Allenes rotation

Tius and co-workers elegantly applied a variant of the Nazarov reaction to the preparation of cyclopentenone prostaglandins (Scheme 19.39) [46]. Moreover, it was demonstrated that the chirality of non-racemic allenes is transferred to an sp3-hybridized carbon atom. Preparation of allenic morpholinoamide 214 and resolution of the enantiomers by chiral HPLC provided (-)- and (+)-214. Compound (-)-214 was exposed to the vinyllithium species 215 to afford a presumed intermediate which was not observed but spontaneously cyclized to give (+)- and (—)-216 as a 5 1 mixture. Compound (+)-216 was obtained with an 84% transfer of chiral information and (-)-216 was obtained in 64% ee. The lower enantiomeric excess of (—)-216 indicates that some Z to E isomerization took place. This was validated by the conversion of 216 to 217, where the absolute configuration was established. The stereochemical outcome of this reaction has been explained by conrotatory cyclization of 218 in which the distal group on the allene rotates away from the alkene to give 216. 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

The property of chirality is determined by overall molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. The examples in Scheme 2.2 include allenes (entries 1 and 2) and spiranes (entries 7 and 8). Entries 3 and 4 are examples of separable chiral atropisomers in which the barrier to rotation results from steric restriction of rotation of the bond between the aiyl rings. The chirality of -cyclooctene and Z, -cyclooctadiene is also dependent on restricted rotation. Manipulation of a molecular model will illustrate that each of these molecules can be converted into its enantiomer by a rotational process by which the ring is turned inside-out. ... 

As the pump shaft rotates, the shaft packing rotates with it due to friction. (In more complex mechanical seals, the shaft-packing element is secured to the rotating shaft by Allen screws.) There is also friction between the spring. 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

Butler and co-workers have taken a unique approach to study the unimolecular dissociation of the vibrationally and rotationally hot allyl radical.150-152 They have examined the secondary C-H dissociation of the allyl radicals that are produced with high internal energies above the allyl dissociation thresholds in the primary photodissociation of allyl chloride and allyl iodide at 193 nm. The production of allene versus propyne (both at mass 40) from the secondary dissociation of the hot allyl radicals are... 

Fig. 4 Ground-state and excited-state energies of the TICT complexes thioflavin T (a) and 9-(dicyanovinyl)-julolidine (DCVJ) (b) as a function of the intramolecular rotation angle (data from Stsiapura et al. [13] and Allen et al. [14]). In both cases, energy levels were determined by quantum mechanical simulations. For thioflavin T, the energy difference between Si and S0 corresponds to approximately 400 nm in the planar state and 470 nm in the twisted state. In the case of DCVJ, the energy differences correspond to 310 and 960 nm, respectively. The DCVJ energy levels reflect a rotation around the vinyl double bond...

As shown in Fig. 10, the lA2 state is predicted to be the lowest singlet state of 4a.55,57 The CASSCF vibrational analysis reveals one imaginary frequency for this state, corresponding to an out-of-plane a2 vibration, which leads to the cyclic allene 3a.57 Thus,1 A2-4a is a transition state for the enantiomerization of 3a. The analogous process of internal rotation about the C-C bonds in allene is also predicted to occur via an open-shell1A2 transition state.72... 

As an example, which will also lead us to the concept of qualitative completeness, consider the allene skeleton, as shown in Fig. 8, and for the moment consider only achiral ligands. Besides the unit element, the symmetry group of the skeleton, "Dm, consists of the rotation operations (in permutation group notation) (12)(34), (13)(24), and (14)(23), plus the improper rotations (1)(2)(34), (12)(3)(4), (1324), and (1432). 

For readers unfamiliar with these techniques, it might be helpful at this point to work out an example in some detail. We choose that of the allene skeleton, already discussed somewhat in this section, and at first we limit ourselves to achiral ligands, so that G = S4. The character table for S4 is shown in Table 1. In this case, the subgroup is just D2a, and its rotational subgroup is D2. Table 2 shows the classes of T>za, the number of elements in each, the class of S4 and of S4 to which each belongs, and the character of each for the representation, T< >. 

Aminoallenes constitute an important class of functionalized allenes with interesting chemical properties. They are known as attractive substrates for constructing three- to six-membered azacycles [78]. In 1999, Ohno and co-workers reported the stereoselective synthesis of chiral a-aminoallenes 179 and 181 by RCu(CN)M-medi-ated anti-SN2 substitution of chiral 2-ethynylaziridines 178 and 180 (Scheme 4.47) [79]. The X-ray data and specific rotations of the allenes were consistent with a net anti-S- 2 substitution reaction. 

Trialkyl- or triarylallenyltin compounds can also be prepared by Sn2 displacement of propargylic mesylates with various stannylcopper reagents in THF (Eq. 9.82) [71]. This reaction is postulated to proceed by an anti Sn2 pathway based on the stereochemical relationship between the enantioenriched mesylate and the allenic product (Eq. 9.83). The allene obtained from the reaction of the mesylate of (R)-3-phenyl-l-propyn-3-ol with Ph3SnCu was assigned the (P) configuration from a consideration of the observed optical rotation and an application of Brewster s rules [71]. 

The [2 + 2]-cycloaddition of allene proceeds via a stepwise diradical mechanism rather than a concerted one-step mechanism. The allenes come together in a crossed configuration. The bond formation between the central sp carbon atoms is accompanied by a simultaneous conrotatory twisting leading to a perpendicular 2,2 -bisallyl diradical 3. Rotation about the central bond of 3 gives the planar diradical and a disrotatory closure leads to the formation of dimer 2. The stereochemistry of some of the following examples is explained by this mechanism. 

The benzannulated analog 115 was likewise synthesized from 114 (Scheme 20.24) [56, 63], However, unlike 109, thermolysis of 115 resulted in its slow decomposition without the formation of the cycloaromatized adduct 116. The lack of propensity for 115 to undergo the Myers-Saito cyclization reaction was attributed to unfavorable steric interactions between the diphenylphosphinyl group and the aryl ring of the benzannulated enyne-allene system, causing the allenic moiety to be rotated out of the plane defined by the aryl ring and preventing the cyclization reaction. 

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