Synthesis of Hypervalent Pentavalent Carbon and Boron Compounds Yohsuke Yamamotoa and Kin-ya Akibab a Department of Chemistry, Graduate Schoolof Science,Hiroshima University, b Advanced ResearchCenter for Scienceand Engineering ,Waseda University, ReceivedAugust 9, 2004 Abstract:Various carbon and boron compoundsbearing a 1,8-disubstitutedanthracene skeleton were syn- thesizedand characterizedby X-ray analysis.They showedthree typesof structuresbased on the kinds of substituents.The first one is symmetricaland is a loosepentacoordinate structure, which has the sp2carbon or boron atom and the two weak apicalinteractions. The next one is an unsymmetricaltetracoordinate structure,which has the sp3 central atom. The last one is symmetricaland is a tight pentacoordinatestruc- ture, which resultedfrom the specialfeature of the fluorine substituentson the boron. The existenceof hypervalentinteraction was provedby the Atoms In MoleculesTheory, experimental electron distribution analysisand a comparisonamong the structuresof tight and loosepentacoordinate species. The synthesis and structuresof hypervalentcarbon compoundswith a 2,6-bis(aryloxymethyl)benzeneligand are also dis- cussed. mation of a sulfonium salt, where one of the phenylthio I. Introduction groups stays unaffected. They concluded that the desired Pentacoordinate carbon compounds should be classified pentacoordinate carbon species should be the transition state into electron-deficient and electron-rich species based on the in the 'bell-clapper' (bond-switching) rearrangement between number of formal valence electrons around the central car- the two sulfonium salts. Forbus and Martin claimed the syn- bon. Electron-deficient pentacoordinate carbon compounds thesis and direct observation of a pentacoordinate carbon such as the methonium ion (CH5+),1 transition-metal com- species using the dication of the 1,8-bis(tolylthio)anthracenyl plexes bearing C-H agostic interaction or a bridging alkyl ligand, where the carbocation at C-9 is contained in a ben- group,2 carboranes,3 and a carbon atom in a metal cluster zene ring bearing the methoxy group at the 2•Œ,6•Œ-positions.8 cage4 have eight electrons formally assigned around the cen- The 13C NMR chemical shift of the central carbon (ƒÂ 109 tral carbon, and the pentacoordinate structure is made possi- ppm; dication) was shifted to higher-field from the reduced ble by the interaction between a sigma bond (2 center 2 elec- electronically neutral anthracene compound (ƒÂ 118 ppm). tron bond: 2c-2e) with a vacant orbital of proton and so Although they carefully tried to prove the existence of penta- forth. In contrast, electron-rich pentacoordinate carbon coordinate carbon compounds with NMR, which showed a species such as the transition states of the bimolecular nucle- symmetrical spectrum reasonable as a pentacoordinate ophilic substitution (SN2) reaction have ten electrons formally species in solution, the compound had not been character- assigned around the central carbon atom based on an inter- ized by X-ray analysis. In 1984, Lee and Martin reported action of a vacant 2p orbital of the central carbon atom with direct observation of hypervalent penta- (ƒÂB -20 ppm) and two lone-pair electrons (4 electrons) or an interaction hexacoordinate (ƒÂB -123 ppm) boron compounds with the between a vacant C-X ƒÐ* orbital and a lone pair of a nucle- use of "B NMR.9 The "B NMR chemical shift ((ƒÂB -123 ophile. The interaction leads to the formation of a linear 3 ppm) was the highest value ever observed for any known center 4 electron (3c-4e) "hypervalent"5 bonding system; boron compound with only first-row elements bonded to the thus, they are called hypervalent compounds. central boron atom. Then, the Hojo group attempted to pre- Theoretical calculations showed that the transition state pare a pentacoordinate carbon compound by stabilizing the of the SN2 reaction should be D3h trigonal bipyramidal carbocation bonded at the C-1 of fluorene by coordination (TBP). Due to the fundamental importance of SN2, there of the two methylthio groups at C-9.10 However, in the solid have been a variety of efforts to stabilize the TS and even to state the two C+-S bond distances were different, with the prepare model compounds of TS. Breslow undertook to shorter one (1.94A) close to that of a covalent C-S bond solve the problem using trityl cation derivatives bearing some (1.81 A), forming a sulfonium salt.11 Kudo detected CLi6, o-methylthiomethyl substituents.6 The compound came out which was proposed by the Schleyer group,12 by Knudsen- as sulfonium salts, and no positive evidence for the presence effusion mass spectrometry.13 However, the details of the of pentacoordinate carbon compounds could be found. bonding nature of CLi5 and CLi6 have not yet been clarified. Martin and Basalay attempted to stabilize the pentacoordi- Recently we reported the synthesis and the crystal struc- nate carbon species by the 1,8-bis(phenylthio)anthracenyl lig- ture of 1,8-dimethoxy-9-dimethoxymethylanthracene mono- and.7 At room temperature, the peak of two methyl groups cation (1) as the first fully characterized model compound for on the carbocation at C-9 was equivalently observed. When the SN2 transition state.14a In addition, we reported the syn- the temperature was lowered, two singlet peaks of the two theses and X-ray structures of several carbon and boron 14b-e methyl groups were separately observed because of the for- compounds (2) bearing a newly synthesized rigid tridentate 1128 (62) J. Synth . Org . Chem ., Jpn . anthracene ligand (Scheme 1). Here we summarize our recent ried out with lithium 4,4' -di-tert-butylbiphenylide (LDBB) research, especially on the structures of the carbon and the followed by the reaction with BrCF2CF2Br. Since the reac- corresponding boron compounds with several newly synthe- tion gave 4 in up to 30% yield even under careful reaction sized tridentate ligands. The bonding nature between the conditions, we recently improved the route via a 9-trifluo- central carbon (or boron) and the two coordinating atoms is roacetoxy intermediate. The yield of 4 was improved to 51%. elucidated and discussed based on the several X-ray struc- The synthesis of the boron compounds (2) is straightforward tures with several different substituents, theoretical DFT cal- as illustrated in Scheme 1. culations, and the accurate experimental X-ray electron-den- For the synthesis of 9-bromo-1,8-bis(dimethylamino)- sity distribution analysis. anthracene (8) a totally different route was necessary.41d Bro- mide substitution15 of commercially available dichloroan- 2. Syntheses of Ligand Precursors Bearing the Anthracene thraquinone followed by reductionl 6 afforded dibromoan- Skeleton and the Corresponding Carbon and Boron Compounds throne. Deprotonation followed by methylation gave 1,8- dibromo-9-methoxyanthracene (9) in 58% yield. The Pd(0)- The synthesis of ligand precursors, 1,8-dimethoxy-9-tri- mediated coupling reaction with various nucleophiles fluoromethanesulfonyloxyanthracene (3) and 9-bromo-1,8- (Bu3SnNMe2, Me3SnNMe2, LiNMe2 and HNMe2) did not dimethoxyanthracene (4), is illustrated in Scheme 1. The give the expected 1,8-bis(dimethylamino)-9-methoxyan- introduction of a carbon atom at the 9-position was success- thracene (10); instead, only a mono-dimethylaminated prod- ful from 1,8-dimethoxy-9-trifluoromethanesulfonyloxyan- uct was obtained in most cases, and reduction of the C-Br thracene (3) by Pd-catalyzed CO insertion with DMSO as a bonds took place in some cases. However, 9 could be convert- solvent and Pd(PPh3)4 as a catalyst in 52% yield.14a The ed to 10 in 79% yield by heating a HNMe2-THF solution of expected cation (1) could be prepared by treatment of the 9 up to 150•Ž in a pressure-resistant vessel with a Ni(0)-cat- ester (5) with excess Me3O+BF4-. However, for introduction alyst, which is a modified Buchwald method.17 LDBB (lithi- of a boron atom, 9-bromoanthracene precursor (4) had to be um di-tert-butylbiphenylide) reduction of the methoxy group synthesized.14b 9-Bromo-1,8-dimethoxyanthracene 4 was at the 9-position of the anthracene skeleton worked well for synthesized by a selective C-O bond cleavage reaction from 10 to afford a novel anthracene ligand precursor, 9-bromo- 9-trifluoroacetoxy compound (6) as a key step (Scheme 1). 1,8-bis(dimethylamino)anthracene (8), in 51% yield after Initially, several reduction conditions were examined for reaction with BrCF2CF2Br. 9-phosphatel4b with a couple of one electron-reducing To synthesize an anionic hypervalent pentacoordinate reagents (K/NH3, lithium naphthalenide, etc.). The reduction carbon compound, we designed a novel ligand bearing two of the phosphate under K/NH3 (Birch reduction condition) deprotectable methoxymethoxy groups as a trianion equiva- gave a 9-protonated product, which should be generated by lent. Synthesis of 9-bromo-1,8-bis(methoxymethoxy)- the reaction of the 9-potassium derivative with ammonia. anthracene (12) was achieved with stepwise oxygenation with To prevent the trapping of a proton, THE was chosen as an the oxaziridine reagent18 and gaseous O2 as illustrated in aprotic solvent, and the reduction of the phosphate was car- Scheme 3.14e Scheme 1. Synthesis of new anthracene ligand precursors (4 and 5) bearing two oxygen atoms at 1,8-positions and their application to synthesis of hypervalent carbon (1) and boron (2) compounds 3 5 1 6 4 2 Vol.62 No.11 2004 (63) 1129 Scheme 2. Synthesis of a new anthracene ligand precursor (8) bearing two nitrogen atoms at 1,8-positions and its application for synthesis of boron compounds (7) 9 10 8 7 Scheme 3. Synthesis of new deprotectable ligand 12 and the anionic species (11) 12 13 11 After quantitative regeneration of the lithium derivative the plane of the anthracene, one of the lone pairs of each by the reaction of 12 with one equivalent of n-BuLi in ether, oxygen atom should be directed toward the empty p-orbital 9-lithio-1,8-bis(methoxymethoxy)anthracene was reacted of the central carbocation at the 9-position and the other with gaseous hexafluoroacetone to give the adduct alcohol.