Rhodium boryl complex

Rhodium boryl complexes have been the subject of intense research effort, reflecting not only fundamental issues of structure and bonding but also their... 

Table 9 Selected data for structurally characterized rhodium boryl complexes ...

Hartwig JF, Cook KS, Hapke M, et al. Rhodium boryl complexes in the catalytic. Terminal functionabzation of alkanes. J Am Chem Soc. 2005 127 2538-2552. 

The reaction of HBpin in toluene in the presence of RhCl P(/-Pr)3 2(N2) (1 mol%) at 140 °C resulted in a mixture of (borylmethyl)benzene (69%) and bis(boryl)methyl benzene (7%), along with several products arising from aromatic C-H borylation (ca. 15%).345 Rhodium-bpy complexes catalyzed the borylation at the benzylic C-H bond.351 Pd/C was found to be a unique catalyst for selective benzylic C-H borylation of alkylbenzenes by B2pin2 or HBpin (Equation (70)).360 Toluene, xylenes, and mesitylene were all viable substrates however, the reaction can be strongly retarded by the presence of heteroatom functionalities such as MeO and F. Ethylbenzene resulted in a 3 1 mixture of pinacol 1-phenylethylboron and 2-phenylethylboron derivatives. 

Initial reports on the borylation of alkanes using isolated transition-metal-boryl complexes date back to 1995, when Hartwig showed that Cp Re(CO)2(Bpin)2 converts pentane to 1-borylpentane with high regioselectivity. " The catalytic C-H borylation of alkanes with Cp Re(CO)3 using photochemical activation was demonstrated soon thereafter (equation 25). Also, an efficient thermal process that involves the use of rhodium catalysts has since been developed (equation 26). It is interesting to note that this methodology is not restricted to small molecules, but has recently been exploited for the direct side-chain functionalization of polyolefins. ... 

Scheme 21 Synthetic routes to the crystallographically characterized rhodium boryl hydride and bis (boryl) complexes 9.8-9.13...

Recently, it was also shown [74] that pentamethylcyclopentadienyl rhodium and iridium complexes with labile dative ligands catalyze the borylation of alkanes at the terminal position under thermal conditions (Scheme 9). It was proposed that Ir(V) and Rh(V) boryl complexes act as intermediates in these reactions. Reaction of Cp IrH4 with HBpin gives Cp Ir(Bpin)(H)3 and Cp Ir(Bpin)2(H)2. Both of the two Ir(V) boryl complexes react with octane to give octylboronate ester [75], though it is still unclear regarding the role... 

The reactions conducted with ligand-less cationic systems are proposed to occur through the rhodium(I) boryl complexes shown in Scheme 16.12, but fewer data are available to support this mechanism. Theoretical studies have implied that insertion of the olefin into the metal-boryl linkage occurs and that this step is followed by oxidative addition of the borane and reductive elimination to form the C-H bond of the product and the starting rhodiiun(I) boryl complex. 

Teltewskoi M, Panetier JA, Macgregor SA, Braun T (2010) A highly reactive rhodium(I)-boryl complexes as a useful tools for C-H bond activation and catalytic C-F bond borylation. Angew Chem Int Ed 49(23) 3947-3951... 

For other neutron diffraction smdies of coordinated B—H bonds, see (a) Lam WH, Shimada S, Batsanov AS, et al. Accurate molecular strucmres of 16-electron rhodium hydrido boryl complexes low-temperature single-crystal X-ray and neutron diffraction and computational smcbes of [(PR3)2RhHCl(Boryl)] (Boryl=Bpin, Beat). Organometallics. 2003 22 4557-4568 (b) Hebden TJ, Denney TJ, Pons V, et al. O-Borane complexes of iridium syntliesis and structural characterization. J Am Chem Soc. 2008 130 10812-10820. 

KaUane SI, Braun T, Braun B, Mebs S. Versatile reactivity of a rhodium(I) boryl complex towards ketones and imines. Dalton Trans. 2014 43 6786-6801. 

KaUane SI, Braun T. Catalytic borylation of SCF3- unctionaIized arenes by rhodium(I) boryl complexes regioselective C-H activation at the ortho position. Angew Chem Int Ed Engl. 2014 53 9311-9315. 

PubUl-UUdemolins C, Poyatos M, Bo C, Fernandez E. Rhodium-NHC complexes mediate diboration versus dehydrogenative borylation of cyclic olefins a theoretical explanation. Dalton Trans. 2013 14 746-752. 

The most recent catalysts that operate under thermal conditions were then based on the premise that a Cp M fragment with ligands that dissociate under thermal conditions could be a catalyst for alkane borylation. After a brief study of Cp IrH4 and Cp Ir(ethylene)2, Dr. Chen studied related rhodium complexes. Ultimately, he proposed that the Cp Rh(ri" -C6Me6) complex would dissociate CeMce as an iimocent side product, and that Cp Rh(Bpin)2 from oxidative addition of pinBBpin (pin=pinacolate) would be the active catalyst. The overall catalytic... 

An extensive array of chiral phosphine ligands has been tested for the asymmetric rhodium-catalyzed hydroboration of aryl-substituted alkenes. It is well known that cationic Rh complexes bearing chelating phosphine ligands (e.g., dppf) result in Markovnikoff addition of HBcat to vinylarenes to afford branched boryl compounds. These can then be oxidized through to the corresponding chiral alcohol (11) (Equation (5)) ... 

Examples for o-phenylene scaffolds for bis-carbene ligands come from the research groups of Peris [344,345] and Herrmann [346]. Synthesis of the bis-imidazolium salt is achieved by reaction of a,a -xylene dichloride and the N-substituted imidazole. The rhodium(l) and iridinm(I) complexes can then be made by addition of the imidazolium salt to a solution of [M(cod)Cl]2 (M = Rh, Ir) in ethanol or acetonitrile (with NEtj as auxiliary base) (see Figure 3.108). The rhodium complexes were used successfully in the hydrosi-lylation of styrene [344] whereas both the rhodium and iridium complexes were used for the direct borylation of arenes making functionalised arylboronic acid esters accessible by a simple one-pot reaction [346]. 

Complexes of the group 9 metals, especially rhodium and iridium, represent one of the more numerous families on which a systematic appraisal of struc-ture/bonding properties for the boryl ligand can be based. In part, this reflects the involvement of such systems not only in earlier work on metal-catalyzed hydroboration chemistry [2-5,35,113-123], but in more recent studies of di-boration [124,125], and the activation of C-H bonds in both saturated [9,10, 126-135] and unsaturated hydrocarbons [9,10,50,51,127,129,134,136-159]. 

Scheme 19 The continuum of electronic structure between limiting descriptions of rhodium complexes as boryl hydrides or as a boranes...

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