Hydroboration kinetic studies

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

The characteristic features of hydroboration of alkenes—namely, regioselec-tivity, stereoselectivity, syn addition, and lack of rearrangement—led to the postulation of a concerted [2 + 2] cycloaddition of borane353,354 via four-center transition state 37. Kinetic studies, solvent effects, and molecular-orbital calculations are consistent with this model. As four-center transition states are unfavorable, however, the initial interaction of borane [or mentioned monobridged dimer, Eq. (6.56)] with the alkene probably involves an initial two-electron, three-center interaction355,356(38, 39). 

Kinetic studies on simple olefins [ 6] have shown that the reaction with a dialkylborane in an ether solvent is of first order with respect to olefin and first order with respect to the borane dimer. It therefore seems probable that the transition complex should be represented as in Fig. 24, with a molecule of ether involved in coordination with the second B-H unit. It is significant that hydroboration occurs only in a solvent able to. perform this role. 

In solvents like carbon tetrachloride, hexane, benzene, and diethylether, 9-BBN exists exclusively as the dimer. However, in THF and MejS an equilibrium between the (9-BBN)2 dimer and solvent-complexed 9-BBN monomer (9-BBN-solvent) is observed [7]. From the kinetic studies (vide infra), it has been proved that the 9-BBN monomer is actually the hydroborating agent. For convenience, 9-BBN is represented in the shorthand notation as shown in [B]. 

Kinetic studies provide valuable information in the areas of both mechanistic and synthetic chemistry concerning the effects of substituents in alkenes and alkynes. The effects of substituents that donate or withdraw or polarize electrons of C=C or C C provide information regarding the mechanism of hydrobora-tion. On the other hand, relative rates of hydroboration of substituted or unsubstituted C=C or C=C give synthetic chemists improved means of predicting the selective hydroboration of C=C or C C or their functionalized derivatives. 9-BBN has proven to be the best candidate for the investigation of mechanism and kinetics of hydroboration because ... 

The hydroboration kinetics are studied by addition of alkenes to the solution of (9-BBN)j in the solvent maintained at 25 °C. The reaction mixtures are pumped through a sodium chloride IR cell. The rates of the disappearance of B-H bridges of (9-BBN)j at 1,570 cm are monitored by quantitative IR spectrometry. The absorbance is recorded on chart paper. 

Kinetic studies of hydroboration are difficult to perform because all three B-H bonds can add to an alkene. Despite this, studies have shown most alkenes react at very similar rates. When more than one alkene is present in a reactant, terminal alkenes are preferred, presumably due to steric considerations. In many cases the reaction is zero order in alkene. The lack of large substituent effects on the rate and the zero-order dependence on the alkene indicate that the dissociation of the dimer or the BHs solvent complex to monomeric BH3 is the rate-determining step. After the dissociation, the mechanism shown in Scheme 10.4 commences. 

Brown and co-workers have continued their extensive studies of the mechanisms of hydroboration. Some of this work has recently been reviewed. Kinetic studies have been reported of the reaction of 9-borabicyclo[3.3.1]nonane dimer (3) with alkenes, alkynes, aldehydes, and ketones, in various solvents and with various Lewis bases and Br0nsted acids. Earlier studies suggested that the reaction of (3) with alkenes in a variety of solvents proceeded via the initial dissociation as in equation (4) to the monomer (4) followed by the hydro-... 

Study of the kinetics of the reaction of the dimer sym-tetrasiamyldiborane with olefins has established that the reaction is second order, first order in each component [11], The hydroboration reaction involves cis addition of the boron-hydrogen linkage to the double bond and the hydroboration reaction is very powerfully catalyzed by ether solvents. Consequently, it was proposed that the hydroboration reaction involved a transition state in which the ether solvent serves to solvate and stabilize the leaving disiamylborane group (Fig. 1) [1]. 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

The calculations discussed so far are for reaction of monomeric BH3 with alkenes in the gas phase. In solution the borane is most likely to be a dimer or, in ether solvents such as THF, a borane-solvent complex. It is difficult to study the kinetics of borane addition in solution because the reaction is complicated by three addition steps (one for each B-H bond), three redistribution equilibria (in which borane and the alkyl boranes exchange substituents), and five different monomer-dimer equilibria involving all the species with at least one B—H bond. In the hydroboration of 2,3-dimethyl-2-butene with diborane in THF, the reacting species is most likely a borane-THF complex. The reaction was foxmd to be second order overall, first order in alkene and first order in BH3-THF. The Eg was foimd to be 9.2kcal/mol, while the activation entropy was —27 eu. These results stand in contrast to the value of 2 kcal/mol determined for AH for the reaction of BH3 with ethene in the gas phase. °... 

Computational studies of the formation of ( )-5-stannyl homoallylic alcohols by an allene hydroboration-aldehyde allylboration sequence show that the kinetic allene hydroboration product is less stable and isomerizes to the more sterically congested a-stannylallylborane with the C—Sn to boron a—n hyperconjugation interaction sufficiently stabilizing to override the steric congestion (Scheme 77). " ... 

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). ... 

The hydroboration of alkynes with (9-BBN)2 exhibits kinetics similar to those for the hydroboration of alkenes. The kinetics are studied (1) by pumping the reaction mixture through a sodium chloride IR cell and monitoring the disappearance of B-H bridges of (9-BBN)2 at 1,570 cm" by quantitative IR spectrometry [Ij and (2) by quenching the aliquots of the reaction mixture periodically with excess methanol and analyzing by GLC for residual alkyne. For the more... 

Soderquist and coworkers have reported [19] that hydroboration with 9-BBN of 1-silylacetylenes, having bulkier groups on the Si, place the boron exclusively at the internal position of the alkyne as a kinetically less preferred process. Consequently, triisopropylsilyl-substituted alkynes afford the novel P-silylated vinyl-boranes, which on oxidation with alkaline hydrogen peroxide affords the corresponding ketones (Eq. 7.11) in excellent yields, a truly noteworthy example of the stability imparted to this functionality by the triisopropyl substitution on silicon [20]. It should be noted that earlier studies have shown failures to obtain (3-ketosilanes from suitable organoprecursors due to their known sensitivity to bases and nucleophiles [21]. 

B-alkyl-9-borabicyclo[3.3.1]nonanes undergo olefin-alkyl group exchange when refluxed with an olefin in THF. Kinetic and competition studies support a dehydroboration-hydroboration process rather than a concerted mechanism. Ab initio M.O. calculations show that the reaction between C2H4 and BH3 proceeds through a two-step process. A loose three-center (C—B—C) tt complex is formed which is then transformed into the product via a four-center transition state in a rate-determining step. 

Dilsopinocampheylborane derivatives continue to be attractive as reagents for enantloselective processes. Studies on the secondary kinetic isotope effect of deuterium on enantioselective hydroborations with (+)(Ipc)2BH have provided significant experimental evidence which provides a test for any detailed explanation of the process. The reagent has been used in a systematic study with representative heterocycles bearing an endocycllc double bond in order to establish the asymmetric induction achieved. It turns out that the reaction provides a simple and efficient method of synthesising heterocyclic boronates... 

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