Allene geometry

The potential carbodicarbene C(C(NMe2)2 2 has been known for a long time but no complex has been reported [101, 102]. It adopts a linear allene geometry in the free state but according to theoretical analysis exhibits a strong nucleophilic central carbon atom [4, 97] and can be seen as an allene with a hidden divalent carbon(O) character emerging in the presence of electron deficient electrophiles. Based on these findings a new field of chemistry will be opened and the number of compounds with a coordination mode should increase in the future. 

In this case, the reversible coordination of tin with aldehyde favors the closed transition state for Felkin-Anh addition in 339, whereas the jS-chelation model of 340 introduces destabilizing steric interactions owing to placement of the methyl substituent of the chiral allene. Transmetalations of chiral allenylstannanes with InBrs occur with net retention of allene geometry (Scheme 5.2.64). Thus, the starting (P)-286 can also be utilized for a stereoselective reaction with the corresponding (5 )-aldehyde (Scheme 5.2.72, bottom). The enantiomeric alcohol ent-337 is produced via the closed transition state... 

Fig. 14. Hiickel energy level diagrams for the two allene geometries of 9 and 10...

We have emphasized this point elsewhere - in connection, not only with the allene geometry, but also in connection with the geometries of both main group and transition metal complexes. In terms of moments it is those walks of the type shown in 12 and 15 which allow one ligand orbital to see another via a central atom orbital which destabilize the busy orbital geometry at the half filled point. For other electron counts however, the busy/idle orbital combination may well be stable as indicated both by Schleyer s electron deficient allenes and the transition metal oxo examples. 20 and 21... 

Structural studies show allene to be nonplanar As Figure 10 7 illustrates the plane of one HCH unit is perpendicular to the plane of the other Figure 10 7 also portrays the reason for the molecular geometry of allene The 2p orbital of each of the terminal car bons overlaps with a different 2p orbital of the central carbon Because the 2p orbitals of the central carbon are perpendicular to each other the perpendicular nature of the two HCH units follows naturally... 

FIGURE 10 7 Bonding and geometry in 1 2 propa diene (allene) The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases... 

Because of the linear geometry required of cumulated dienes cyclic allenes like cycloalkynes are strained unless the rings are fairly large 1 2 Cyclononadiene is the smallest cyclic allene that is sufficiently stable to be isolated and stored conveniently... 

Allen, W. D., Csaszar, A. G., 1993, On the Ab Initio Determination of Higher-Order Force Constants at Nonstationary Geometries , J. Chem. Phys., 98, 2983. 

Fig. 15 The experimental geometries of allene- -HC1 and allene- -C1F, drawn to scale. The n-electron model for allene is also shown. The angles 2- - H and 2- - Cl, respectively, where is the centre of the C C bond, are both close to 90°, as required by rule 2. The hydrogen and halogen bonds both show small non-linearities. See Fig. 1 for key to the colour coding of atoms... 

These reactions point to the polar character of 179-Za and 293-Za. Already the first calculations resulted in a virtually planar geometry and thus in the conclusion that 179-Zj and likewise 293-Zj are the ground states [144]. In consequence, the allene structures 179 and 293 are not energy minima. Utilizing a higher level of theory,... 

Free radical addition of HBr to buta-1,2-diene (lb) affords dibromides exo-6b, (E)-6b and (Z)-6b, which consistently originate from Br addition to the central allene carbon atom [37]. The fact that the internal olefins (E)-6b and (Z)-6b dominate among the reaction products points to a thermodynamic control of the termination step (see below). The geometry of the major product (Z)-(6b) has been correlated with that of the preferred structure of intermediate 7b. The latter, in turn, has been deduced from an investigation of the configurational stability of the (Z)-methylallyl radical (Z)-8, which isomerizes with a rate constant of kiso=102s 1 (-130 °C) to the less strained E-stereoisomer (fc)-8 (Scheme 11.4) [38]. 

With 141, the allenic Jt-bond that is distal to the ester group added to furan spontaneously upon concentration [122]. The observed site-selective addition can be attributed to steric reasons. The transition state geometry 143 is destabilized by a repulsive non-bonding interaction between H-3 and the exo-methylene group. 

---