Metalloenediynes: Ligand field control of thermal Bergman cyclization reactions

Citation
Pj. Benites et al., Metalloenediynes: Ligand field control of thermal Bergman cyclization reactions, J AM CHEM S, 122(30), 2000, pp. 7208-7217
Citations number
84
Categorie Soggetti
Chemistry & Analysis",Chemistry
Journal title
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
ISSN journal
00027863 → ACNP
Volume
122
Issue
30
Year of publication
2000
Pages
7208 - 7217
Database
ISI
SICI code
0002-7863(20000802)122:30<7208:MLFCOT>2.0.ZU;2-7
Abstract
We report the preparation and thermal reactivities of unique Cu(I) and Cu(I I) metalloenediyne complexes of the flexible 1,8-bis(pyridine-3-oxy)oct-4-e ne-2,6-diyne ligand (bpod, 1). The thermal reactivities of these metalloene diynes are intimately modulated by metal oxidation state. Using differentia l scanning calorimetry (DSC), we demonstrate that the [Cu(bpod)(2)](+) comp lex (2) undergoes Bergman cyclization at 203 degrees C, whereas the Cu(II) analogue (3) is substantially more reactive and cyclizes at 121 OC. Similar results are also observed for mixed ligand [Cu(bpod)(pyridine)2](+/2+) ana logues 4 (194 degrees C) and 5 (116 degrees C), suggesting that both comple xes of a given oxidation state have comparable structures. The Cu(bpod)Cl-2 compound (6) exhibits a cyclization temperature (152 degrees C) midway bet ween the those of Cu(I) and Cu(II) complexes, which can be explained by the propensity for cis-CuN2Cl2 structures to exhibit dihedral angle distortion . The oxidation-state-dependent thermal reactivity is unprecedented and ref lects the influence of the ligand field geometry on the barrier to enediyne cyclization. On the basis of X-ray structures of Cu(pyridine)lt complexes, 2 and 4 are proposed to be tetrahedral. In contrast, the electronic absorp tion spectra of 3 and 5 each show a broad envelope that can be Gaussian res olved into three ligand field transitions characteristic of a Cu(II) center in a tetragonal octahedral environment. This structural assignment is conf irmed by the EPR spin Hamiltonian parameters (g(parallel to)/A(parallel to) (cm) = 134 (3), 138 (5)) and is consistent with crystallographically chara cterized Cu(pyridine)(4)X-2 structures. Molecular mechanics calculations ha ve independently derived comparable tetrahedral and tetragonal structures f or 2 and 3, respectively, and determined the average alkyne termini separat ion to be [a] = 4.0 Angstrom for 2 and 3.6 Angstrom for 3. Thus, the tetrah edral geometries of the copper centers in 2 and 4 increase the distance bet ween alkyne termini relative to the tetragonal Cu(II) geometries of 3 and 5 , and are therefore responsible for the increase in the thermal cyclization temperatures. The DSC and spectroscopic data for 6 support these conclusio ns, as the latter suggests a distorted four-coordinate structure in the sol id state, and a six-coordinate geometry in solution, which gives rise to an intermediate Bergman cyclization temperature. Overall, our results emphasi ze the utility of newly emerging metalloenediyne complexes for controlling thermal Bergman cyclization reactions and provide insights into designing n ovel, pharmacologically useful metalloenediyne compounds.