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Downloaded by TUFTS UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch006
Metal-Mediated Making and Breaking of Carbon-Carbon Bonds in Aromatic Hydrocarbons John J. Eisch Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13901
The interaction of zero- or low-valent lithium and nickel reagents with unsaturated hydrocarbons of the alkyne or arene type has been explored as a method of making carbon-carbon bonds in polynuclear aromatic hydrocarbons. Such reactions have been shown to proceed via two principal types of intermediates: (1) radical-anions, which are paramagnetic systems resulting from single-electron transfer; and (2)π-complexesor metallocycles, which are diamagnetic systems aris ing from a net oxidative addition or electron-pair transfer. The en ergetics of such metal-unsaturated substrate interactions is analyzed in terms of a Born-Haber cycle. Examples of useful carbon-carbon bond formations studied in this work are (1) the oligomerization of alkynes, (2) the cyclization of aromatic hydrocarbons, (3) the desulfurization of sulfur heterocycles, and (4) the dimerization of cyclobutadienoid systems.
THE INTERACTION OF ZERO- OR LOW-VALENT METALS with aromatic nuclei is a reaction of great generality encompassing the alkali-metal adducts re ported by Schlenk and Bergmann (1) in 1928 as well as arene-chromium complexes elucidated by Fischer and Hafher (2) and Zeiss and Herwig (3) in 1956. Not only can arenes interact with metals, but alkynes can be cyclotrimerized to arenes by using metal catalysts. The trimerization of alkynes to benzenes (2) by (Ph P) (CO) Ni (Ph denotes phenyl) is shown in Scheme I (4). Because of the thermodynamic stability of the aromatic ττ-electron configuration, the driving force for such oligomerization of alkynes is readily 3
2
2
0065-2393/88/0217-0089$06.00/0 © 1988 American Chemical Society
In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
POLYNUCLEAR AROMATIC COMPOUNDS
Downloaded by TUFTS UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch006
90
comprehensible. However, the similar cyclotetramerization of certain al kynes to the nonaromatic cyclooctatetranes (3) by nickel(II) cyanide or nickel(O) complexes is, in contrast, astonishing (4). Yet, a closer study of the behavior of low-valent transition metals, such as nickel, and of alkali metals, such as lithium, reveals that metals are able to disrupt aromatic systems almost as readily as they can induce their for mation. The possibility of full (Li) or partial (Ni) electron transfer from a metal to a sufficiently low-lying antibonding ir-molecular orbital of the re agent hydrocarbon is basic to both the formation and disruption of an aromatic nucleus by a low-valent metal reagent, M(0). For oligomerizations of alkynes (e.g., Scheme I), two extreme types of intermediates (4 and 5 in Scheme II) can be generated by a one- or two-electron transfer: metal-radical anion pairs (4) or metal-dianion pairs (5). In a less extreme extent of electron transfer, 4 and 5 may be better represented as covalent ττ-complexes 6 and 7 (Scheme II). The ultimate products arising from intermediates 3 and 4 can be produced either catalytically from transition metals or stoichiometrically from main-group metals.
(R-C=C-R) M2+
2-
R-C=C-R
M
°
•»
(R-C=*C-R)~
1
5
4
ι
R-C=C-R
R-C===C-R
V
ft 6
Scheme II The extent of electron transfer between an organic substrate and a metal is a complex function of the metal's sublimation energy (Si) and ionization
In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
6.
EISCH
91
Metal-Mediated Making and Breaking of C-C Bonds
potential (I), as well as the organic substrate's sublimation energy (S ) and electron affinity (£) (5). In terms of Gibbs free energy, one can analyze the free-energy solution potential (P) into the foregoing energy inputs and note that energy-releasing terms, namely, ion solvation (H and H ) and ion association (A), are necessary to yield a negative P. Such a Born-Haber treatment for the solution potential of a metal-hydrocarbon adduct is shown in Figure 1. This figure shows how crucial the role of the donor solvent is in solvating alkali-metal cations in such adduets. Such metal-cation solvation is the principal free-energy compensation (-G) for the positive S χ and J investments. In the case of transition metals, such energy compensation can also be provided by donor ligands on the metal center, such as phosphines (H ), or from the association of partially charged metal and organic ligand centers (A). This association can be considered as the back bonding invoked in transition-metal ττ-complexes. A n important extension of such electron transfer from a metal center to a hydrocarbon substrate is that the metal need not be in the zero-valent state. Rundle (6) pointed out that an electronic kinship exists between metal lattices and organometallic compounds: Both systems are electron-deficient; that is, they have too few valence electrons to fill the available low-energy 2
Downloaded by TUFTS UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch006
x
2
x
R
Mg
9
-P
-P =
donor solvent
S + I + Ε - Η - H- A 2
Λ
2
Figure 1. Born-Haber cycle for the formation of the carbon-metal bond. Subscripts: d, dissolved; s, solid.
In Polynuclear Aromatic Compounds; Ebert, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.
92
POLYNUCLEAR AROMATIC COMPOUNDS
bonding orbitals. The resulting electron-deficient systems are therefore ex pected to display greater electron derealization, similar to their metal coun terparts. Thus, the associated (RLi) , where R is an alkyl group, should have an enhanced tendency to lose electrons, as does its corresponding cluster, L i . Indeed, a number of instances in which organolithium compounds have reacted by electron transfer have been observed. Especially noteworthy i n this regard is teri-butyllithium (ί-BuLi), which causes the reductive dimer ization of diphenylacetylene (8) in tetrahydrofuran (THF) (7), as shown in Scheme III. The formation of 10 clearly shows that 9 was generated by electron transfer and then underwent dimerization. Compound 10 is also the product formed from the alkyne and lithium metal. n
Downloaded by TUFTS UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch006
n
Ph-C=C-Ph
•
(Ph-C=C-Ph)"Li+
8
9
Phv
/Ph C—C
ι
Ph-C
C-Ph
Li
l(
10 Scheme III
In this and other reactions, terf-butyllithium shows a greater tendency to undergo electron transfer than its isomer, n-butyllithium. n-Butyllithium, for example, does not form 10 from 8 but adds instead across the C = C linkage (7). This behavior suggests that the ionization potential of the carbanionlike R group in R L i is an important determining factor in such electron transfer: M e C ~ L i — ^ M e C- + L i 3
+
3
+
(1)
In this view, the lower ionization potential of the tert-butyl anion in com parison with that of the η-butyl anion is ascribed to the greater stability of the tert-butyl radical (
