Synthesis of Aldehydes and Ketones

Abstract

Carbonyl compounds are an important class among organic molecules. Literature records several methods for their synthesis. However, there are very few methods to convert carbon-carbon unsaturation to carbonyl compounds. Hydroboration of acetylenes, followed by oxidation provides a novel method for carbonyl synthesis. It has been noted that regioselectivities achieved in the monohydroboration of internal acetylenes with thexylborane [1], disiamylborane [1], dicyclohexylborane [1], and catecholborane [2] are similar to, but less pronounced than, that realized by 9-BBN [3]. The B-alkenyl-9-BBN derivatives undergo oxidation to the corresponding ketones or aldehydes under aprotic conditions with trimethylamine N-oxide [4, 5] or under protic conditions by inverse addition to buffered hydrogen peroxide [3]. The inverse addition, i.e., the slow addition of the B-alkenyl-9-BBN in THF to the buffered H 2 O 2 , suppresses the otherwise undesirable protonolysis reaction and favors the oxidation pathway to the desired aldehyde or ketone. Synthesis of AldehydesThe monohydroboration of 1-alkyne (1 mole) with 9-BBN (0.5 mole), followed by oxidation affords the corresponding aldehyde (Eq. 7.1) [3] with no evidence of the presence of the isomeric-2-alkanones. (7.1) B-Alkyl-9-BBN on carbonylation with carbon monoxide in the presence of potassium triisopropoxide affords the corresponding aldehyde or alcohol (Scheme 7.1) [6] in excellent yields. 7 Synthesis of Aldehydes and Ketones 7 Synthesis of Aldehydes and Ketones 214 Scheme 7.1 Similar reactions of 9-BBN with alkenes and dienes, followed by carbonylation in the presence of lithium trimethoxyaluminohydride afford the corresponding alcohols or aldehydes (Chart 7.1) [7a]. Chart 7.1The results are summarized in Table 7The method is extended to introduce aldehyde groups [7b] into the molecules containing other functional groups. The mild reducing agent lithium-tri-t-butoxyaluminohydride reduces the carbonyl intermediate from B-R-9-BBN and does not reduce the esters or nitriles (Chart 7.2; Table 7.2) [7b].