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ñþ, A>°_C_CHl h2o x0_c_ch IF1
ArCHO + 2CH3COOH
(See Problem 6.19(/) for hydroboration.)
The vinyl Ñ with more H’s is converted into C=0. Alkenes can also be transformed into dialkyl carbonyls by a carbonylation-oxidation procedure.
1. ñî + h2o
2. H->0-j, NaOH
5. Oxidative Cleavages
Ozonolysis of alkenes (end of Section 6.4) and cleavage of glycols (Section 14,11) afford carbonyl compounds. These reactions, once used for structure determinations, have been superseded by spectral methods.
BY REDUCTIONS OF ACID DERIVATIVES, RCOX, OR NITRILES, RC=N
Acid chlorides, R(Ar)COCl, are reduced to R(Ar)CHO by H2/Pd(S), a moderate catalyst that does not reduce RCHO to RCH2OH (Rosenmund reduction). Acid chlorides, esters (R(Ar)COOR), and nitriles (RC^N) are reduced with lithium tri-;-butoxyaluminum hydride, LiAlH[OC(CH3)3]3, at very low temperatures, followed by H20. The net reaction is a displacement of X by :H_,
RCOC1 + :H“ —- RCHO + ÑÃ
See Section 16.3 for preparation of acid derivatives.
BY HYDROLYSIS AND HYDRATION OF COMPOUNDS AT —C=0 OXIDATION LEVEL
1. — CX2—, —C(OCOR)2, and — C(OR)2 (Acetal or Ketal)
These groupings are hydrolizable to the —C=0 group. (See Section 15.4 for acetal chemistry.)
See Section 8.2 for direct hydration and for net hydration through formation of vinylboranes by hydroboration.
Problem 15.9 Which is the only aldehyde that can be prepared by HgS04-catalyzed hydration of an alkyne? -4
Since the addition of H20 to C=C is Markovnikov regiospecific, RC==CH or RC^CR must give ketones. Only HC=CH is hydrated to give an aldehyde, CH3CHO.
BY FRIEDEL-CRAFT ACYLATIONS OR FORMYLA TIONS OF ARENES
Friedel-Crafts acylations of arenes with RCOCl or anhydrides (RC—Î—CR) in the presence of AlCl3 give good yields of ketones.
Problem 15.10 Suggest a mechanism for acylation of ArH with RCOCl in AlCl3. The mechanism is similar to that of alkylation:
(1) RCOCl+ AICI3--
(2) RC=6: + ArH
> RC=0: + AICI4 acylonium ion
Problem 15.11 Can formylation of an arene, ArH, with an acid chloride be employed to prepare ArCHO? M
No. The needed acid chloride is the hypothetical “formyl chloride,” HCOCl. But this compound cannot be realized; attempts to prepare it from formic acid (HCOOH + SOCl2) yield only mixtures of HCl and carbon monoxide, :C=0:.
Arenes can be formylated by generating the active intermediate, 'O^C—H, from reagents other than HCOCl. The Gatterman-Koch reaction uses a high-pressure gaseous mixture of CO and HCl.
AlCVCuC'l + ËãÍ
CO + HCl ----------- 0=C—H ---------- ArCHO
BY ACYLATION OR HYDROFORMYLA TION OF ALKENES
1. Oxo Process
This is an industrial hydroformylation for synthesizing aliphatic aldehydes, RCHO.
RCH=CH2 + CO + H2 —-----v RCH2—CH2CHO + RCH—CH3
Î R' 1 Î
11 1 -HCl II
This is a Markovnikov addition initiated by RC+=0:, an acylonium cation.
BY COUPLING REACTIONS
1. Carboxylic Acids and Their Derivatives, with Organometallics
(a) R'—Ñ—Cl + R2CuLi —> R'—Ñ—R (cf. Corey-House reaction, Section 4.3) Î Î
an acid chloride a ketone
Ô) |CftH51 MgBr + ( WN 2'h|o^\ /—jy—+nh4
--- 1 -------------------- 0
a nitrile cyclohexyl phenyl