Borataalkenes, boraalkenes, and the η2-B,C coordination mode in coordination chemistry and catalysis

Abstract

Borataalkenes and boraalkenes are the boron-containing isoelectronic analogues of alkenes and vinyl cations respectively. Compared with alkenes, the borataalkene and boraalkene ligand motifs in transition metal coordination chemistry are relatively underexplored. In this review, the synthesis of borataalkene and boraalkene complexes and other transition metal complexes featuring the η²-B,C coordination mode is described. The diversity of coordination modes and geometry in these complexes, and the spectroscopic and structural evidence supporting their assignments is disclosed as well as computational analysis of bonding. The applications of the borataalkene ligand motif in synthetic organic homogeneous catalysis, especially those involving geminal bis(pinacolatoboronates) and 1,4-azaborines, are discussed.

Figure 1

Zeise’s salt and borataalkenes.

Figure 2

Electronic structure and binding modes of borataalkenes and alkenes.

Figure 3

Other types of complexes featuring η²-B,C coordination.

Figure 4

ORTEP illustrations of 124, 125, and 126 with 50% thermal probability ellipsoids. Hydrogens omitted for clarity. Counteranion in 125 omitted for clarity.

Scheme 1

η¹ and η² tantalum borataalkene complexes.

Scheme 2

Synthesis of η¹-C borataalkene Ti(IV) complexes.

Scheme 3

Synthesis of Group 6 η¹-C borataalkene complexes.

Scheme 4

Sadighi’s synthesis of Cu(I) borataalkene complexes.

Scheme 5

Reactions of zinc and copper borataalkene compounds.

Scheme 6

Pd-catalyzed couplings of geminal bis(boronates).

Scheme 7

Cu-catalyzed couplings of geminal bis(boronates).

Scheme 8

Synthesis of Rh(I) borataalkene complex 29 via deprotonation of 27.

Scheme 9

Owen’s synthesis of Rh(I) η²-B,C borataalkene complex.

Scheme 10

Ozerov’s synthesis of Ir(I) η²-B,C borataalkene complex.

Scheme 11

Synthesis of Au(I) borataallene complex via silyl migration.

Scheme 12

Synthesis of Au(I) boryl phosphorus ylide complexes and computational analysis of bonding.

Scheme 13

Synthesis of 9-borataphenanthrene Cr(0) and Au(I) complexes.

Scheme 14

Martin’s synthesis of group 11 borataalkene complexes.

Scheme 15

Structural and calculated parameters for complexes 62, 63, and 56.

Scheme 16

Synthesis of stabilized parent boryl anion 64, and ligation to coinage metals.

Scheme 17

Structural, spectroscopic, and computational analysis of 65–67.

Scheme 18

Bertrand’s borataalkene synthesis via B-H deprotonation strategy.

Scheme 19

Synthesis of Au(I) borataalkene complex 73.

Scheme 20

Azaborataallene complexes.

Scheme 21

CAAC-stabilized diborane 79 as a precursor to borataalkene complexes.

Scheme 22

Deprotonation pathway to boraalkene complexes.

Scheme 23

Boraalkene complexes and selected structural and spectroscopic data.

Scheme 24

Synthesis of 1-borabutadiene complex 92.

Scheme 25

Diborabutadiene complex 95.

Scheme 26

Synthesis and reactivity of 1-borabutadiene complex 97.

Scheme 27

η²-B,C_ipso and η¹-C_ipso interactions in borylmetallocenes a) Synthesis of borylferrocenes. b) Qualitative depictions of selected stabilized MOs from a bent, C_s-symmetric borylferrocene. c) Synthesis of Group 4 borylindenyl complexes.

Scheme 28

Collection of ηⁿ-bound complexes bearing ambiphilic ligands

Scheme 29

Reversible H₂ activation by complex 111.

Scheme 30

Hydrodechlorination of chlorobenzene catalyzed by 116.

Scheme 31

E-X activation by Fe complex 106.

Scheme 32

Synthesis of pyridine-1,4-azaborine platinum(II) complexes.

Scheme 33

Selectivity in palladium-catalyzed 1,3-enyne hydroboration.

Scheme 34

Mechanism of trans-hydroboration of 1,3-enynes.

Scheme 35

trans-Cyanoboration of 1,3-enynes.

Scheme 36

cis-Carboboration of 1,3-enynes.

Scheme 37

trans-Hydroalkynylation of 1,3-enynes.