Alkynes hydroboration reactions

Professor Stone s paper points out that the reactivity of [ (ti-C5H5) (OC) 2WsCR] towards transition metal complexes is similar to that of an alkyne. It would be of interest to examine this compound and several of its derivatives which contain OW double bonds with respect to their reactivity patterns towards the BH3 group to determine if reactions analogous to the hydroboration reaction of alkynes and olefins would occur (1) or reactions similar to the attempted hydroboration described below would take place. 

BBN hydroborates internal alkynes cleanly (Equation B1.13) and thus for this reaction it is superior to borane which tends to give intractable polymers when added to alkynes. The reaction is less useful for terminal alkynes as monohydroboration can only be achieved if an excess of alkyne is used. 

The hydroboration reaction is also useful with alkynes. As is shown in the following example, the product after treatment with basic hydrogen peroxide is an enol. As we have seen before, the enol cannot be isolated because it spontaneously tautomerizes to a ketone. This provides another way to hydrate alkynes to produce ketones. 

If the hydroboration reaction is to be used to convert 1-alkynes into aldehydes, some way to stop the addition at the vinylborane stage is needed. The problem is that there is not enough steric hindrance at the end carbon of the vinylborane. The solution is to build extra steric hindrance into the other alkyl groups attached to the boron of the vinylborane. A borane, R2BH, with two bulky R groups already attached to the boron is used as the hydroboration reagent. One such reagent is prepared by the reaction of two equivalents of 2-methyl-2-butene (also known by the common name of isoamylene) with borane to produce a dialkylborane called di si amyl borane (a shortened version of diisoamylborane) ... 

Instead of adding two hydrogen atoms to an alkynyl silane we could add H and SiMe3 to a simple alkyne by hydrosilylation (addition of hydrogen and silicon). This is a cis addition process catalysed by transition metals and leads to a tram (E-) vinyl silane. One of the best catalysts is chloroplatinic acid (H2PtCl6) as in this formation of the E-vinyl silane from phenylacetylene. In this case photochemical isomerization to the Z-isomer makes both available. Other than the need for catalysis, this reaction should remind you of the hydroboration reactions earlier in the chapter. The silicon atom is the electrophilic end of the Si-H bond and is transferred to the less substituted end of the alkyne. 

Dichloroborane and monochloroborane etherates or their methyl sulfide complexes have been prepared by the reaction of borane and boron trichloride [44]. The hydroboration of alkenes with these borane reagents is, however, usually very slow because of the slow dissociation of the complex. Dichloroborane prepared in pentane from BCI3 and trimethylsilane is unusually highly reactive with alkenes and alkynes hydroboration is instantaneous at -78 °C (Eq. 20) [45]. 

In the case of unsymmetrical alkynes the regioselectivity is only modest. Use of excess borane-THF results in dihydroboration. Many of the problems encountered in hydroboration reactions of borane-THF can be overcome by the use of more selective reagents such as dialkylboranes or dihaloboranes. 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

Hydroalumination. The treatment of alkynes with diisobutylaluminum hydride in hydrocarbon solvents results in a aT-addition of the Al-H bond to the triple bond to produce stereodefined alkenylalanes. The hydroalumination of alkynes is more limited in scope than the corresponding hydroboration reaction of alkynes (see Chapter 5) with regard to accommodation of functional groups and regioselectivity. Whereas hydroalumination of 1-alkynes is highly regioselective, placing the aluminum at the terminal position of the triple bond, unsymmetrically substituted alkynes produce mixtures of isomeric alkenylalanes. 

Reduction of mono- or disubstituted alkynes with diisobutylaluminum hydride proceeds, like the corresponding hydroboration reaction, via stereoselective cw-addition of the Al-H bond to the triple bond. Hydrolysis of the resultant tram-alkenylalanes produces 1-alkenes and cA-alkenes, respectively. 

In many catalytic processes and transition metal mediated reactions, a-bor-ane complexes have been shown to be intermediates. The bis(borane) complex Cp2Ti( 72-HBcat/)2 (HBcaT = HBcat-4-f-Bu) is a highly active catalyst for the hydroboration of vinylarenes [37]. A mechanism, shown in Scheme 3, has been proposed for the Ti-catalyzed hydroboration on the basis of a detailed mechanistic study [37]. Theoretical calculations provided further support to the proposed reaction mechanism and showed that the reductive elimination step, giving the product molecules, is rate-determining [38]. In the Cp2Ti(CO)2 catalyzed hydroboration of alkynes [36,37], the proposed reaction mechanism (Scheme 4) also involves a a-borane complex similar to 11 and 14. In the titanium-catalyzed decaborane-olefin hydroborations [47,48], a-borane complexes were also considered as intermediates. In the Cp2MH (M = Nb, Ta) mediated hydroboration reactions of olefins [39,41], Smith and his coworkers observed several interesting cr-borane complexes, such as 21-23 discussed above. 

Diborylated organoboranes with a BCB backbone such as (17) are usually prepared by double hydroboration of terminal alkynes (equation 29). The treatment of the diborane B2pin2 with diazomethane derivatives provides an interesting alternative that allows for the synthesis of species pinB-CR2-Bpin (18, R=H, Ph equation 30), which are not accessible via hydroboration reactions. ... 

Syn hydroboration of internal alkynes tends to give a mixture of two possible regioisomers. In cases where 1-halo-l-alkynes are used as internal alkynes, the reaction is nearly 100% regioselective placing B at the halogen-bound carbon. The resultant (Z)-a-haloaIkenylboranes can be used to prepare (i) (Z)-l-alkenylboranes (Type IV) [59], (ii) (Z)-a,P-disubstituted alkenylboranes (Type V) [60], and (iii) ( )-a,P-disubstituted alkenylboranes (Type VI) [61] as summarized in Scheme 3.9. 

How are alkynes prepared Alkynes react as Br0nsted-Lowry bases in acid-base reactions with acids such as HBr or with aqueous acid. Alkynes react as Lewis bases with dihalogens such as Brg and in hydroboration reactions (see Chapter 10). Alkynes often come from reactions involving other alkynes, using the fact that a terminal alkyne is a weak acid, as introduced in Chapter 6 (Section 6.2.6). Treatment of a terminal alkyne with a base such as sodium... 

Hydroboration of Fluoroalkenes. Dibromoborane-dimethyl sulfide (DBBS) has demonstrated to be an excellent hydrob-orating agent. Based upon kinetic and mechanistic studies, it is clear that the hydroboration reaction exhibits a second-order kinetics (detaching of Me2S from the boron center follows a dissociative pathway, while the hydroboration process follows an associative mechanism).DBBS hydroborates a wide variety of alkenes and alkynes. Perfluoroalkylethylenes and 2, 3, 4, 5, 6 -pentafluorostyrene undergo Markovnikov hydroboration (>92% regioselectivity) (eq 12). ... 

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