Brewster’s rules

Chiral allenylstannanes can be prepared by Sjv2 displacement of propargylic halides sulfinates or sulfonates with tin cuprates (Table 14)78. The nonracemic propargylic mesylate (74) afforded a nonracemic allene, [a]D —570, whose configuration was assigned by application of Brewster s rules (equation 38)78. Displacements on the steroidal mesylates 75 and 76 afforded the allenic products with complete inversion of configuration (Scheme 32)78. 

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 focus of the earlier chapter was a brief outline of the sources of chiral amino compounds, application of Brewster s rules for the assignment of the absolute configurations to a few chiral amines, a discussion of the ORD and CD in the visible (380-780 nm) and near-ultraviolet (200-380 nm) spectral regions of chiral amino compounds and some of their derivatives, and how the observed Cotton effects (CEs) in their ORD curves and CD spectra relate to their conformational preferences and absolute configurations. 

Furthermore it is interesting to notice that connecting monomeric units, the carbon atoms of which are ordered in the conformation with the highest optical activity according to Brewster s rules, one after the other in an isotactic manner, the conformation of the principal chain is of the right- or of the left-handed helix type (100). 

Allenylstannanes are readily prepared by Sn2 displacement of propargylic mesylates by stEmnyl cuprates (Table 46) [82]. When the reaction was performed on an enan-tioenriched (/ )-mesylate the allene was deduced on the basis of the optical rotation and by use of Brewster s rules to be of the (M) configuration (Eq. 66). Thus it was surmised that the displacement had proceeded with inversion of stereochemistry. 

A less clear-cut demonstration of the nucleophilicity of [Pt(PPh3)2] comes from its reaction with optically active 8-(a-bromoethyl)quinoline, as shown in (20). The configuration of the product was deduced by Brewster s rules (Sokolov, 1976). However, prior coordination of the nitrogen atom to the metal could predefine the pathway. 

Brewster s rules (J. H. Brewster, J.A.C.S., 1959, 81, 5475) are a set of semi-empirical rules relating molecular chirality to sign of molecular rotation. For a molecule containing a chiral centre Cabde, the sign of rotation is predicted from the polarizabilities of the four groups abde. This rule has been used to predict the absolute configurations of a number of deutero-compounds where d = and e= H. 

The Cotton effects of many olefins have been correlated with Mills and Brewster s rules, using the recently derived olefin octant rule. The latter fails, however, in the case of methylenesteroids, and it may be that some twisting is necessary in the olefin for satisfactory application of the rule. 

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