Page 23 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

1.1 NUCLEOPHILES AND ELECTROPHILES: THE S 2 PARADIGM
N 3
− − −
H , BH 4 , AIH 4
− − CO 2 Et
RLi , RMgBr , RC C , HC
CO 2 Et
− − +
N , R 3 P, CN , N
R 3 N NR
− − −
ROH, RO , HO , HOO
− − −
RSH , RSR′, RS , S C N S C N
− − − −
F , Cl , Br , I
Some common nucleophiles, with the nucleophilic atoms indicated in blue.
Figure 1.2

1
substitution. If this carbon atom is stereogenic, such an inversion of configuration may be
discerned experimentally, as in the example below; otherwise the inversion is not detectable,
even though it occurs.

−
− Br
(1.2)
−
RS Br RS
H
H
Me Me
′
Several common nucleophiles are depicted in Figure 1.2, where R and R denote alkyl
groups. Many of them are nitrogen-based, such as ammonia, amines (RNH ), and azide
2
−
−
(N ), or oxygen-based, such as water, alcohols (ROH), and alkoxide (RO ) and carboxy-
3
−
−
late (RCO ) anions. Sulfur-based nucleophiles such as thiols (RSH), thiolates (RS ), and
2
′
thioethers (RSR ) are also widely used in chemical synthesis. Triphenylphosphine, a phos-
phorus nucleophile, is an important reagent in organic synthesis, as well as an important
transition-metal ligand. Halide ions are widely employed as both nucleophiles and leaving
groups. Hydride is used both as a base (typically as NaH or KH) and as a nucleophile (often
− −
in complexed forms such as BH 4 or AlH ).
4
Carbon nucleophiles play a central role in organic chemistry, as they form the basis of
carbon–carbon bond formation. A few are shown in Figure 1.2, including such carbanionic
species as organolithiums (RLi), Grignard reagents (typically written as RMgBr), and the
− −
cyanide (CN ) and acetylide (R–C≡C ) anions. Other examples such as enolates, enols,
and enamines will be briefly discussed in Section 1.15.
Some common electrophiles are shown in Figure 1.3. These include protons and posi-
tively charged metal ions, electron-deficient species such as trivalent group 13 compounds
(e.g., BF ,AlCl ), the cationic carbon in carbocations, the halogen-bearing carbon in alkyl
3
3
1 A stereogenic center is an atom in a molecule for which interchanging any two of its substituents leads to a
different stereoisomer. The term was introduced by Mislow and Siegel in an important foundational paper on
modern stereochemical concepts and terminology: Mislow, K.; Siegel, J. J. Am. Chem. Soc. 1984, 106, 3319–3328.