Diazomethane umpolung atop anthracene: an electrophilic methylene transfer reagent

Formal addition of diazomethane's terminal nitrogen atom to the 9,10-positions of anthracene yields H2CN2A (1, A = C14H10 or anthracene).

Diazomethane is infamous for the dangers associated with its use. 1 Despite its synthetic versatility, diazomethane's high toxicity and propensity to explode should give a chemist pause before committing to its use.In an effort to offer an alternate methylene source using an anthracene-based strategy, [2][3][4][5][6][7][8] we report herein the synthesis and some initial reactivity studies of H 2 CN 2 A (1, A ¼ C 14 H 10 or anthracene), a molecule conceived as a formal adduct between diazomethane and anthracene.An initial survey of the reactivity patterns of 1 has revealed it not to be a simple substitute for diazomethane, instead characterizing it as a unique electrophilic methylene source.Its electrophilicity differentiates 1 from common metal-free methylene transfer reagents such as diazomethane and methylene triphenylphosphorane.
Synthesis of hydrazone 1 proceeded from Carpino's hydrazine H 2 N 2 A upon paraformaldehyde treatment in a biphasic diethyl ether-water mixture, 9,10 providing the target molecule in 74% isolated yield (Scheme 1).An X-ray diffraction study of its structure revealed expected metrical data. 11ydrazone 1 was found to be an air-stable and crystalline solid, easily manipulable and displaying no propensity for detonation upon heating or shock.The solid was found to be volatile by thermogravimetric analysis (TGA), which showed gradual sample evaporation up to 120 C without any discrete mass-loss events that would be expected from its fragmentation into diazomethane and anthracene.Within a sealed capillary, 1 melted without explosion (116-119 C).Aer heating the melt to 140 C, NMR spectroscopic analysis of the resolidied solid showed 74% recovery of 1 with 26% anthracene production.][4][5] Having established 1 to pose a low explosion risk, we were encouraged to proceed to test its reactivity as a methylene synthon.Our initial investigations rapidly uncovered contrasting reactivity patterns vis-à-vis those characteristic of diazomethane.For example, methylation of carboxylic acids, a hallmark of diazomethane reactivity, 12 did not proceed upon treatment with excess pivalic acid, acetic acid, or triuoroacetic acid.These experiments were informative, and led us to consider more closely the electronic structure of 1.
Hydrazones are known to be carbon ambiphiles; 13 however, 1 did not demonstrate nucleophilicity.Such behavior is not unexpected, as the p CN is known to be polarized away from the carbon center, although less so than an imine p CN or a ketone p CO bond. 14The polarization of this bond suggests that 1 should be expected to exhibit moderate electrophilicity at its methylene carbon.This would effectively induce umpolung of the diazomethane unit as diazomethane generally reacts as a carbon nucleophile. 15he predicted reversal of philicity was initially conrmed by successful methylene transfer in the reaction between 1 and H 2 CPPh 3 .Combination of these two reagents in benzene-d 6 yielded ethylene in 21% yield over 12 h in concert with anthracene, triphenylphosphine, and, presumably, dinitrogen.The reaction was found to produce several unidentied byproducts by NMR spectroscopy, explaining the low yield of ethylene; however, isotopic labelling of the ylide led to H 2 C] 13 CH 2 from 1 and H 2 13 CPPh 3 , and H 2 C]CD 2 from 1 and D 2 CPPh 3 , conrming ethylene formation through the unication of the electro-and nucleophilic methylene units.Although the yield was low, this mode of reactivity was instructive for our further studies.The electrophilicity of 1 lent itself well to the synthesis of Nheterocyclic olens from N-heterocyclic carbenes (NHCs). 16In benzene-d 6 solution, 1 reacted with nucleophilic IPr (1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene) to yield the corresponding olen in 70% yield aer 13 h at 80 C. 16 As a nucleophile with increased electrophilicity, the Bielawski N,N 0 -diamidocarbene ("DAC") was found to react in essentially quantitative yield to form a new C]C bond over 24 h at 22 C. 17 This mode of reactivity differs markedly from that of diazoalkanes, which have been documented to react with NHCs at their electrophilic N-terminus to produce azines with a new C]N-N]C moiety. 18Heating 1 with triphenylphosphine or tricyclohexylphosphine has not yielded the analogous yieldes, suggesting a modest Lewis acidity at the carbon center of 1.
It is rare for diazomethane to be used in transition metal chemistry for the synthesis of a stable methylidene complex. 19n fact, the use of diazoalkanes in d-block chemistry is oen complicated by their propensity for side reactions other than alkylidene delivery. 20,21The reactivity differences between 1 and diazomethane thus encouraged us to attempt the use of 1 in methylidene complex synthesis to see if engagement of the terminal nitrogen in bonding to anthracene subdues deleterious alternate reaction pathways.
We identied [W(ODipp) 4 ] (2, ODipp ¼ 2,6-diisopropylphenoxide) 22,23 as a d 2 transition metal complex well poised to behave as a methylene acceptor. 24Complex 2 is synthetically easy to access, and its square-planar geometry features a nucleophilic lone pair of electrons housed in a metal-centered d z 2 -like orbital, analogously to related tantalum and molybdenum singlet d 2 species. 8,25Treatment of 2 with excess 1 gave facile formation of the anticipated methylidene complex [2]CH 2 ] aer mild heating in benzene to 55 C for 35 h (Scheme 2).Characteristically deshielded proton and carbon resonances of the CH 2 unit were found by NMR spectroscopy: 1 H d 8.95 ppm and 13 C d 232.9 ppm with scalar coupling constants of 2 J WH ¼ 156.0 Hz, 1 J WC ¼ 185.0 Hz, and 1 J CH ¼ 155.6 Hz.7][28][29] The success of 1 in this capacity was exciting, as the rarity of terminal, isolable methylidene complexes 30 makes new methods for their generation welcome developments.
Crystallization from pentane at À35 C overnight enabled an X-ray diffraction study of [2]CH 2 ] (Fig. 1, le) that conrmed the molecular structure.Although the data were not of high quality, the coordination geometry about the tungsten center was unambiguously identied to be intermediate between square pyramidal and trigonal bipyramidal (s ¼ 0.48), 31 and the alkylidene bond was identied with a W/C interatomic distance of 1.864(4) Å.4][35] Compound [2]CH 2 ] was not found to react productively with ethylene or 1-hexene upon heating to 70 C in benzene-d 6 for 18 h, conrmed by a lack of isotopic migration from [2]  13 CH 2 ] to the olens. 36Under these conditions, [2]CH 2 ] also did not react with mesitaldehyde or 4,4 0 -Scheme 2 Comparative reactivity of W(ODipp) 4 ( 2 dimethylbenzophenone to form [2^O] and the corresponding olens.Despite this, [2]CH 2 ] is notable as an example of a methylidene complex with aryloxides as the exclusive supporting ligands.1][42] Treating a solution of 2 with H 2 CPPh 3 (1 equiv.)An X-ray crystallographic study of [MePPh 3 ][2^CH] revealed a W/C interatomic distance of 1.749(1) Å and a square pyramidal (s ¼ 0.21) coordination geometry about tungsten.A search of the CSD revealed this to be the rst catalogued example of a structurally characterized metal methylidyne in an all-oxygen ligand environment, and the rst catalogued example of a tungsten(VI) methylidyne complex.
As interest in metal methylidene species is rapidly growing both in homogeneous and heterogeneous catalysis, [37][38][39][43][44][45] we hope 1 can be further exploited in their syntheses. Compoud 1 has also shown promise in formation of new C]C bonds with H 2 CPPh 3 and NHCs, and may nd use in construction of terminal olens.
in THF at 25 C rapidly consumed 50% of 2 and formed the methylidyne salt [MePPh 3 ][2^CH].Doubling the amount of H 2 CPPh 3 gave total consumption of 2 and provided [MePPh 3 ][2^CH] in 49% isolated yield (Scheme 2).Variation of the stoichiometry and temperature of this reaction did not lead to conditions for [2]CH 2 ] formation, indicating competitive deprotonation of intermediate [2]CH 2 ] by Brønsted basic H 2 CPPh 3 .Such acid-base chemistry is postulated to play a critical role in the formation of surfacebound alkylidenes and alkylidynes for alkane and olen metathesis, 37,38 meaning [2]CH 2 ] serves also as an interesting reactivity model for alkylidyne synthesis mediated through proton transfer.This was corroborated by independent deprotonation of [2]CH 2 ] with H 2 CPPh 3 , and highlights the utility of 1 as a weakly Brønsted basic source of methylene.Protonation of [MePPh 3 ][2^CH] using lutidinium triate presents a complementary route to [2]CH 2 ].