Helical Descriptors—M and

As another example, consider triphenylborane (Eq. 6.1, where a, b, and c are just labels of hydrogens so that you can keep track of the rotations shown). Triphenylborane cannot be fully planar because of steric crowding, and so it adopts a conformation with all three rings twisted in the same direction, making a right- or left-handed propeller. The M or P descrip- 

Rotation about the C-B bonds of triphenylborane is relatively facile, and the motions of the rings are correlated in the sense shown (Eq. 6.1). In Eq. 6.1 the arrows denote the direction of bond rotation, not the helical direction. Two rings rotate through a perpendicular conformation while one moves in the opposite way. This two-ring flip reverses helicity and, in a substituted case (now a, b, and c in Eq. 6.1 are substituents), creates a new diastereomer. 

---

The 2,3-dihydro-l //-l, 4-bcnzodiazepin-2-ones exist as an equilibrium mixture of two conformers, designated by the helical descriptors M and P, that reflect chirality associated with the 7 -N(l)-C(7)-C(8) dihedral angle (Scheme 1). The interconversion barrier between the M- and P- atropoisomers is dependent upon the size of the N-l substituent with AG = 12.3, 18.0, 21.1, and >24 kcal mob1 for H, Me, /-propyl, and /-butyl, respectively, a function of increasing... 

The first step for a successfid memory of chirality reaction should involve deprotonation of the stereogenic center in the enantiopure reactant (S)-substrate forming a conformationally chiral reactive (Af)-intermediate with high enantioselectivity. The helical descriptors (A/)- and Py are used to describe the chirality of the intermediates (see section 1.3.8). Most importantly, the conformationally chiral (M)-intermediate should not racemize easily on the time scale of the desired subsequent reaction. The final criterion is that the conformationally chiral intermediate must... 

M-), (P-) Stereochemical descriptors (M = minus, P = plus) introduced to describe the chirality of helical molecules. Extension of the CIP system to planar chirality gave an alternative description aR/aS for helical molecules such as helicenes, aR invariably = (M) and aS = (P), but for compounds showing planar chirality the reverse, with pR = (P) and pS = (M). Best avoided. See Section 7.3.2. 

Figure 1. Top P2]2i2i-unit cell (a = 9.535(2), b = 9.782(2), c = 16.428(3) A) of 15, left- and right-handed arrangement middle left-handed (A/) and right-handed helix P) of 15 where M and P are descriptors for helicity (M = minus and P = plus) bottom (l/ ,45)-16 and (15,4/f)-16.

In the section on "Helical Descriptors" (part of Section 6.1.2), we showed an allene and two related structures and gave M / P assignments. Show that the same assignments are obtained if you sight down the opposite end of the axis shown. 

Screw-like structures are found above all for molecules such as helicenes, 1,3-disubstituted allenes, 2,2 -disubstituted biphenyls, etc., displaying C2 symmetry. With the aid of the 4-point figures indicated in the formulae the absolute helicities (P or M) can straightforwardly be ascertained. The currently still customary descriptors aS (or Sa) and aP (or Pa) regrettably correspond only inversly to the much more generally applicable helicity symbols P and M. 

The author is well aware of the fact that many chemists still advocate use of the traditional RIS symbols also for at least some, if not all, compound types with helical stereogenic units, even if these are not in concordance with the proper helicity descriptors as in the case of axially chiral compounds where A M and A P. For the sake of consistency,... 

P and M descriptors a helical, propeller or screw-shaped structure (e.g. S has a helical chain) can be right- or left-handed and is termed P ( plus ) or M ( minus ), respectively. This is illustrated with P)- and (M)-hexahelicene ... 

---