Structurally Defined Ring-Opening and Insertion of Pinacolborane into Aluminium-Nitrogen Bonds of Sterically Demanding Dialkylaluminium Amides

Dialkylaluminium amides iBu(2)Al(TMP) and iBu(2)Al(HMDS) can perform catalytic hydroboration of ketones with pinacolborane to form the expected boronic esters. However, repeating the same reactions stoichiometrically without a ketone leads unexpectedly to ring-opening of pinacolborane and insertion of its open chain into the Al-N(amido) bond. To date there has been limited knowledge on decomposition pathways of HBpin despite its prominent role in hydroboration chemistry. X-ray crystallography shows these mixed Al-B products [iBu(2)Al{OC(Me)(2)C(Me)(2)O}B(H)(NR2)](2) (NR2=TMP or HMDS) form dimers with an (AlO)(2) core and terminal B-N bonds. Since the bond retention (B-H) and bond breaking (B-O) in these transformations seemed surprising, DFT calculations run using M11/6-31G(d,p) gave an energy profile consistent with a sigma-bond metathesis mechanism where London dispersion interactions between iBu and (amide) Me groups play an important stabilising role in the final outcome.

Automated summary

Dialkylaluminium amides iBu(2)Al(TMP) and iBu(2)Al(HMDS) catalyze the hydroboration of ketones to form boronic esters, but stoichiometric reactions without ketones lead unexpectedly to ring-opening of pinacolborane. X-ray crystallography reveals the formation of dimeric Al-B products, while DFT calculations support a sigma-bond metathesis mechanism involving London dispersion interactions.