Cyclic four-center transition

Addition takes place through the initial formation of a n complex, which changes into a cyclic four-center transition state with the boron atom adding to the less hindered carbon atom. The dashed bonds in the transition state represent bonds... 

STEP 1. Reaction of a nucleophile and an electrophile to form a new covalent bond. The addition of borane to an alkene is initiated by coordination of the vacant 2p orbital of boron (an electrophile) with the electron pair of the pi bond (a nucleophile). Chemists account for the stereoselectivity of hydroboration by proposing the formation of a cyclic, four-center transition state. Boron and hydro-... 

Independent interpretation of mass spectroscopic data by Wolovsky 66> and Ben-Efraim, Batich, and Wasserman 6 ) indicate that cyclic olefins subjected to disproportionation conditions form interlocking rings (catenanes) as well as cyclopolyolefins. The proposed scheme involves four-center transition state and Mobius-Strip approach... 

At this point in our studies we favor a five-membered cyclic transition state, quite analogous to the four-center transition state (Reaction 5), except that no o- bonds of the attacking electrophile are broken. The fourth bonding center is the carbon sp3 orbital as in structures 25 and 26. 

Higher temperatures and polar solvents are considered to switch the reaction mechanism of transmetalation from a four-centered transition state (Se2 (cyclic)) to a back-side attack of the palladium(II) complex (Se2 (open)) (Fig.1). 

The site-selective addition of nitrosyl chloride to an f-configurated double bond, determined by kinetic studies18 with acyclic and cyclic (E)- and (Z)-alkenes in chloroform or by the reactions with (Z,E, )-1,5,9-cyclododecatriene19 and (Z,E)-, 5-cyclodecadiene"fl in dichloromethane, can be explained in terms of conformational stability in the four-centered transition state or by a syn-type addition however, the configuration of the products of such reactions was not elucidated. 

The stereochemistry is also affected by the solvent polarity (Scheme 26). The reaction using (5)-l-phenyl-l-(trifluorosilyl)ethane of 38% ee at 60 °C resulted in retention (23% ee, S) in THF, but inversion (8% ee, R) in HMPA-THF (1 10). Higher temperature and polar solvents are considered to change the reaction mechanism of transmetallation from a four-centered transition state [Se2 (cyclic)] to a backside attack of the paUadium(II) complex [Se2 (open)] (Scheme 27). 

While the four-center transition structure for BH3 addition is a widely used model, other reaction pathways have also been considered. In a synthesis of optically active (-)-l-butanol-l-d, Streitwieser and co-workers used the optically active borane formed from diborane and (-l- )-a-pinene (R2BH) to carry out the hydroboration-oxidation of (Z)-l-butene-l-d. To explain the observed stereochemistry of the reduction, they proposed that hydroboration involves a n complex between R2BH and the alkene (Figure 9.41). Note the close resemblance of such a complex to the cyclic structures... 

It requires a four-membered transition state involving a ring of the past asymmetric center, the gegen ion, and the two atoms of the new monomer. The presence of the established asymmetric center in the cyclic four membered ring determines the stereochemistry of the new asymmetric center. Steric or electronic repulsion of the substituents in the transition state causes isotactic steric control. 

The mechanism of hydroboration involves a four-center, cyclic transition state. 

Why does alkene hydroboration take place with non-Markovnikov rc o-chemiatry, yielding the less highly substituted alcohol Hydroboration differs from many other alkene oddition reactions in that it occurs in a single step without a carlK>cation intemnediate. We can view the reaction as taking place through a four-center, cyclic transition state, as shown in Figure 7.6 Cp. 244 j. Since both 0-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry is observed. 

The mechanism and the stereoselectivity of the reaction7,11-13 are affected not only by the structure of the alkene, but also by the method of preparing the nitroso chloride and by the solvent. Different mechanisms may be followed two-stage anti addition (NO Cle), particularly in polar solvents such as sulfur dioxide, or syn molecular addition via a four-centered cyclic transition state, favored in low polarity solvents such as dichloromethane and carbon tetrachloride. 

The cis addition of nitrosyl chloride to substituted cyclic enol ethers and O-protected glycals, rationalized as either molecular addition through a four-centered polar transition state or by radical mechanism, generally takes place with complete simple and induced diastereose-lectivity45-47. 

Restricting consideration to sipafacial-suprafacial geometries, two limiting confbimations are possible for the six-membered cyclic transition state, a four-center or chair conformation with staggered allyl units which resembles chair cyclohexane, and a six-center or boat confomnation with... 

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