Bis- boryl

The cA-bis(boryl)alkenes 482 are obtained by bis-boration of terminal alkynes catalysed by a Pt complex [185]. Pd and Rh complexes are inactive. Then the (Z)-1,2-diphenylalkene 483 is prepared by Pd-catalysed Suzuki-Miyaura coupling of 482 with iodobenzene. 

Herberich, G. E., Fischer, A., Wiebelhaus, D., Bis(boryl)metallocenes. 1. 1,1 -bis(diisopropylboryl)cobaltocenium cation A novel anion-binding ligand. Organometallics 1996,15, 3106-3108. 

The mechanism proposed for aromatic C-H borylation of aromatic compounds 1 by B2pin2 3 catalyzed by the Ir-bpy complex is depicted in Scheme 3 [6-9]. A tris(boryl)Ir (III) species [5, 6, 11] 6 generated by reaction of an Ir(I) complex 5 with 3 is chemically and kinetically suitable to be an intermediate in the catalytic process. Oxidative addition of 1 to 6 yields an Ir(V) species 7 that reductively eliminates an aromatic boron compound 4 to give a bis(boryl)Ir(III) hydride complex 8. Oxidative addition of 3 to 8 can be followed by reductive elimination of HBpin 2 from 9 to regenerate 6. 2 also participates in the catalytic cycle via a sequence of oxidative addition to 8 and reductive elimination of H2 from an 18-electron Ir(V) intermediate 10. Borylation of 1 by 2 may occur after consumption of 3, because the catalytic reaction is a two-step process - fast borylation by 3 then slow borylation by 2 [6],... 

In 1993, Ishiyama et al.60 reported the synthesis of isomerically pure cis-1,2-bis(boryl)alkenes 53a-57a from their corresponding alkynes 53-57 (Scheme 12)60,61 via platinum complexes. The solvents did not play an important role in the reaction, but a comparison of the reaction rates at 50 °C revealed that the addition was apparently accelerated in polar... 

The same reaction produced different alkenes 58-6563 (Scheme 13). These reactions were not catalyzed by rhodium(I) or palladium(0). Tetra(methoxo)- and bis(pinacolato)diboron(4) added to both terminal and internal alkynes in the presence of a catalytic amount of Pt(PPh3)4 to provide stereo defined pinacolato r/s-bis(boryl)alkenes 58-65 in excellent yields. Because reagents and reaction conditions were sufficiently mild,... 

Bis(pinacolato)diborane(4) selectively adds to terminal aikenes and cyclic aikenes having internal strain to provide bis(boryl)alkanes in 76-86% yields 85-89 in the presence of a catalytic amount of Pt(dba)2 at 50 °C67 (Scheme 16). It is interesting to mention that Pt(dba)2 directed 1,2-addition to certain conjugated dienes, whereas 1,4-addition through a 7i-allyl-platinum(II) intermediate is an energetically more favorable process. The 1,4-addition to penta-1,3-diene at 80 °C with Pt(PPh3)4 gives 90, but the same reaction with Pt(dba)2 selectively produced the 1,2-addition product 91 at room temperature (Scheme 16). 

Benzyl alcohol, dehydrogenation of, 279 Bifunctional dicarboxylic acids, 85 Biphasic olefin dimerization, 274-276 Bis(boryl) platinum compounds, 228 Bis(catecholato)diborane(4) compounds, 195, 200-201... 

Potential difference in reactivity between two G-B bonds allowed the transformation of l,2-bis(boryl)-l-alkenes to 1-alkenylboranes via a cross-coupling with the aryl, 1-alkenyl, benzyl, and cinnamyl halides (Equation (23)).211-213 This tandem procedure synthetically equivalent to a yy/z-carboboration of alkynes was used for synthesizing Tamoxifen derivatives via stepwise double coupling with two of the G-B bonds.212,213 Hydrogenation of the resulting bisborylalk-enes with a chiral rhodium catalyst is synthetically equivalent to an asymmetric diboration of alkenes (Equation (24)).214... 

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. 

Insertion of isonitriles and carbenoids into the B-B and B-Si bonds have recently been developed as a method for synthesizing geminal bis(boryl) and boryl-silyl compounds (Scheme 24). The insertion of alkyl and aryl isonitriles into the B-Si bond proceeded at room temperature to provide monomeric (boryl)(silyl)imines 227.388,389 Alkylidene... 

The intramolecular version was studied to control the selectivity of the insertion 268.433 Cyclotrimerization of bis(boryl) acetylene provided a novel hexa(boryl)benzene 269 (cat represents catecholato group).434,435... 

The boron-Wittig reaction has been carried out by bis(boryl)methyllithium 309 generated in situ from tris(boryl)-methane 308 (Equation (91 )).469 The geminal dichromium reagent 3 12470 was found to be an excellent alternative yielding 1-alkenylboronic esters 313470-472 with high //w/ -selectivity (Equation (92)). 

An iridium(l) complex, generated from l/2[Ir(OMe)(COD)]2 and 4,4 -di-/-butyl-2,2 -bipyridine (dtbpy), catalyzed the direct borylation of 2-substituted pyrroles in stoichiometric amounts relative to 2,2 -bi-l,3,2-dioxaborolane 785 in hexane at room temperature (Equation 188) <2003ASC1103>. The pyrrolylborates 786 from regioselective C-H activation at the 5-position were formed in high yields. Similar borylation of unsubstituted pyrrole with an equimolar amount of borolane 785 regioselectively provided 2,5-bis(boryl)pyrrole 787 (Equation 189). 

Various derivatives of bis(boryl)benzene isomers (37), (38), and (39) are readily obtained through metathesis reaction of boron hahdes with the respective Grignard, trimethylsilyl, or trimethylstannyl reagents. Especially the 1,2-disubstituted derivatives (37) have been studied extensively in molecular recognition and as activators in olefin polymerization (Sections 7.2 and 7.3.3), whereas species (38) and (39) have primarily been used for the design of new electronic materials (Section 7.1). 

B2(cal)2 adds readily to the ruthenium zero oxidation state compounds, Ru(CO)L(PPh3)3 (L = CO, CN-p-tolyl), to give six coordinate bis(boryl)-complexes in which both the two phosphine ligands, and the two boryl ligands, arc mutually cix (see Scheme 2).6 This reaction is not general since the corresponding osmium complexes do not react directly with B2(cat)2, however, bis(boryl)-osmium... 

The resulting compounds may be converted to other bis(boryl) derivatives by a variety of metathetical reactions appropriate to organoboron dihalides, by oxidative hydrolysis to wfc-diols, or, in certain cases, to hydrocarbons by protonolysis. 

The addition reaction has been observed with a variety of olefins, acetylenes, and with cyclopropane 23). Both B2CI4 and B2F4 can add to suitable unsaturated molecules. Unsuccessful attempts to achieve similar reactions with tetraamino and tetraalkoxydiboron compounds and with 1,2-bis-(dimethylamino)-l,2-diethyldiborane(4) have been noted 13, 68). Evidence for formation of bis(boryl) derivatives from the reaction of mixtures of B2[N(CH3)2]4 and BCI3 with olefins has been mentioned (115) the reaction was presumed to involve a mixed dimethylaminochlorodiboron derivative. Lewis base complexes of B2CI4 are unreactive toward olefins (56). 

Demonstration of cis addition with acetylene is unequivocal, in that the stereochemistry of the resulting bis(dihaloboryl)ethylene has been shown directly by spectroscopic evidence (90) and by conversion of the initially produced cis isomer to the trans compound by photochemical isomerization (27). Stereochemical inferences in the olefin addition reactions are based on the assumption that hydrolysis and oxidation of the bis(boryl) compounds to the corresponding diols occurs with retention of configuration, as is the case in closely related systems 19). The NMR spectrum of the addition product of B2CI4 with 1,3-cyclohexadiene is also consistent with cis addition (120). 

There is no clear evidence for formation of stable bis(boryl) derivatives from the interaction of diboron tetrahalides with molecules containing multiple bonds other than C=C or Cr C. Such reactions as occur may in some cases involve initial diboration, but as they are of limited interest from the organometallic viewpoint, they will be mentioned only briefly. 

Binger reported the first synthesis of the derivative l,3,4,5-tetraethyl-2,3-dihydro-2-methyl-l//-l,3-diborole, which involved thermolysis of m-3,4-diethyl-3,4-bis(diethylboryl)hexane at 160 °C <1968AG288>. Recently, Siebert and co-workers heated hexakis(diethylboryl)benzene S3 at 180°C in a stream of argon and isolated l,2,3,4-bis(l, 3 -diborole-2, 3 -dihydro)-5,6-bis(boryl)benzene 21, accompanied by evolution of ethane (Equation 6) <2000EJI1177>. This compound represents the first example of a benzo derivative with two condensed 1,3-diborole rings. 

The Pd(0)-catalyzed addition of the B-S bond to alkynes (thioboration) regio- and stereoselectively produces (Z)-2-(organothio)-l-alkenylboron reagents (Scheme 2-25) [63]. Tlie addition of (tetraalkoxy)diboron to alkynes to give cw-bis(boryl)alkenes (diboration) is catalyzed by a platinum(O) catalyst [34]. 

Another resin-capture approach has been pubhshed in relation to the synthesis of tetrasubstituted ethylenes via Suzuki coupling reactions (Scheme 20) [42, 53]. A 25-member hbrary was synthesized using five alkynes, five aryl halides, and a polymer-bound aryl iodide. The alkynes 55 were converted into bis(boryl)alkenes 56 in solution, and the crude intermediates were used in Suzuki reactions with an excess of aryl halide. When all of the bis(boryl)alkene 56 had been consumed, the aryl iodide resin 59 was added to the reaction mixture and the reaction continued on the solid support. Side products such as 58, arising from a double Suzuki reaction, remained in solution and could be washed away. Compounds 60 were cleaved from the polymer using trifluoroacetic acid and products 61 were obtained in > 90% purity. 

Although group 5 organometallic systems have been found to be of relevance in transition-metal catalyzed hydroboration reactions, structurally authenticated group 5 boryl complexes remain relatively few in number. Smith and co-workers, for example, have probed the mechanisms for the formation of niobium and tantalum mono- and bis(boryls) from propylene complex precursors, with concomitant formation of propyl boronate esters [31,32]. Of particular interest from a structural viewpoint are the relative merits of alternative bonding descriptions for metal(V) boryl bis(hydrides) as borohydride complexes or as mono(hydride) a-borane systems [31-34]. 

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

---