Transition state hydroboration

BBN+4-methylpent-2-ene C2 and 9-BBN+4-methylpent-2-ene C3 are transition states for hydroboration by 9-BBN at the C2 and C3 positions, respectively. Which transition state is the lower energy Calculate the ratio of major to minor regioproducts at room temperature. Is this reaction likely to be more or less regioselective than the corresponding reaction involving BH3 Explain your reasoning. 

Bond density surface for 9-BBN-i4-methylpent-2-ene at C2 reveals to what extent bonds are formed in hydroboration transition state. 

The transition state for the hydroboration requires that the boron atom and the hydrogen atom add to the same face of the double bond => a syn addition. 

The investigation of factors affecting facial selectivity in the hydroboration of steroidal -alkenes revealed the facial (a vs /3) stereoselectivities of hydroboration of androst-5-enes (69) and B-norandrost-5-enes (70) do not parallel the difference between the calculated force-field energies for a- and jS-cyclobutane models (71)-(74). This finding appears to suggest that the facial selectivity is not determined by the four-centre transition state but by the relative ease of formation of the initial tt-complex. ... 

Hydroboration is a sterospecific syn addition. The addition occurs through a four-center transition state with essentially simultaneous bonding to boron and hydrogen. Both the new C—B and C—H bonds are, therefore, formed from the same side of the double bond. In molecular orbital terms, the addition is viewed as taking place by interaction of the filled alkene n orbital with the empty p orbital on boron, accompanied by concerted... 

This is attributed to the unfavourable steric interactions which arise in the transition state that is required for antiperiplanar migration of the exocyclic substituent.143 Some examples of synthesis of alcohols by hydroboration-oxidation are included in Scheme 4.8. More vigorous oxidizing agents such as Cr(VT) reagents effect replacement of boron... 

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). 

Correlations of ionization potentials (IP) versus relative reactivities of a variety of alkenes towards bromination, oxymercuration and hydroboration clearly show that the reaction rate decreases as the IP is increased29 the transition states of the ratedetermining steps of oxymercuration and hydroboration are similar, but different from that of bromination29. 

The hydroboration of acetophenone in the chiral solvent (S)-methyl lactate exhibits moderate enantioselectivities. A six-membered transition state (11) involving the ketone, the borane, and the lactate as the only chiral source is proposed. Molecular modeling explains the experimentally observed enantioselectivities.311... 

Borane ). This reagent is commercially available or prepared by hydroboration of (-)-a-pinene (16) with 9-BBN (17).10 The stereoselectivity of carbonyl group reduction with (S)-Alpine Borane is explained via six-membered transition state 18. 

MM has been used to study the transition states involved in SN2 reactions, hydroborations, cycloadditions (mainly the Diels-Alder reaction), the Cope and Claisen rearrangements, hydrogen transfer, esterification, nucleophilic addition to... 

The hydroboration occurs from the less hindered face (i.e., the convex face). Transition state B is destabilized by the electron-withdrawing effect of oxygen. 

Hydroboration-reduction of enones.2 Hydride reduction of a carbonyl group can be used to induce asymmetric intramolecular hydroboration of a double bond via a cyclic transition state. Thus reaction of the enone 1 with thexylborane (1 equiv.) followed by oxidation provides the 1,5-diol 2 with high 1,4-syn selectivity. A similar reaction with the homologous enone provides a 1,6-diol with modest 1.5-syn selectivity (syn anti = 6.6 1). 

Thus hydroboration is not a pericyclic reaction, because the boron atom makes use of two AOs. Similarly, the reaction between a carbene and a double bond is not pericyclic because the carbon atom uses two AOs. Cheletropic reactions are not pericyclic either. Consider, for example, the fragmentation of 3-cyclopentenone to give CO and butadiene. Not only does the expelled carbon atom use two AOs to bond with its neighbours, but also the oxygen atom, which is an intervening atom (it was initially linked to the carbon atom by a double bond, which becomes a triple bond in CO) is exocyclic in the transition state. 

No more than a hydroboration is The transition state for a Woodward-Hoffmann-allowed reaction is aromatic in character, whereas a forbidden one is antiaromatic. However, the rules of aromaticity only apply when each atom of the annulene uses one, and only one, AO to bind to its neighbors. This is not the case here each fluorine atom uses two AOs, one for bonding to the other F atom and one for the incipient bond to C. 

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]. 

The characteristic features of hydroboration are consistent with a concerted four-centre transition state carrying charges on the participating atoms (Figure Bl.l) and this model adequately rationalizes the majority of hydroboration results. 

The efficiency of asymmetric hydroboration is high if one approach trajectory leads to severe steric interactions between the hydroborating reagent and the alkene and the approach trajectory to the other face of the alkene involves relatively insignificant steric interactions, i.e. the energy difference between the two transition states is large. It should be noted, however, that if both approaches involve major steric interactions then a decrease in overall reactivity will be observed. 

The two transition states for the addition of a homochiral hydroborating reagent to the two faces of a prochiral alkene are diastereoisomeric and of different energy. 

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