Mechanistic Features of Boron−Iodine Bond Activation of B−Iodocarboranes

Abstract

Oxidative addition of the B-I bond of 9-iodo-m-carborane to [(Ph 3 P) n Pd] (n ) 3, 4) is reversible, the equilibrium being shifted to the Pd(0) and the iodocarborane. In the presence of [(Ph 3 P) 4 Pd] and [Bu 4 N]Br in THF, 9-iodo-m-carborane undergoes halide exchange to produce 9-bromo-m-carborane. Coordinatively unsaturated Pd(0) and hydrido Pd(II) species generated upon thermal decomposition of [(Ph 3 P) 2 Pd(Ph)(O 2 CH)] and [(Ph 3 P) 2 Pd 2 (Ph) 2 (µ-O 2 CH) 2 ] reduce 9-iodo-m-carborane to m-carborane with 100% selectivity. The thermal decomposition of [(Ph 3 P) 2 Pd 2 (Ph) 2 (µ-O 2 CH) 2 ] in the presence of excess 9-iodo-m-carborane and PhI (1:1) results in the formation of m-carborane (3%) and [(Ph 3 P) 2 Pd 2 (Ph) 2 (µ-I) 2 ] (97%), whose structure was confirmed by single-crystal X-ray diffraction. X-ray analysis of 9-iodom-carborane and m-carboran-9-yl(phenyl)iodonium tetrafluoroborate shows that in the iodonium salt the B-I bond is longer by ca. 0.03 Å than in the iodocarborane. In contrast, the C-I bond distances in carboran-9-yl(phenyl)iodonium tetrafluoroborate (2.111(2) Å) and in iodobenzene (2.098(4) Å) are only marginally different. The elongation of the B-I bond, not the C-I bond, likely contributes to (i) the enhanced reactivity of B-carboranyl(phenyl)iodonium cations toward nucleophiles and (ii) the remarkably high selectivity of these S N reactions that occur exclusively at the boron atom. A new crystallographic form of 9,10diiodo-m-carborane is reported.

Linear homobimetallic palladium complexes

“…Both the Pd1-I2 (2.699(1) Å ) and the Pd1-C19 distances (2.04(1) Å ) ( Fig. 1) agree well with those separations reported for other trans-configurated iodo-palladium-carbon units in which likewise the Pd-I bond is trans to a C-sp 2 donor atom of high trans-influence [11][12][13][14][15][16].…”

“…3). This coordination geometry is typical for molecules containing a (Ph 3 P) 2 Pd entity [14,16]. A comparison with compound 9a shows that the angles between the palladium coordinated aromatic rings and the palladium coordination plane are in the same range (84.7°C 6 H 4 and 68.6°C 4 H 4 N 2 ).…”

When Are Tricoordinated PdIISpecies Accessible? Stability Trends and Mechanistic Consequences

“…[40,41] This experimental evidence suggests that, as suspected, the values of Gibbs energy are overestimated and a more realistic picture should bring them somewhat closer to the DE values.…”

Synthesis and Properties of Linear, Branched, and Cyclic Metallacarborane Oligomers

“…The latter method is elsewhere [7]; here we concentrate on palladium-catalyzed Negishi coupling [8], which in general is assumed to involve formation of square planar palladium complex intermediates. In an exploratory reaction with trimethylphosphine [5], the Pd complex formed from (C 6 H 6 )Fe(Et 2 C 2 B 4 H 3 -5-I) underwent B-P elimination to afford a trimethylphosphonium salt (Scheme 2), in contrast to a recent study of Pd-catalyzed cross-coupling in 1,7-C 2 B 10 H 11 -9-I in which B-I reductive elimination was observed [9].…”