Studies into the mechanism of CO-induced N2 cleavage promoted by an ansa-hafnocene complex and C-C bond formation from an observed intermediate

Carbonylation of the hafnocene dinitrogen complex, [Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-(t)Bu)Hf](2)(μ(2), η(2), η(2)-N(2)), yields the corresponding hafnocene oxamidide compound, arising from N(2) cleavage with concomitant C-C and C-N bond formation. Monitoring the addition of 4 atm of CO by NMR spectroscopy allowed observation of an intermediate hafnocene complex with terminal and bridging isocyanates and a terminal carbonyl. (13)C labeling studies revealed that the carbonyl is the most substitutionally labile ligand in the intermediate and that N-C bond formation in the bridging isocyanate is reversible. No exchange was observed with the terminal isocyanate. Kinetic data established that the conversion of the intermediate to the hafnocene oxamidide was not appreciably inhibited by carbon monoxide and support a pathway involving rate-determining C-C coupling of the isocyanate ligands. Addition of methyl iodide to the intermediate hafnocene resulted in additional carbon-carbon bond formation arising from CO homologation following nitrogen methylation. Similar reactivity with (t)BuNCO was observed where C-C coupling occurred upon cycloaddition of the heterocumulene. By contrast, treatment of the intermediate hafnocene with CO(2) resulted in formation of a μ-oxo hafnocene with two terminal isocyanate ligands.     

Once cleaved C-C bond was reformed: reversible C-C bond cleavage of dihydroindenyltitanium complexes

The once cleaved carbon-carbon bond of the Cp moiety in 2 was recombined in indene products. Aslo, we propose a novel mechanism for the cleavage of the carbon-carbon bond of the Cp moiety.     

Mechanistic insight into the cleavage of an aromatic C-C bond by tungsten

The pathway for the cleavage of an aromatic C-C bond in quinoxaline by a tungsten(II) complex [W(PMe(3))(4)(η(2)-CH(2)PMe(2))H] is explored by performing detailed DFT calculations. The real active complex was found to be [W(PMe(3))(2)(η(2)-CH(2)PMe(2))H] rather than [W(PMe(3))(4)]. The key step in the whole reaction is the reductive elimination of two hydrides that are located originally on quinoxaline (see scheme).     

Iron-catalyzed C-H and C-C bond cleavage: a direct approach to amides from simple hydrocarbons

Something functional: The title reaction proceeds in the presence of azide and water to deliver amides in high yields, and it can be used in a ring-expansion strategy to generate lactams. A mechanism is proposed based on experimental results. This reaction offers a new approach to functionalizing simple and readily available hydrocarbons. DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.     

Catalytic C-C bond cleavage and C-Si bond formation in the reaction of RCN with Et3SiH promoted by an iron complex

Catalytic C-C bond cleavage of acetonitrile and C-Si bond formation have been attained in the photoreaction of MeCN with Et3SiH in the presence of an iron complex, Cp(CO)2FeMe. This catalytic system can be applied for arylnitrile C-C bond cleavage.     

Iron-catalyzed C2-C3 bond cleavage of phenylpyruvate with O2: insight into aliphatic C-C bond-cleaving dioxygenases

Iron(II)-phenylpyruvate complexes of tetradentate tris(6-methyl-2-pyridylmethyl)amine (6-Me3-TPA) and tridentate benzyl bis(2-quinolinylmethyl)amine (Bn-BQA) were prepared to gain insight into C-C bond cleavage catalyzed by dioxygenase enzymes. The complexes we have prepared and characterized are [Fe(6-Me3-tpa)(prv)][BPh4] (1), [Fe2(6-Me3-tpa)2(pp)][(BPh4)2] (2), and [Fe2(6-Me3-tpa)2(2'-NO2-pp)][(BPh4)2] (3), [Fe(6-Me3-tpa)(pp-Me)][BPh4] (4), [Fe(6-Me3-tpa)(CN-pp-Et)][BPh4] (5), and [Fe(Bn-bqa)(pp)] (8), in which PRV is pyruvate, PP is the enolate form of phenylpyruvate, 2'-NO2-PP is the enolate form of 2'-nitrophenylpyruvate, PP-Me is the enolate form of methyl phenylpyruvate, and CN-PP-Et is the enolate form of ethyl-3-cyanophenylpyruvate. The structures of mononuclear complexes 1 and 5 were determined by single-crystal X-ray diffraction. Both the PRV ligand in 1 and the CN-PP-Et ligand in 5 bind to the iron(II) center in a bidentate manner and form 5-membered chelate rings, but the alpha-keto moiety is in the enolate form in 5 with concomitant loss of a C-H(beta) proton. The PP ligands of 2, 3, 4, and 8 react with dioxygen to form benzaldehyde and oxalate products, which indicates that the C2-C3 PP bond is cleaved, in contrast to cleavage of the C1-C2 bond previously observed for complexes that do not contain alpha-ketocarboxylate ligands in the enolate form. These reactions serve as models for metal-containing dioxygenase enzymes that catalyze the cleavage of aliphatic C-C bonds.     

Ring opening of methylenecycloalkenes via the C-C bond cleavage

3-Methylenecycloalkene-1,1-dicarboxylates underwent facile ring opening via the cleavage of the unactivated C-C bond to yield aliphatic alkenes. The reaction (intramolecular deallylation) was catalyzed by "Ni-H" species generated in situ from a NiX2(phosphine) n/R3Al mixture and proceeded under ambient conditions. In addition, the skeletal rearrangement of a diene could be carried out by a one-pot cycloisomerization/deallylation sequence.     

Mn-promoted aerobic oxidative C-C bond cleavage of aldehydes with dioxygen activation: a simple synthetic approach to formamides

A novel Mn-promoted aerobic oxidative C-C bond cleavage of aldehydes with dioxygen activation has been developed. The usage of molecular oxygen (1 atm) as oxidant, reactant, and an initiator to trigger this transformation makes this transformation very green and practical. A plausible radical process is proposed on the basis of mechanistic studies. Furthermore, this method provides a practical, neutral, and mild synthetic approach to formamides, which are important units in biologically active molecules.     

Electrochemical cleavage of a benzylic C-C bond in arylaliphatic compounds

Electrochemical cleavage of a benzylic C-C bond in arylaliphatic compounds and the effect of the structure of their alkyl and aryl fragments on the process are studied. Cleavage was found to be the most effective for substituted benzenes and anisoles with side chains bearing vicinal methoxy- and hydroxy-groups in the a- and -positions. Cleavage was moderate for cior -methoxy(hydroxy)- and (,-dialkoxyalkyl)arenes. Electrolysis carried out in methanol results in formation of PhCH₂OMe and PhCH(OMe)₂ from PhCH₂R and PhCH(OMe)R, benzaldehyde from (1,2-dihydroxypropyl)benzene, acetophenone from 2-phenylhexan-2-ol, 4-MeOC₆H₄CH(OMe)₂ from 4-(1,2-dimethoxypropyl)anisole, and 2-(MeO)₂CHC₆H₄CH(OMe)₂ from 1,2-dimethoxyindan.Translated fromIzvestiya Akademii Nauk, Seriya KMmicheskaya, No. 1, pp. 140–143, January, 1993.     

A simple procedure for C-C bond cleavage of aromatic and aliphatic epoxides with aqueous sodium periodate under ambient conditions

The sodium periodate mediated oxidative cleavage of the C-C bond of 12 epoxides is reported with yields of the corresponding carbonyl compounds in up to 91%. This is a two-step reaction that proceeds through a rate-limiting epoxide opening to a vicinal diol that is cleaved in situ to the corresponding carbonyl compound. This method serves as a chemoselective alternative to ozonolysis.     

Recent advances for the photoinduced C-C bond cleavage of cycloketone oximes

Nitriles are widely existed in many bioactive compounds, and they can be easily transformed into other functional groups. Therefore, the synthesis of nitriles under cyanide-free conditions is of significant importance. Recent advances for the synthesis of nitriles through photoinduced C-C bond cleavage of cycloketone oximes classified by the type of C-X bond forming are summarized. Various compounds possessing nitriles can be efficiently accessed via this method.     

FeCl2-promoted cleavage of the unactivated C-C bond of alkylarenes and polystyrene: direct synthesis of arylamines

Ironing it out: an efficient and convenient nitrogenation strategy involving C-C bond cleavage for the straightforward synthesis of versatile arylamines is presented. Various alkyl azides and alkylarenes, including the common industrial by-product cumene, react using this protocol. Moreover, this method provides a potential strategy for the degradation of polystyrene.     

Oxidative cleavage of the C-C bond in processes involving the heteroaromatization of 9R-sym-nonahydro-10-oxa(chalcogena)anthracenes

The reaction of 9-(2-methoxyphenyl)- and 9-(2-thienyl)-sym-nonahydro-10-selena(thia)anthracenes with trifluoroacetic acid causes their heteroaromatization with the elimination of substituents from the positions of the heterorings. A similar transformation of these compounds, as well as their oxygen and 9-benzyl-substituted analogs, occurs during anode electrochemical oxidation. The stepwise character of the cleavage of the C-C bond, which includes one-electron oxidation of the chalogenapyrans and subsequent fragmentation of the cation-radical intermediates, is substantiated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 900–904, July, 1991.     

Homogeneous catalytic oxidation of light alkanes: C-C bond cleavage under mild conditions

The combined oxidation of CO and C₂–C₄ alkanes (associated petroleum gas and natural gas components) under the action of oxygen in trifluoroacetic acid solutions in the presence of rhodium and copper chlorides was accompanied by the oxidative degradation of C-C bonds in a hydrocarbon chain with the formation of carbonyl compounds, alcohols, and esters. For butane and isobutane, the reaction path with C-C bond cleavage was predominant. The buildup curves of isobutane oxidation products (both with the retention and with the degradation of the chain) were S-shaped and characterized by the same induction period; they did not pass through a maximum. A reaction scheme was proposed to reflect the main special features of the mechanism of transformations occurring in the O₂/Rh/Cu/Cl⁻ oxidation system.     

C-C bond insertion of a complexed phosphinidene into 1,6-methano[10]annulene

Reaction of electrophilic phosphinidene complex [MePW(CO)5] with 1,6-methano-[10]annulene results in the sole formation of the isomeric C-C insertion products 6 c (main) and 6 d (minor). The single-crystal X-ray structure of the complexed 1,7-methano-3-phospha[11]annulene (6 c) shows a syn-W(CO)5 group at the exo bent phosphorus. The structure displays C-C bond alternation without bonding between the bridgehead carbon atoms. Density functional theory calculations indicate 6 c to result from a concerted disrotatory ring opening of an undetected tricyclic exo-syn phosphirane intermediate. The endo-anti phosphirane cannot undergo ring expansion, due to the high barrier that is associated with an intramolecular antara-antara retro Diels-Alder reaction. The stabilizing effect of transition-metal coordination is discussed.     

Reaction of a double bond of zirconacyclopentadienes: formation of 1,2,3,5-tetrasubstituted benzenes via the C-C bond cleavage.

Reactions of zirconacyclopentadienes with dichlorocarbene afforded cyclopropanation products in good yields. In a similar manner, reactions of zirconacyclopentadienes with a Simmons-Smith type cyclopropanation reagent also gave vinylcyclopropanes in good yields after hydrolysis. Before hydrolysis, the formation of 1-zirconabicyclo[3.1.0]hexene was observed by 13C NMR spectroscopy. The 1-zirconabicyclo[3.1.0]hexene deriatives reacted with CO to produce 1,2,3,5-tetrasubstituted benzene derivatives in good yields via the C-C bond cleavage of the cyclopropane rings. This is in sharp contrast to the formation of usual 1,2,3,4-tetrasubstituted benzene derivatives from zirconacyclopentadienes.     

C-C bond formation via C-H bond activation: synthesis of the core of teleocidin B4

The core of teleocidin B4, a complex fragment of a natural product containing two quaternary stereocenters and a penta-substituted benzene ring, was synthesized in four C-C bond-forming steps starting from tert-butyl derivative 1. The first step involved alkenylation of the tert-butyl group with a vinyl boronic acid, followed by the successful annulation of the cyclohexane ring to the benzene nucleus via an intramolecular Friedel-Crafts reaction. The third step required a diastereoselective oxidative carbonylation of the geminal dimethyl group, followed at last by indole assembly via the alkenylation of the phenol nucleus, to afford the teleocidin B4 core. Noteworthy is the fact that steps 1 and 3 critically depended on the directing role of the aniline nitrogen (directed C-H bond functionalization).