Page 214 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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500 Organic Chemistry, Synthesis

regioselective. Oxidation of the boron carbon bond with
hydrogen peroxide generates the desired alcohol: OH
H
O 3 HO 3 H O 2
3CH 3 CH CH 2 BH 3 (CH 3 CH 2 C 2 ) 3 B
H 2 O 2 /OH (17)
3CH 3 CH 2 CH 2 OH (13)
Treatment of ozone or hydrogen peroxide with very strong
When addition in a Markovnikov sense is required, acids, such as hydrogen fluoride–antimony pentafluoride
oxymercuration can be very effective and is not accom- orfluorosulfuricacid–antimonypentafluoride,isproposed
panied by rearrangements. Addition of an electrophilic to lead to the formation of protonated ozone or hydrogen
mercury species to an olefin, followed by trapping of peroxide, which are highly electrophilic reagents.
the intermediate cation, leads to the formation of an
oxygenated organomercurial. The carbon mercury bond
may be cleaved reductively with sodium borohydride: 4. Hydroxymethylation

OAc Bysimpleanalogywithhaloalkylationchemistry,alcohols
CH 3 CH 2 CH CH 2 Hg(OAc) 2 CH 3 CH 2 CH CH 2 HgOAc can also be introduced by carbon carbon bond forming
reactions. The addition of formaldehyde to a nucleophilic
OH
carbon such as the alpha carbon of an enol or organometal-
NaBH 4
CH 3 CH 2 CH CH 3 Hg (14)
lic reagent leads to hydroxymethylation:
MgBr CH 2 OH
2. Reduction of Carbonyl Compounds H /H 2 O

HCHO
Aldehydes, ketones, carboxylic acids, and carboxylic acid
derivatives can be reduced to alcohols. Historically, cat- (18)
alytic reduction with hydrogen and a catalyst such as plat-
Inadifferentapproach,treatmentofanalkylboroncom-
inum oxide or Raney nickel was employed to form the hy-
pound with carbon monoxide leads to carbon monoxide
droxylic product in high yields. More recently, the more
insertion in the alkyl carbon boron which can be reduced
convenient metal hydride reducing agents such as lithium
to form the hydroxymethyl group:
aluminumhydrideorsodiumborohydrideandtheirderiva-
tives have been employed to form alcohols at ambient or O
R H
subambient temperatures: BH 3 CO
3 C C (RCH 2 CH 2 ) 3 B (RCH 2 CH 2 ) 2 BCCH 2 CH 2 R
H H
O
OH LiBH 4 H 2 O RCH 2 CH 2 CH 2 OH (19)
NH 4 Cl/H 2 O
R R LiAlH 4 R H
The Williamson ether synthesis, in which treatment of
R (15) an alkyl halide with the alkali metal salt of an alcohol
yields an ether, appends an alkoxy chain to an alkyl halide.
A variety of additional methods exist for the reduction
of carbonyl compounds to alcohols. A typical example CH 3 I CH 3 CH 2 CH 2 CH 2 ONa
is the Meerwein–Pondorf–Verley reduction of carbonyls NaBr
CH 3 CH 2 CH 2 CH 2 OCH 3 (20)
with aluminum alkoxides:
OH Alkoxymethylation is possible by reaction of substituted
O ethers with an appropriate nucleophile.

H CH 3
Al(OCH(CH 3 ) 2 ) 3
C. Amination
(16)
Selective formation of amines is extremely important for
the synthesis of many natural products and biologically ac-
3. Electrophilic Oxygenation
tive compounds. The introduction of amines in the pres-
Contemporary work has focused on the development of ence of other functionality is a particularly challenging
methods for the electrophilic oxygenation of alkanes, synthetic problem. The selectivity required for practical
alkenes, and arenes: utility is not generally associated with the more rigorous