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

R"' n n
X/X_J 2 ~ ~
Organoboron compounds
( i) RCH=CH2 + TlX3 --- CH-CH -Tl-X --- RCHCH 2X + TlX +X-
o
e.g.
+ TL(OAo) 3
AcOH
25°
CXOAo
O Ac
(50%) + other products.
(ii) ArCH=CHCHO
ArCH=CHCH(OMe) 2
/CH(OMe) 2
ArCH
"'-cH (0Me) 2
Fig. 3.17 Two pathways for thallation of alkenes.
(b) THALLATION. Thallium(III) is a rather stronger oxidizing agent than
mercury(II). Ee(TP+;rl+)=l.25V; Ee(Hg 2 +/Hg)=0.8SV. Consequently
thallation of alkenes is often followed by oxidation (loss of thallium(I) ). Severa!
pathways have been recognized, of which only two are mentioned here.
Thallation may be accompanied by l, 2-migration of substituents on elimination
of TlX.
3.8 Organoboron compounds
Mononuclear organoboron compounds are of two types. First there are those
which contain three-coordinate trigonal planar boron atoms. These include the
trialkyl and triarylboranes, R 3 B, and numerous derivatives R"BX 3 "(n = 1-3) in
which R is formally replaced by a monovalent group X (X = halogen, OH, OR',
SR', NR~). Generally these compounds are monomeric (contrast the Al analogues),
but when X= H, dimerization through three-centre B-H-B bridges is
normally observed. The boron atom has an empty 2p orbita!. which may to a
greater or lesser extent take part in n-bonding with substituents such as
-NR~. OR' or F (p. 73).
Trialkylboranes can be prepared from the reaction of Grignard, organolithium
65