_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The first three periodic groups
This provides a useful route to unsymmetrical ketones, or to aldehydes starting
from lithium formate. (Grignard reagents do not react with carboxylate anions,
apart from formates.) Aldehydes can also be obtained from formate esters or,
better, orthoformate esters under mild conditions.
Organolithium reagents generally add in 1, 2-fashion to af3-unsaturated
carbonyl compounds. This addition takes place rapidly at low temperatures and is
kinetically controlled.
The lithium adduct (A) is formed irreversibly; lithium is strongly bound to
oxygen. With the Grignard reagent the formation of (A) is reversible and the
thermodynamically more stable product (B) gradually accumulates. Often a
mixture of 1, 2- and 1, 4-addition products is obtained. Steric effects are
important, favouring 1, 4-addition. Essentially exclusive 1, 4-addition results
from the use of organocopper reagents or copper catalysts (p. 89) (Fig. 3.9).
(b) 'UMPOLUNG'. The carbonyl group in aldehydes and ketones is polarized
b+ 0-
:=;c=O. It is therefore susceptible to nucleophilic attack at carbon. Methods ha ve
been developed by which this polarization is effectively reversed so that the
carbon atom itself becomes the nucleophilic centre. Such an inversion is known
as 'umpolung'. An example is provided by the 1, 3-dithiane system. An aldehyde
may be converted into a cyclic dithioacetal by reaction with propane-1, 3-dithiol
in the presence of an acid. The two adjacent electronegative sulphur atoms make
the .)c-H bond of this acetal rather acidic. Treatment with butyllithium
therefore affords a lithio derivative in which the carbon atom is susceptible to
electrophilic attack. The 1, 3-dithiane system is reconverted into a carbonyl group
by acid hydrolysis in the presence of mercury(II) ions, which complex with the
dithiol. The RCO group in the original aldehyde is thus 'equivalent' to R-C=O
(Fig. 3.10).
(c) ADDITION TO ALKENES. The first observation of the addition of an
organometallic compound to an alkene was ma de by Ziegler in 1927.
Ph 2 CMe·K+ + PhCH=CHPh----+ Ph 2 C(Me)-CH(Ph)-CHPh·K+
red
yellow
Normally, however, alkyl sodium and potassium reagents metallate alkenes
rather than add to them. Organolithium or Grignard reagents are also unreactive
towards C-C multiple bonds. Vigorous conditions are needed to cause
organolithiums to add to 1-alkenes unless the latter are activated e.g.
46
R'Li
CS CH 2R'
'1-
s