Metal boryl bonds

Insertion into metal boryl bonds is represented by two examples described for a late and an early transition metal complexes. Platinum diboryl complexes react with alkynes to give the diborylated alkene (Scheme 6.61, a) [191]. A titanium(II) metallocene with a coordinated ethylene reacts with borane to give, via insertion of the alkene into a M-B bond and subsequent j6-H ehmination, a complex with a coordinated vinylborane (Scheme 6.61, b) [192]. Both processes can be made catalytic. 

Rablen PR, Hartwig JF, Nolan SP. First transition metal-boryl bond energy and quantitation of large differences in sequential bond dissociation energies of boranes. J Am Chem Soc. 1994 116 4121-4122. 

Synthesis, reactivity, and structure of transition metal boryl compounds, derivatives of B,N- and B,0-heterocycles with B—M bond 98CRV2685. 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

This section reviews the developments in the chemistry of monoborane complexes of the transition metals especially borohydride and hydridoborate complexes. Although such complexes are not strictly metallaboranes in the sense that they are not cluster species, they are included here as they share many similarities with polyborane species of the transition metals such as three-center two-electron bonding. Additionally, as will be shown in Section 3.04.3.1 borohydride species can also be intermediates in the formation of larger M By clusters. In this chapter, three-coordinate monoborane species, which are best considered as cr-complexes between a transition metal and HBR2 or metal-boryl (M-B) species, are not considered. 

Protic reagents such as primary amines, alcohols, and water led to the corresponding substituted borylene complexes [n-BX (C5H4Me) Mn(CO)2 2] (13, X = NHtBu 14, X = NHPh 15, X = OMe 16, X = OEt 17, X = Oz Pr 18, X = OH) in high yields of up to 94% (Fig. 3).100 102 It should be noted that due to the kinetic lability of the metal-boron bond such reactions at the boron center with retention of the M-B linkage are very rare for both boryl- and borylene complexes and were observed subsequently by Roper only in the case of one particular boryl complex.103... 

Comparatively recent is the isolation of stable metal-boryl complexes by the oxidative addition of B—B bonds, e.g., to Pt°or Rh1 ... 

In this article emphasis has been placed on a review of crystallographic, spectroscopic and quantum chemical studies which shed light on the fundamental structure and bonding characteristics of the boryl ligand. The chemistry of transition-metal boryl complexes was reviewed extensively in 1998 [11,16-18], and a number of further review articles have been written in the interim [19-24]. Herein, we focus primarily on literature reports which have been published in the period 1998-2006, with due reference made to key systems reported in the pre-1998 period. The material includes three-... 

Finally, we note that the chapter ordering in the current Volume places borylene complexes before boryl complexes. We have ordered the chapters according to the degree of metal-boron bonding, from highest to lowest, rather than following a historical perspective. 

Transition-metal-boryl complexes contain a covalent bond between a metal and three-coordinate boron. Boryl complexes are a subset of the variety of ligands containing a single boron atom that would encompass borane, boryl, and borene ligands. Of this group, boryl complexes are the most abundant. 

Boryl complexes participate in a variety of catalytic processes, including transition-metal-catalyzed hydroboration and diboration of olefin and acetylene C-C ir-bonds (Chapter 16) and tihe functionalization of alkane and arene C-H bonds (Chapter 17). The chemistry of these complexes has been developed primarily since 1990. A body of literature on metal-boiyl complexes was published in the 1960s, but the structures of these compounds were apparently misassigned in the absence of modem X-ray crystallographic and NMR methods. Boryl complexes of all transition metals except for group 3 (Sc, Y, and La) and group 4 (Ti, Zr, Hf) metals - have been isolated. A majority of transition-metal boryl-complexes contain late metals. 

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