Page 427 - Advanced Organic Chemistry Part B - Reactions & Synthesis

400 – – – –
+
+
+
+
Li HB( CHCH CH ) Li HB[ CHCH(CH 3 2 3 Na HB( CHCH CH ) K HB( CHCH CH )
) ]
3 3
2
3 3
2
2
3 3
CHAPTER 5
CH 3 CH 3 CH 3 CH 3
Reduction of L-selectride LS-selectride N-selectride K-selectride
Carbon-Carbon Multiple
Bonds, Carbonyl
Groups, and Other Derivatives of aluminum hydrides in which one or more of the hydrides is
Functional Groups
replaced by an alkoxide ion can be prepared by addition of the calculated amount of
the appropriate alcohol.
LiAlH + 2 ROH LiAlH (OR) + 2 H 2
2
2
4
LiAlH + 3 ROH LiAlH(OR) + 3 H 2
3
4
These reagents generally show increased solubility in organic solvents, particu-
larly at low temperatures, and are useful in certain selective reductions. 75 Lithium
tri-t-butoxyaluminum hydride and sodium bis-(2-methoxyethoxy)aluminum hydride
(Red-Al) 76 are examples of these types of reagents that have synthetic use. Their
reactivity toward carbonyl groups is summarized in Table 5.3.
Closely related to, but distinct from, the anionic boron and aluminum hydrides
are the neutral boron (borane, BH ) and aluminum (alane, AlH ) hydrides. These
3
3
molecules also contain hydrogen that can be transferred as hydride. Borane and alane
differ from the anionic hydrides in being electrophilic species by virtue of the vacant p
orbital and are Lewis acids. Reduction by these molecules occurs by an intramolecular
hydride transfer in a Lewis acid-base complex of the reactant and reductant.
+ –
O O MR 2 O MR 2
MH + C C H R C H
R 2
R R R R
R
Alkyl derivatives of boron and alane can function as reducing reagents in a similar
fashion. Two reagents of this type, disiamylborane and diisobutylaluminum hydride
(DiBAlH) are included in Table 5.3. The latter is an especially useful reagent.
Diborane also has a useful pattern of selectivity. It reduces carboxylic acids
to primary alcohols under mild conditions that leave esters unchanged. 77 Nitro and
cyano groups are relatively unreactive toward diborane. The rapid reaction between
carboxylic acids and diborane is the result of formation of a triacyloxyborane inter-
mediate by protonolysis of the B−H bonds. The resulting compound is essentially a
mixed anhydride of the carboxylic acid and boric acid in which the carbonyl groups
have enhanced reactivity toward borane or acetoxyborane.
3 RCO H + BH 3 (RCO 2) B + 3 H 2
2
3
O O
+ –
RC O B(O CR) 2 RC O B(O CR) 2
2
2
Diborane also reduces amides to amines (see Section 5.3.1.2).
75
J. Malek and M. Cerny, Synthesis, 217 (1972); J. Malek, Org. React., 34, 1 (1985).
76 Red-Al is a trademark of the Aldrich Chemical Company.
77
N. M. Yoon, C. S. Pak, H. C. Brown, S. Krishnamurthy, and T. P. Stocky, J. Org. Chem., 38, 2786
(1973).