Ketone hydroboration-oxidation

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Hydroboration-oxidation of an internal alkyne forms a ketone hydroboration-oxidation of a terminal alkyne forms an aldehyde. Either BH3 or R2BH can be used for internal alkynes R2BH should be used for terminal alkynes. 

The hydroboration/oxidation sequence is complementary to the direct, mercury(ll)-catalyzed hydration reaction of a terminal alkyne because different products result. Direct hydration with aqueous acid and mercury(IJ) sulfate leads to a methyl ketone, whereas hydroboration/oxidation of the same terminal alkyne leads to an aldehyde. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Hydroboration-oxidation of 1,4-di-f-butylcyclohexene gave three alcohols 9-A (77%), 9-B (20%), and 9-C (3%). Oxidation of 9-A gave a ketone 9-D that was readily converted by either acid or base to an isomeric ketone 9-E. Ketone 9-E was the only oxidation product of alcohols 9-B and 9-C. What are the structures of compounds 9A-9E ... 

Hydroboration-oxidation of 429 yielded a separable 3 1 mixture of two alcohols (432 433 = 1 3), and subsequent oxidation of alcohol 433 gave the ketone... 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

The starting material for the present synthesis was Wieland-Miescher ketone (24), which was converted to the known alcohol (25) by the published procedure [10], Tetrahydropyranylation of alcohol (25) followed by hydroboration-oxidation afforded the alcohol (26), which on oxidation produced ketone (27). Reduction of (27) with metal hydride gave the alcohol (28) (56%). This in cyclohexane solution on irradiation with lead tetraacetate and iodine produced the cyclic ether that was oxidized to obtain the keto-ether (29). Subjection of the keto-ether (29) to three sequential reactions (formylation, Michael addition with methyl vinyl ketone and intramolecular aldol condensation) provided tricyclic ether (30) whose NMR spectrum showed it to be a mixture of C-10 epimers. The completion of the synthesis of pisiferic acid (1) did not require the separation of epimers and thus the tricyclic ether (30) was used for the next step. The conversion of (30) to tricyclic phenol (31) was... 

Fig (5) The ketoalcohol (34) prepared from ketone (33) has been converted to methoxyabietatriene(38), by standard organic reactions. Hydroboration, oxidation of (38), followed by oxidation, yielded the ketone (39), which is converted to the alcohol (40), by metal hydride reduction. On subjection to transanular oxidation with lead tetracetate and benzene alcohol (40) fiimishes (41) whose conversion to pisiferic acid has already been described. 

The a,p-acetylenic ketones can be synthesized in good yields by the selective mono-hydroboration-oxidation process of conjugated diynes. The monohydroboration of conjugated diynes with disiamylborane places boron preferentially at the internal triple position of the diyne system. The resultant organoboranes on treatment with sodium hydroxide and 30% H202 afforded the a,P-acetylenic ketones (Eq. 33) 79). 

The hydroboration-oxidation of internal alkynes produces ketones. 

Hydroboration/oxidation can also be used to form the ketone from 4-octyne. 

Diketo diester 512 has played a key role in one of Paquette s approaches to the pentagonal dodecahedrane. Direct hydroboration-oxidation of502 provided as the principal product the unwanted isomer 511 (49 %) rather than 512 (30 %). This complication was circumvented by the improvisation of the cross-corner oxygenation sequence outlined in Scheme 75.420,42 The iodolactonization of diacid 510 proceeded with high efficiency to give 513 which underwent cleavage to 514 in the presence of methanolic sodium methoxide at room temperature. Oxidation to a-iodo ketone 515 followed by reductive deiodination with zinc-copper couple and ammonium chloride in methanol solution furnished isomerically pure 512. 

Subsequent hydroboration of endocyclic enol ether 5 proceeded stereoselectively to give alcohol 6, which was further oxidized with tetra- -propylammorrirrm perrutherrate (TRAP) and A-me% lmorpho-line 7V-oxide (NMO) (Ley et al. 1994) to afford ketone 7. Oxidative removal of the /r-methoxyberrzyl (PMB) group followed by treatment with ethanethiol and zinc triflate effected cyclization of the D-ring... 

Hydroboration-oxidation of an alkenylborane derived from a symmetrically substituted alkyne yields a single ketone, whereas unsymmetrically disubstituted alkynes furnish mixtures of ketones. 

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