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].
