The reactions of acetylenes with benzyne−nickel(0)
complexes
Ni((1,2-η)-4,5-X2C6H2)L2
(X = H, L2 = 2PEt3 (1), dcpe
(2); X = F, L2 = 2PEt3
(3), dcpe (4)) to give substituted
naphthalenes have been investigated further in an effort to understand
the factors that
control the successive insertions. Complex 1 reacts
with a mixture of tert-butylacetylene
and 3-hexyne in a 1:3 molar ratio to give approximately equal amounts
of 1,2-diethyl-3-tert-butylnaphthalene (11), the exclusive product
of cocyclization, and 1,2,3,4-tetraethylnaphthalene (10), which results from double insertion of
3-hexyne. Double insertion of 1,7-octadiyne into the benzyne−nickel bond of 1 occurs
exclusively in an intramolecular fashion
to give 1,2,3,4-tetrahydroanthracene (14) in 48% isolated
yield. These observations are
consistent with the following postulates: (1) the first insertion
occurs under steric control
and forms a nickelaindene in which the substituted carbon atom is bound
to nickel; (2) the
second insertion occurs in the nickel−vinyl bond of this species, not
the nickel−aryl bond.
The second insertion is also under steric control, but the
regiospecificities for tert-butylacetylene and 1,7-octadiyne are opposite. Addition of
PEt3 or PPh3 suppresses (though
not completely) the catalytic cyclotrimerization of methyl propiolate
in the presence of 3 in
favor of stoichiometric formation of a mixture of dimethyl
6,7-difluoronaphthalenedicarboxylates, the ratio of the 1,3- and 2,3-isomers, 8 and
9, being 1:3 (PEt3) and 1:4
(PPh3).
For tert-butyl propiolate in the presence of
PPh3, the ratio of 1,3- to 2,3-isomer changes
to
1.6:1. These observations support the idea that for acetylenic
esters both steric and electronic
factors influence the second insertion. The isolated nickelaindene
Ni{C6H4C(CO2Me)C(CO2Me)}(dcpe) (18) reacts with
methyl propiolate, probably under electronic control, to
give
trimethyl 1,2,3-naphthalenetricarboxylate as the only product of
cocyclization.