Diboration olefins

Iverson, C.N. and Smith, M.R., Efficient olefin diboration by a base-free platinum catalyst, Organometallics, 16, 2757, 1997. 

Diboration provides another means of obtaining organoboranes. Studies [30-36] have been focused on the diboration reactions of alkynes and olefins with pinacol ester derivatives catalyzed by Pd(0) and Pt(0) metal complexes. Interestingly, it has been shown that Pt (0) complexes catalyze cis addition of the B-B bond in pinacol ester derivatives to alkynes but not to olefins. On the other hand, Pd (0) complexes do not catalyze diboration reactions, neither for alkynes nor for olefins. 

Haloboration Reactions. The haloboration of carbon-carbon triple bonds provides another entry point for the synthesis of organoboranes. A wide variety of haloboranes including BBrs, 9-BBN-Br, and 9-BBN-l has been found to react with terminal alkynes to produce (Z)-2-halo-l-aIkenylboranes. The reaction occurs in a stereo-, regio-, and chemoselective fashion specifically with terminal alkynes and has been used to synthesize numerous substituted olefins and related compounds. Diboration reactions of alkynes with B2CI4 are also well known. However, more convenient transition-metal-catalyzed procedures with the less reactive aUcoxy substituted diboranes B2(OR)4 have recently been developed. 

The transition-metal-catalyzed diboration of alkynes can be directed to give highly selective di-addition yielding 1,2-diborylated olefins (equation 24). Depending on the catalyst. 

Many different routes are available for the synthesis of vinylboranes and several of them are shown in Scheme 7. Hydroboration and diboration reactions of alkynes and borylated alkynes provide access to the frill series of mono-, di-, tri-, and tetraborylated olefins. 1,2-Diborylated olefins (33) are obtained via diboration of alkynes and 1,1-diborylated olefins (34) are accessible through hydroboration of borylalkynes. An alternative route to 1,1-disubstituted products involves the diboration of carbenoids formed in situ from vinylhalides and butyl hthium. In certain cases, metal-catalyzed dehydrogenative borylation of olefins may be used. Borylalkynes serve as precursors to triborylated (35) and tetraborylated (36) olefins. Thus, the sparingly soluble tetraborylethylene derivative (36) forms in good yield through platinum-catalyzed diboration of diborylacetylene in toluene at 40 °C if the base-free catalyst [Pt(cod)2] is used. If the reaction, however, is performed at higher temperature, ftnther diboration of (36) leads directly to the hexaborylated ethane (23) shown above. Intramolecular B-O interactions were postulated for (36) based on HF-SCF calculations. ... 

Alkynes including acetylene 23), propyne 117), 2-butyne 90, 118), di-ferf-butyl acetylene 117), and diphenylacetylene 112) react readily at room temperature with 1 mole of B2CI4 to form the corresponding vicinal bis(dihaloboryl)olefins. In the case of acetylene, Urry 101) has mentioned the formation at 80° C of a solid complex from which the reagents can be recovered and which slowly reacts to form the 1 1 diboration product. Addition of a second mole of B2CI4 to the olefinic 1 1 addition product has been described only in the case of acetylene. The product, 1,1,2,2-tetrakis-(dichloroboryl)ethane is a solid, m.p. 29°-30°C, stable to 200° C and monomeric in the vapor 24). Addition of B2F4 to acetylene in 1 1 ratio has been... 

The reaction of B2CI4 with ferrocene is of interest in comparison to the reaction of the halide with benzene which, as noted previously, also reacts by substitution and does not undergo an olefin-like diboration 39). 

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. 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

This chapter focuses on the subset of these reactions that have been studied most intensively and that draw from the stoichiometric reactions presented earlier in this text. Thus, the first sections of this chapter highlight certain aspects of hydrocyanation, hydrosilylation, disilylation, hydroboration, diboration, silylborations, and hydroami-nation. The last section presents aspects of palladium-catalyzed oxidation and metal-catalyzed oxidative amination of olefins. 

Scheme 1 Postulated activation of CI2B-BCI2 with olefins toward diboration reaction.

Scheme 8 Diboration of olefins with B2cat2 by Pt 0)-olefin complexes.

The B2pin2 activation by ds-[Pt(r -CH2=CH2)(PPh3)2] was also found to be an effective catalyst precursor for the 1,4-diboration of a,P-unsaturated ketones at 80 °C for 12 h (Scheme 11), while [Pt(PPh3)4] as catalyst precursor required 20 h at 110 °C to add B2pin2 to the same activated olefins. A second generation of platinum system... 

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. 

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