No abstract

No abstract

The reaction of AlMe3 or ZnMe2 with hppH (hppH=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidine) and then with tBuLi affords [Li8(H)m(hpp)6]n+[X]−n, X=[ZntBu3], m=n=1 (the cation core of which is shown); X=[Li(Me2AltBu2)2], m=0, n=2, as the major product in each case. These data reveal that non‐aromatic heterocycle hppH and ZnMe2 can be employed to generate novel hydride‐encapsulation main‐group‐metal clusters, and that related polylithium architectures can also incorporate a central void.

Aromatic compounds can be elaborated by directed lithiation in a number of ways [1] and both ortho and lateral metallation have been employed as synthetic tools generally [2] and in a host of recent total syntheses specifically. [3,4] Whereas ortho metallation occurs both because the directing group can inductively raise the acidity of the ortho hydrogen atom and also because the incoming organometallic substrate closely approaches this position, lateral metallation results from the directing function coordinating an organometallic substrate whilst conjugatively withdrawing electrons from a benzylic group. Consequently, the processes are competitive and, as such, result from the presence of similar directing agents. Recently, ring substitution and lateral-group branching [5] have been employed, in addition to the use of a-silyl lateral groups, [6] as means of controlling the regioselectivity of deprotonation. The use of deuterium as a protecting group at kinetically acidic positions has been reported both for amides [7] and N-heterocyclic systems. [8] Overall, studies to date have clearly established that, for either class of reaction, amide-type groups are among the most useful directors of reaction.[2]Transformations of ortho-and laterally lithiated tertiary amides have been investigated, [9] with directing effects having been attributed to the rate-determining deprotonation of a substrate-organolithium complex. [10][11][12] However, it is only very recently that solid-state structural evidence has been presented in support of the nature of lithiated intermediates in either reaction pathway. For 1 and 2 ortho lithiation has led to the characterization of solid-state dimers, 3, and isostructural N,N-diisopropyl-2-lithionaphthamide-THF complex, 4, respectively. These are based on core C···Li interactions, support of the metals coming from (amide)OÀLi bonding with concomitant modulation of the sterically induced twist angle between the amide and the plane of the aromatic ring.[13] Contrastingly, the laterally deprotonated salt of 5 reveals a tris(thf) solvate, 6, in which the metal center is only coordinated by O atoms with no C···Li interaction, thus allowing the amide and aromatic planes to be near to perpendicular in the solid state.[14] These data suggest a link between the number of donor atoms per solvent molecule (solvent denticity) and reaction chemoselectivity and lead us to report here on the competitive deprotonation of 2-ethyl-N,N-diisopropyl-1-benzamide, 7.Treatment of 7 in THF/toluene at À78 8C with 1 equivalent of tBuLi gave a maroon solution from which, on storage at À30 8C, crystals were deposited. Surprisingly, in light of previous work, [2,12,15] these were identified as N,N-diisopropyl-2-ethyl-6-lithiobenzamide-THF, 8, by X-ray crystallography. [16] In accordance with our own recent studies, [13] 8 forms a solid-state dimer based on the stabilization of each metal component in a {(CLi) 2 } core (C2ÀLi1 = 2.345(5) , C2A À Li1 = 2.187(5) ] by an amide-O center (O1 À Li1 = 1.972(5) ) and one THF m...

The new very-intense vertical-axis Laue diffractometer (VIVALDI) at the Institut Laue-Langevin has been used to probe the single-crystal structure of [(t-Bu2AlMe2)2Li]-[{Ph(2-C5H4N)N}6HLi8]+, with data proving that the molecular main-group-metal cluster cation component incorporates interstitial hydride. Variations in nitrogen coordination mode and distortion of the metal core are in accord with octahedral hydride coordination (H−Li = 1.92(2), 2.04(2), 2.07(2) Å), two metal centers being nonbonding with respect to hydride (H···Li = 2.86(2) Å).

Dimethylzinc reacts with an excess of N-2-pyridylaniline 6 to give the homoleptic species, Zn[PhN(2-C(5)H(4)N)](2) 8. Single crystal X-ray diffraction reveals a solid-state dimer based on an 8-membered (NCNZn)(2) core motif. Zn[CyN(2-C(5)H(4)N)]Me (Cy =c-C(6)H(11)) 10, prepared by the combination of ZnMe(2) with the corresponding cyclohexyl-substituted pyridylamine, is also dimeric in the solid state but reveals a central (ZnN)(2) metallacycle. Employment of (p-Tol)NH(2-C(5)H(4)N)(p-Tol = 4-MeC(6)H(4)) 11 yielded the tris(zinc) adduct Zn(3)[(p-Tol)N(2-C(5)H(4)N)](4)Me(2) 12, which incorporates a central chiral molecule of 'Zn[(p-Tol)N(2-C(5)H(4)N)](2)' 12a, that bridges two 'Zn[(p-Tol)N(2-C(5)H(4)N)]Me' 12b units. A similar trimetallic structure is noted when the pyridylaniline substrate 11 is replaced with the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH), affording Zn(3)(hpp)(4)Me(2) 13. Spectroscopic studies point to retention of the solid-state structure of in hydrocarbon solution. Reaction of 13 with dimesityl borinic acid, Mes(2)BOH (Mes = mesityl), affords Zn(3)(hpp)(4)(OBMes(2))(2) 14 in which the trimetallic core is retained. This reactivity is in contrast to the closely related reaction of dimeric Zn[Me(2)NC[N(i)Pr](2)]Me 15 with Mes(2)BOH, which yielded Zn[Me(2)NC[N(i)Pr](2)][OBMes(2)].Me(2)NC[N(i)Pr][NH(i)Pr] 16 as a result of protonation at the guanidine ligand in addition to the Zn-Me bond.

The sequential treatment of Lewis acids with N,N'-bidentate ligands and thereafter with ButLi has afforded a series of hydride-encapsulating alkali metal polyhedra. While the use of Me3Al in conjunction with Ph(2-C5H4N)NH gives Ph(2-C5H4N)NAlMe2 and this reacts with MeLi in thf to yield the simple 'ate complex Ph(2-C5H4N)NAlMe3Li.thf, the employment of an organolithium substrate capable of beta-hydride elimination redirects the reaction significantly. Whereas the use of ButLi has previously yielded a main group interstitial hydride in which H- exhibits micro6-coordination, it is shown here that variability in the coordination sphere of the encapsulated hydride may be induced by manipulation of the organic ligand. Reaction of (c-C6H11)(2-C5H4N)NH with Me3Al/ButLi yields [{(c-C6H11)(2-C5H4N)N}6HLi8]+[(But2AlMe2)2Li]-, which is best viewed as incorporating only linear di-coordination of the hydride ion. The guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) in conjunction with Me2Zn/ButLi yields the micro8-hydride [(hpp)6HLi8]+[But3Zn]-.0.5PhMe. Formation of the micro8-hydride [(hpp)6HLi8]+[ButBEt3]- is revealed by employment of the system Et3B/ButLi. A new and potentially versatile route to interstitial hydrides of this class is revealed by synthesis of the mixed borohydride-lithium hydride species [(hpp)6HLi8]+[Et3BH]- and [(hpp)6HLi8]+[(Et3B)2H]- through the direct combination of hppLi with Et3BHLi.

Reaction of 2-ethyl-N,N-diisopropyl-1-naphthamide 3 with Bu t Li in tetrahydrofuran (thf) affords a laterally metallated derivative which exists as a tris(thf) solvated monomer with no Li-C interaction and an sp 2 hybridised carbanionic centre in the solid-state; NMR spectroscopy suggests that this structure is viable in solution but that Li-C bonded atropisomers are also possible and calculations corroborate these data.