Alkylboronic oxidation

A mild one-pot procedure based on a platinum-catalyzed diborylation of 1,3-butadienes (see Eq. 30) gives doubly allylic boronate 144, which adds to an aldehyde to form a quaternary carbon center in the intermediate 145 (Eq. 105). The use of a tartrate auxiliary in this process leads to good levels of enantiose-lectivity in the final diol product, which is obtained after oxidation of the primary alkylboronate intermediate. Although examples of aliphatic, aromatic, and unsaturated aldehydes have been described, enantioselectivities vary widely (33 to 74% ee), and are good only for aliphatic aldehydes. An intramolecular variant of this interesting tandem reaction is also known. ... 

Perillyl aldehyde (entry 11 in Table 13.3) is typically obtained from the corresponding alcohol via Oppenauer-type oxidation by using alkylboron compounds... 

Computational studies performed on model complexes in collaboration with Hall and coworkers suggest that alkane borylation may occur by a ej-bond metathesis pathway (Scheme 3) [48]. The proposed mechanism for the borylation of alkanes by 1 begins with elimination of HBpin to generate the 16-electron complex Cp Rh(Bpin)2. This complex then forms a <7-complex (3) with the alkane. The vacant p-orbital on boron then enables cr-bond metathesis to generate a o-borane complex (4). Reductive elimination of the alkylboronate ester product and oxidative addition of B2pin2 then regenerate 1. 

In 2006, Yu et al. combined pyridinyl-directed C-H activation and C-C bond formation with alkylboronic acids (see Section 10.5.4.2).23 The success of this transformation relied on the combination of palladium acetate (10 mol%), benzoquinone (1 equiv), and silver oxide or carbonate (0.5 equiv) in a protic solvent, but an excess of boronic acid (3 equiv) was required (Scheme 10.9). Interestingly, in this reaction silver oxide played a dual role as promoter for the transmetallation step and as cooxidant with benzoquinone. 

Brown and coworkers have described an alternative synthesis of chiral alkylboronic esters. In this synthesis prochiral alkenes are hydroborated with monoisopinocamphenylborane to yield isopinocam-phenylalkylboranes which are then readily transformed to chiral alkyllraronic esters (Scheme 39). Homologation with dichloromethyllithium, followed by reduction with potassium triisopropoxyborohy-dride (KIPBH) and oxidation, finely yields B-chiral alcohols (Scheme 40). These alcohols are not easily prepared by other methods. Aldehydes can be prepared by homologation from chiral alkylboronic esters with LiCH(OMe)SPh and oxidation (Scheme 41). ... 

Alkylborinic esters, obtained from alkylboronic esters and an organometallic reagent, are converted into the corresponding ketones by the reaction with dichloromethyl methyl ether in the presence of a hindered base, followed by oxidation with hydrogen peroxide in pH 8 buffer or with anhydrous trimethylamine Ar-oxide18. 

Because the oxidation step in the hydroboration-oxidation synthesis of alcohols takes place with retention of configuration, the hydroxyl group replaces the boron atom where it stands in the alkylboron compound. The net result of the two steps (hydroboration and oxidation) is the syn addition of —H and —OH. We can review the anti-Markovnikov and syn aspects of hydroboration-oxidation by considering the hydration of 1-methylcyclopentene, as shown in Fig. 8.3. 

These reactions are likely to occur by oxidative addition through a radical mechanism, as evidenced by the loss of stereochemistry of the starting alkyl halide during the coupling process (Equation 19.14b). Despite the radical mechanism, some reactions of benzylic electrophiles have been conducted enantioselectively (Equation 19.14c). Even reactions of alkylboron reagents witti secondary alkyl halides catalyzed by nickel complexes have now been reported. These reactions were conducted with nickel precursors in combination with trflns-l,2-cyclohexanediamine (Equation 19.14d). ... 

Hydrobondion RhCl(PPhj)3 catalyzes the addition of the B—H bond in catecholborane (9.30) to alkenes (eq. 9.27). This reaction also goes without catalyst, but the catalytic reaction has usefolly different chemo-, regio-, and stereoselectivities. Oxidative workup of the alkylboron product normally... 

A more recent example from the Yu group uses alkylboronic acids as aUcyl precursors, which enables a broader product scope and even the introduction of strained cyclopropyl groups [82]. Pyridine directing groups enable the complete ort/io-selectivity in the presence of Ag salts, benzoquinone, and air as oxidants (Scheme 23.19). Interestingly, the protocol can be employed for C-H alkylations of both sp and sp C-H bonds. 

Ph(CH2)2, cyclopropyl SCHEME 23.19 Pd-catalyzed oxidative alkylation with alkylboronic acids. 

It is not that convenient to store alkylboronic acids since these compounds are quite oxygen-sensitive. Indeed, 3 is gradually oxidized to generate boric acid and butanol when exposed to air for long periods of time. Furthermore, they also tend to form boroxines under dehydrating conditions, which are themselves oxygen sensitive. 

Compared to aryl- and alkenylboronic adds, alkylboronic acids and esters have found limited use as synthetic intermediates aside for their oxidation into alcohols (Section 1.5.2.1). This is due in part to their inferior shelf-stability. In addition, their trans-metallation with transition metal catalysts such as palladium is presumed to be more difficult than that of the unsaturated and aromatic boronic acid derivatives [296]. For example, alkylboronic adds have long been known to be reluctant substrates in the Suzuki-cross-couphng reaction, and they have become eflfident in this apphcation only very recently with the use of special bases and the advent of new and highly active catalyst systems (Section 1.5.3.1). Perhaps the most synthetically useful class of alkylboronic adds are the a-haloalkyl derivatives popularized by Matteson (Section 1.3.8.4), and their elegant chemistry is described in Chapter 8. 

The treatment of arylboronic acids and esters with alkaline hydrogen peroxide to produce the corresponding phenols was first reported more than 75 years ago [324]. The oxidation of alkyl- and alkenyl- boronic acid derivatives leads to alkanols [40] and alde-hydes/ketones, respectively [85, 257, 279, 316]. With a-chiral alkylboronates, the re-action proceeds by retention of configuration (Equation 53, Figure 1.32) [359, 121]. In fact, the oxidation of boronic acids and esters is a synthetically useful process, mainly in the preparation of chiral aliphatic alcohols via asymmetric hydroboration reactions [300, 302], or from Matteson homologation chemistry [322]. Paradoxically, the... 

Reaction of Cp Re(CO)2(Bpin)2 (16), prepared from Cp Re(CO)j (15) and puiaBa, led to the regiospecific formation of 1-borylpentane in quantitative yield under irradiation of light in pentane. Thus, the catalytic cycle involves oxidative addition of pin2B2 to Cp Re(CO)j with photochemical dissociation of CO, oxidative addition of C-H bond to Cp Re(CO)2(Bpin)2 (16) giving a rhenium(V) intermediate (17), and finally reductive elimination of an alkylboronate with association of CO (Scheme 2.4) [51]. The interaction required for C-H activation of alkane with 16 is not known but higher reactivity of primary over secondary C-H bonds has been reported in both oxidative addition (17) and bond metathesis (18) processes [52]. Isomerization of a sec-alkyl group in Cp Re(H)(R)(CO)(Bpin)2 (17) to an n-alkyl isomer before reductive elimination of pinB-R is another probable process that has been reported in metal-catalyzed hydroboration of internal alkenes [15c]. 

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