Burgi-Dunitz angle

Attack by a nucleophile breaks the re-bond, and the electrons reside on the oxygen atom. This is energetically favourable, as a strong o-bond is formed at the expense of a weaker re-bond. Examination of crystal structures has shown that the nucleophile approaches the carbonyl carbon at an angle of around 107°. This is known as the Burgi-Dunitz angle (see Section 4.10). 

Fig. 5.6 The Salem-Klopman equation applied to the Burgi-Dunitz angle...

Figure 4.6. (a) Orientation of the superimposed carbonyl and nitrogen atoms, (b) Superimposed plot of the N, C, and O atoms of structures A-F, and the variance of the RRC plane from the RRO plane, a is the Burgi-Dunitz angle, 107°. Reprinted with permission from ref. [21], copyright 1973, American Chemical Society. 

Seebach suggested in 1981 [57] that the donor and acceptor are probably synclinal in the transition state. Steric repulsion between Ri and the donor, D, is proposed to orient Ri antiperiplanar to D pyramidalization cf. Figure 3.4 and ref. [150-153]) and tilting of the donor to accomodate the Burgi-Dunitz angle of 107°... 

Burgi-Dunitz angle of 107°, passing the least sterically hindering a-substituent in its approach (transition state E). In the double asymmetric reactions of a-chiral aldehydes with chiral allylboronates 1-3, the a-stereocenter and the chiral boronate auxiliary both influence the stereochemical outcome of the reaction. 

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