 2,3,4,5-tetraphenylnickelole-bis(1,2-diphenylphosphino)ethane (IIIb), either from (E,E)-1,2,3,4-tetraphenyl-1,3-butadien-1,4-ylidenedilithium (I) and the corresponding nickel(II) chloride-phosphine complexes (II) or from the reduction of η4-tetraphenylcyclobutadienenickel(II) bromide dimer (XII) in the presence of phosphines, proceeds in good yields. Nickelole IIIa displays physical and chemical properties consistent with its structure and is a catalyst for the trimerization of diphenylacetylene. Nickelole IIIb is a highly associated structure but in its chemical response to alkynes, HOAc, O2, Br2, NaAlEt2H2 and heat displays the properties of a nickelole, rather than a cyclobutadienenickel(0) complex. Attempts to generate IIIb photochemically from η4-1,5-cyclooctadiene(η4-tetraphenylcyclopentadienone)nickel and diphos failed, but it was shown that structural types, such as η4-tetraphenyl-cyclopentadienone(diphos)nickel (a model for the structure suggested by Hoberg and Richter for IIIb), are unstable. Oligomerizations of diphenylacetylene by bis(1,5-cyclooctadiene)nickel were retarded by conducting the reaction in THF or in the presence of diphos. This retardation permitted the interception of products (cis-stilbene and (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene) diagnostic for the intermediacy of nickelirenes and nickeloles. Deuterium labeling verified the presence of carbon-nickel bonds. These trapping experiments, together with findings on the thermal behavior of nickeloles, are combined into a comprehensive view of the cyclotrimerization, cyclotetramerization and linear polymerization of alkynes by nickel(0).

Stereospecific preparation of 1,2-epoxyalkyllithium reagents via the generalized lithiation of a-heterosubstituted epoxides1

Abstract The alkyldesulfonylation of acetylenic and vinylic sulfones by organolithium and organomagnesium reagents is shown to take place via alkyl radicals arising from electron-transfer processes.

Degenerate and nondegenerate rearrangements of the 7-borabicyclo[..1]heptadiene system: Twofold [1,3] supra-facial sigmatropic migrations and anionic [1,] carbon-boron shifts☆

Abstract A variety of α-heterosubstituted epoxides bearing triphenylsilyl, trimethylsilyl, diethoxyphosphinyl, phenylsulfonyl, phenyl, ethoxycarbonyl and cyano groups was found to undergo stereospecific α-lithiation by use of such bases as n -butyllithium, t -butyllithium or lithium diisopropylamide in solvent combinations of hexane with THF, ethyl ether or TMEDA at temperatures ranging from −78° to −110°C. The formation and stereochemistry of the resulting1,2-epoxyalkyllithium reagents were ascertained by quenching with deuterium oxide, methyl iodide or chloro(trimethyl)silane and analyzing the NMR spectra of the products isolated in 50–100% yields.

The role of nickelole intermediates in the oligomerization of alkynes

Abstract Two notable skeletal rearrangements of the 7-borabicyclo[..1]heptadiene system (III) have been uncovered: (1) a facile, degenerate, net twofold [1,] suprafacial sigmatropic migration of the 7-substituted boro group, observable by NMR spectroscopy when the carbon centers of the borole (I) and alkyne (II) bear suitable groups; and () a nondegenerate, anionic [1,] aryl shift from carbon to boron, converting the 7,7-dimethylborate salt of III into an aryl(dimethyl)pentaarylborate salt. Thus, the degenerate rearrangement was revealed when pentaphenylborole (Ia) was allowed to react with di- p -tolylacetylene (IIb) at 5°C; the resulting 7-borabicyclo[..1] heptadiene (III) formed displayed NMR methyl signals characteristic of p -tolyl groups located at both C 5 and C 6 , as well as C 4 and C 5 . Similarly, the nondegenerate, disruptive anionic isomerization of the 7,7-dimethylborate salt of IIIe or IVe was found to involve migration of either phenyl or p -tolyl groups from carbon to boron, showing that both phenyl (IIIe) and p -tolyl groups (IVe) were located at the bridgeheads in the precursors to VI.

Rearrangements of organometallic compounds : XVII. Synthesis of lactones via the titanium-catalyzed hydromagnesiation of alkenols

Abstract Bis(triphenylphosphine)nickel(II) chloride reacts with E,E, -1,4-dilithio-1,2,3,4-tetraphenylbutadiene to yield a solution of a 2,3,4,5-tetraphenylnickelole complex. This compound reacts promptly with CO to yield tetracyclone, with dimethyl acetylenedicarboxylate to form dimethyl tetraphenylphthalate and catalytically with diphenylacetylene to form hexaphenylbenzene. A similar treatment of (1,2-bis(diphenylphosphino)ethane)nickel(II) chloride with the lithium reagent led to the isolation of (1,2-bis(diphenylphosphino)ethane)bis(2,3,4,5-tetraphenylnickelole), which likewise reacts with dimethyl acetylenedicarboxylate to yield dimethyl tetraphenylphthalate. These results support the interpretation that nickeloles are reactive intermediates in the cyclotrimerization of alkynes by nickel(0) catalysts.

Selective alkali metal and hydrogen reduction of benzene to cyclohexene

Abstract An efficient, two-step synthesis is presented for preparing γ- and δ-lactones from aldehydes or ketones. (1) the addition of vinyl- or allyl-Grignard reagents to the appropriate carbonyl substrate; and (2) the titanium-catalyzed hydromagnesiation of the resulting alkenols with ethyl Grignard reagent and (η 5 -C 5 H 5 ) 2 TiCl 2 , followed by carbonation. The selectivity of hydrometallation observed with 3-butenyl(methyl)vinylcarbinol indicates the importance of alkoxytitanium hydrides in determining the course of reaction.

Rearrangements of organometallic compounds. XIII. Boraaromatic systems. IV. Synthesis of heptaphenylborepin via the thermal rearrangement of heptaphenyl-7-borabicyclo[2.2.1]heptadiene

Les N-methyl polyamines lineaires et ramifiees sont capables de promouvoir la reduction des aromatiques