Biirgi-Dunitz trajectory

The hydride nucleophile prefers to attack the carbonyl group of the borane complex IBBU2 from the top face following the Biirgi-Dunitz trajectory.3 The... 

Any other portions of the molecule that get in the way of (or, in other words, that cause steric hindrance to) the Biirgi-Dunitz trajectory will greatly reduce the rate of addition and this is another reason why aldehydes are more reactive than ketones. The importance of tire Biirgi-Dunitz trajectory will become more evident later—particularly in Chapter 34. 

Although we now know precisely from which direction the nucleophile attacks the C=0 group, this is not always easy to represent when we draw curly arrows. As long as you bear the Biirgi-Dunitz trajectory in mind, you are quite at liberty to write any of the variants shown here, among others. 

Biirgi-Dunitz trajectory The angle of approach of a nucleophile toward a carbonyl carbon, 107° (probably more accurately 105 5°) [78-80]. See Section 4.1. 

CHCH2M moieties would favor b over c and e over /. Then, assuming that the nucleophile approaches the carbonyl along the Biirgi-Dunitz trajectory (Section... 

Figure 5.9. Pyramidalization of the donor and the Biirgi-Dunitz trajectory contribute to destabilization of the Ik topicity combination according to Seebach [57].

Fukui et al. studied the bond interchange in the course of ammonia addition to formaldehyde by means of a localized molecular orbital method, based on the analysis of the semiempirical INDO molecular orbitals of the isolated molecules and of the reacting system [127], Here, the N... C = 0 angle was fixed at 107°, since preliminary calculations gave results in fairly good agreement with the experimental reaction path, i.e. the Biirgi-Dunitz trajectory. 

The actual experimental results can be explained by using models 290 and 291, the two envelope conformations available to 2-substituted cyclopentanones (sec. 1.5.B). In 290, approach of a reagent to the carbonyl at an angle of 110° (the Biirgi-Dunitz trajectory-sec. 6.6.A)285 leads to an interaction with Ha (path b), making path a preferred and this leads to the trans product, which appears to be contradicted by inspection of 291, where approach via path a is inhibited by the R group. Since the R group will be pseudo-equatorial in the... 

In a classic 1976 paper, Baldwin used the Biirgi-Dunitz trajectory to define the well-known rules for the design of cyclizations (the Baldwin rules). However, stereoelectronic factors for a bond formation to alkynes have been controversial. Originally, the rules for alkyne cyclizations were based on the assumption that nucleophiles add to alkynes along an acnte trajectory, instead of the obtuse Biirgi-Dunitz angle of attack. Subsequent experimental and computational analysis suggested that this trajectory is stereoelectronically... 

Figure 9.7. A depiction of the presumed favored direction (the Biirgi-Dunitz trajectory ) of attack of an electron rich species on the carbon of the carbonyl (C=0) (Biirgi, H. B. Dunitz, J. D. Shefter, E. J. Am. Chem. Soc., 1973, 95,5065).

Fig. 24 Scheme of the Biirgi-Dunitz trajectory of addition of a nucleophile N to a carbonyl carbon atom and representation of the n—>jt interaction in conformers gGl(a) and gG3(c) of GABA. (From [157])... 

The reactants first reach the coordination complex 40, a local minimum on the energy hypersurface [94]. In this complex the O-Li-O angle can vary from 145° to 180°. The transition state of carbon-carbon bond formation is calculated to have the half-chair conformation 41. The angle of nucleophilic attack on the carbonyl group is 106.9°, consistent vith the Biirgi-Dunitz trajectory [95] and in accordance vith calculations of Houk and co vorkers [92]. The transition state structure 41 finally collapses to the aldolate 42 vith the lithium atom coordinating the t vo oxygen atoms. The activation barrier of the reaction is calculated to be 1.9 kcal moH and the overall exother-micity is 40.2 kcal moH. ... 

As a reminder, the Felkin-Ahn model can be illustrated with aldehyde 55. The largest substitutent (R) is perpendicular to the C=0 bond, and the nucleophile attacks the carbonyl group at an angle (Biirgi-Dunitz trajectory), eclipsing the sterically least demanding bond (H-C) as in transition state 56. 

---