Alkenes borylations

Scheme 2.22 NCN-Pd complex catalyzed alkene borylation reaction.

Carbon-heteroatom bond-forming reductive elimination from transient intermediates has been proposed as the product release step of a variety of important Pd-catalyzed transformations, including arene and alkane C-H bond functionahza-tion [1,2], ally lie acetoxylation [3], alkene borylation [4], and olefin difunctionalization [5]. Over the past 25 years, a variety of Pd " model complexes have been synthesized to study reductive elimination reactions at Pd centers. For instance, in 1986, Canty reported the first example of a crystallographically characterized organometallic Pd complex, /ac-[(bpy)Pd (CH3)3(l)] (bpy = 2,2 -bipyridine) (1). In addition, his group has demonstrated that this species undergoes facile C-C bond-forming reductive elimination to release ethane (Eq. 1) [6],... 

The stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

The complexes [Cu(NHC)(MeCN)][BF ], NHC = IPr, SIPr, IMes, catalyse the diboration of styrene with (Bcat) in high conversions (5 mol%, THF, rt or reflux). The (BcaO /styrene ratio has also an important effect on chemoselectivity (mono-versus di-substituted borylated species). Use of equimolecular ratios or excess of BCcat) results in the diborylated product, while higher alkene B(cat)j ratios lead selectively to mono-borylated species. Alkynes (phenylacetylene, diphenylacety-lene) are converted selectively (90-95%) to the c/x-di-borylated products under the same conditions. The mechanism of the reaction possibly involves a-bond metathetical reactions, but no oxidative addition at the copper. This mechanistic model was supported by DFT calculations [68]. 

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-... 

Reaction of alkenes with boryl and silyl groups c/r-located at the C=C bond with alcohols is known to proceed in two different ways <1973S309, 1986JOM(310)151> by protolytic cleavage of the B-C bond and by complexation... 

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)) ... 

Bis(pinacolato)diboron reacts with 1-halo-l-lithioalkenes, that is, alkylidene carbenoids, affording 1,1-diboryl-1-alkenes in good yields (Scheme 9).76 The reaction proceeds via formation of a borate intermediate, which is followed by 1,2-migration of the boryl group with elimination of the bromo group. 

Addition of silylpinacolborane to alkenes has been achieved with platinum catalysts (Equation (83)).225 The reaction proceeds regioselectively to provide products in which the silyl groups are attached to the terminal carbon atoms contrary to the silaboration of alkynes. Although no regioisomers are detected, 1-boryl-l-silylalkenes are formed as major byproducts. 

The reaction of silylborane with 1-halo-l-lithio-l-alkenes yields 1-boryl-l-silyl-l-alkenes via borate formation followed by 1,2-migration of silyl group (Equation (90)).76,240 The mechanism seems to be closely related to that proposed for the silaboration of isocyanide (Figure 2). Vinyl-substituted carbenoids, l-chloro-l-lithio-2-alkenes, react with silylpinacolborane to give l-boryl-l-silyl-2-alkanes in good yield (Equation (91)).241 This methodology is applied to the synthesis of l-boryl-l-silyl-2-cyclobutene.2 2 Similar reactions are carried out with other carbenoid... 

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. 

For each case we will also present catalytic analogues, namely (1) the activation of methane to form methanol with platinum, the reaction of certain aromatics with palladium to give alkene-substituted aromatics, and (2) the alkylation of aromatics with ruthenium catalysts, and the borylation of alkanes and arenes with a variety of metal complexes. 

Fig. 2.8 A d irect route to vinylboranes in rhodium-catalyzed hydroborations with phos-phine-free catalysts (including oxidative degradation of a rhodium phosphine). The key intermediate is a rhodium hydride, capable of reversible insertion into the alkene (step A), followed by addition of borane in step B. This leads to reductive elimination of RH in step C followed by boryl migration in step D. A further...

The hydroboration of alkynylchlorosilanes gave chlorodimethylsilyl-diethylborylalkenes such as 57. The alkene possess two electrophilic centres on the silyl and boryl groups and they react with 2-lithiated thiazoles 58 to give the zwitterionic compound 59 which can drawn as resonance structures 59a or 59b <99JOM98>. 

The reactivity of l-silyl-l-boryl-2-alkenes 68 obtained by gem-silylboration of allyl-chloride were studied by Hiyama et al. [36]. Allylsilanes 68, engaged in SMS reactions, afford the desired trans-vinylboranes 69 with good yields (Scheme 13.29). 

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. 

The reaction of pinacolborane with styrenes 127 in the presence of bis(chloro-l,5-cyclooctadienylrhodium) at room temperature provides styrenyl pinacol boronate 128 <1999TL2585, 2002BCJ825>. While hydroboration of alkenes is the predominant reaction with phosphine-containing rhodium catalysts such as Wilkinson s catalyst and Rh(PPh3)2COCl, dehydrogenative borylation dominates over hydroboration in the presence of phosphine-free... 

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,... 

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