Chirality at Atoms Other Than Carbon

Since the most common cause of chirality is the presence of four different substituents bonded to a tetrahedral atom, tetrahedral atoms other than carbon can also be chirality centers. Silicon, nitrogen, phosphorus, and suit fur are all commonly encountered in organic molecules, and all can be chin rality centers under the proper circumstances. We know, for example, thal trivalent nitrogen is tetrahedral, with its lone pair of electrons acting a4 the fourth substituent (Section 1.11). Is trivalent nitrogen chiral Does compound such as ethylmethylamine exist as a pair of enantiomers j 

The answer is both yes and no. Yes in principle, but no in practia Trivalent nitrogen compounds undergo a rapid umbrella-like inversion tha interconverts enantiomers. We therefore can t isolate individual enan tiomers except in special cases. 

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V. Chirality at atoms other than carbon (Section 9.12). 

Section 7 16 Atoms other than carbon can be chirality centers Examples include those based on tetracoordmate silicon and Incoordinate sulfur as the chirality center In principle Incoordinate nitrogen can be a chirality center m compounds of the type N(x y z) where x y and z are different but inversion of the nitrogen pyramid is so fast that racemization occurs vrr tually instantly at room temperature... 

Phosphines metallated at remote atoms other than carbon have also been applied in synthesis, including the bis(phosphinophenyl)amide salt (44), used to prepare various uranium complexes, N-metallated 2-phosphi-noindoles, used to prepare new chiral hybrid phosphino-phosphito... 

The diastereoselectivity in the ene reaction of O2 with chiral alkenes bearing a stereogenic centre at the a-position with respect to the double bond has been extensively studied. Chiral alkenes which bear a substituent on the asymmetric carbon atom other than the hydroxy or amine functionality afford predominately erythro allylic hydroperoxides. The erythro selectivity was attributed to steric and electronic repulsions between... 

Since there are two possible configurations for an asymmetrically substituted carbon atom, a structure containing n such centres will, in theory, possess 2 stereoisomers. The actual number of stereoisomers that exist may be less than this due to steric effects. Compounds that have the same stereochemistry at one chiral centre but different stereochemistry at the others are known as diastereoisomers (diastereomers) a good example is given by the alkaloids ephedrine and pseudoephedrine. Ephedrine (the (1R, 2S) diastereoisomer) is a natural product isolated from Ephedra (the Ma Huang plant) and known to Chinese medicine for over 3000 years. It was used in the last century for the treatment of asthma. Pseudoephedrine (the (IS, 2S) diastereoisomer) is a decongestant and a constituent of several over-the-counter cold and flu remedies (Figure 4.12). 

In organisms ranging from bacteria to humans, there are 20 common amino acids that share structural and stereochemical motifs. In each amino acid there is a central carbon atom bonded to a hydrogen atom, an amine, and a carboxylic acid. At neutral pH, the amines and carbo>qrlic acids exist as ammonium ions and carboxylates, respectively. In addition, for 19 of the 20 amino acids, there is a fourth group on the central carbon other than hydrogen, referred to as a"side chain". The central carbon of these 19 amino acids is therefore a chiral center, and all 19 natural... 

Consider 7.53 and its enantiomer ent-7.53. These are clearly related as object and mirror image and, in the enantiomer, chiraUty, is reversed at each of the two asymmetric carbon atoms. What is the relationship between these two molecules and the enantiomeric pair 7.54 and ent-7.54 Remembering what we said about stereoisomers at the start of the chapter, the relationship between 7.53 and 7.54 is that of diastereoisomers—all relationships that are not enantiomeric are diastereo-isomeric. Diastereoisomers are different compounds, with different chemical and physical properties. Enantiomers have the same physical properties other than their ability to rotate the plane of plane-polarized light and the same chemical reactivity unless they are reacting with a chiral reagent. 

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