Page 813 - Advanced Organic Chemistry Part B - Reactions & Synthesis

In this procedure, dichloromethyl methyl ether is used as the source of the carbonyl 789
carbon. 13
SECTION 9.1
1) Cl CHOCH ,
2 3 O Organoboron
5 3
RCH CH R'CH CH 2 CH CHR (C 2 H ) CO – Compounds
2
IpcBCl 2 IpcBCH CH R IpcB RCH 2 CH CCH CHR′
2 2 2 2 2
(CH ) SiH LiAIH CHR′ 2) CH CH O
3
3 3
Cl 4 CH 2 3) H 2 O , OAc
–
2
Scheme 9.1 shows several examples of one-carbon homologations involving boron
to carbon migration. Entry 1 illustrates the synthesis of a symmetrical tertiary alcohol.
Entry 2 involves interception of the intermediate after the first migration by reduction.
Acid then induces a second migration. This sequence affords secondary alcohols.
O – R OH X
) + O
LiAlH(OCH 3 3 H H 2 2
R 3 B C O R 3 B CHR B CH RB-CHR 2 R CHOH
2
– OH
R R
Entries 3 to 5 show the use of alternative sources of the one carbon unit. In Entry 3,
a tertiary alcohol is formed with one of the alkyl groups being derived from the
dithioacetal reagent. Related procedures have been developed for ketones and tertiary
alcohols using 2-lithio-2-alkyl-1,3-benzodithiole as the source of the linking carbon. 14
Problem 9.3 deals with the mechanisms of these reactions.
Section B of the Scheme 9.1 shows several procedures for the synthesis of ketones.
Entry 6 is the synthesis of a symmetrical ketone by carbonylation. Entry 7 illustrates
the synthesis of an unsymmetrical ketone by the thexylborane method and also demon-
strates the use of a functionalized olefin. Entries 8 to 10 illustrate synthesis of ketones
by the cyanide-TFAA method. Entry 11 shows the synthesis of a bicyclic ketone
involving intramolecular hydroboration of 1,5-cyclooctadiene. Entry 12 is another ring
closure, generating a potential steroid precursor.
Section C illustrates the synthesis of aldehydes by boron homologation. Entry 13
is an example of synthesis of an aldehyde from an alkene using 9-BBN for hydro-
boration. Entry 14 illustrates an efficient process for one-carbon homologation to
aldehydes that is based on cyclic boronate esters. These can be prepared by hydro-
boration of an alkene with dibromoborane, followed by conversion of the dibromo-
borane to the cyclic boronate. The homologation step is carried out by addition of
methoxy(phenylthio)methyllithium to the boronate. The migration step is induced by
mercuric ion. Use of chiral boranes and boronates leads to products containing groups
of retained configuration. 15

Me SiO(CH ) OSiMe 3 O
3
2 3
RCH CH 2 + HBBr 2 RCH 2 CH BBr 2 RCH CH B + LiCHOCH 3
2
2
2
O
SPh
H O
H O , pH 8 O Hg 2+
2
2
CH CH –
RCH 2 2 O RCH 2 CH C B RCH 2 CH B O
2
2
O CHSPh
O
CH 3
CH O
3
13 H. C. Brown, S. V. Kulkarni, U. S. Racherla, and U. P. Dhokte, J. Org. Chem., 63, 7030 (1998).
14 S. Ncube, A. Pelter, and K. Smith, Tetrahedron Lett., 1893, 1895 (1979).
15
M. V. Rangaishenvi, B. Singaram, and H. C. Brown, J. Org. Chem., 56, 3286 (1991).