Rhodium-Promoted anti-Hydroboration of Terminal Alkynes via Combined Stoichiometric Reactions
Year: 2024
Type: article
Abstract: Reactions of RhH{κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]} (xant(P i Pr 2 ) 2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) with 2 equiv of tert -butylacetylene and phenylacetylene lead to the acetylide derivatives Rh(C≡CR){κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]} (R = t Bu, Ph). The C–C triple bond of these compounds undergoes the B–H anti -addition of pinacolborane (HBpin) to produce Rh{( E )-C(Bpin)=CHR- Pro-Z }{κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]} (R = t Bu, Ph), which regenerate Rh(C≡CR){κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]} in the presence of a new alkyne molecule, releasing the respective ( Z )-borylolefin. Complex Rh{( E )-C(Bpin)=CHPh- Pro-Z }{κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]} is unstable in toluene. Initially, the C–C double bond of the borylalkenyl ligand undergoes a E to Z isomerization to produce Rh{( Z )-C(Bpin)═CHPh- Pro-E }{κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]}, which subsequently evolves to the aryl derivative Rh{C 6 H 4 -2-[ E -CH═CH(Bpin)]}{κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]}. The latter reacts with a new phenylacetylene molecule to produce Rh(C≡CPh){κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]} and the ( E )-borylolefin. According to this reactivity, the complex RhH{κ 3 - P,O,P -[xant(P i Pr 2 ) 2 ]} is an effective catalyst precursor for the hydroboration of terminal alkynes to mixtures of ( Z )- and ( E )-borylolefins. The molar ratio between isomers depends on the substituent of the alkyne; para -substituted aryl substituents with electron-withdrawing groups favor Z -borylolefin.
Source: Organometallics
Language: English
Subfield: Organic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W4405802766