Versatile Stereoselective Cycloadditions between Heterocumulenes and Phosphagermaallene Tip(tBu)GeCPMes*: Experimental and Theoretical Investigations

Abstract

Phosphagermaallene Tip(tBu)Ge=C=PMes* 1 (Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl) reacts with phenyl isocyanate and tert-butyl isocyanate by a [2+2] cycloaddition that involves the Ge=C and C=O double bonds to afford 1-oxa-2-germacyclobutanes 2 and 3. With N,N'-dicyclohexylcarbodiimide, a [2+2] cycloaddition is observed between the Ge=C and C=N unsaturations to lead to 1-aza-2-germacyclobutane 6 with exocyclic P=C and C=N double bonds. In sharp contrast, 1 reacts with phenyl isothiocyanate, ethyl isothiocyanate, and carbon disulfide according to a [3+2] cycloaddition that involves the whole Ge=C=P unit and the C=S double bond to give transient phosphagermacarbenes (PGeHCs) 11, 12, and 13. These new PGeHCs undergo C-H insertions into one o-tBu group of Mes* (in the case of 11 and 12) or one o-iPr group of Tip (in the case of 13) with formation of tricyclic compounds 8, 9, and 10, respectively. The reaction mechanisms that involve 1 and the phenyl isocyanate and the phenyl isothiocyanate are described and their regioselectivity is explained by theoretical calculations.

Ketenes and Other Cumulenes as Reactive Intermediates

“…Phenylisocyanate reacts with the phosphagermaallene 310 by [2 + 2] cycloaddition (eq 271). 248 Acyl azide thermolysis gives the 3,4-diarylbuta-1,3-dienyl isocyanate 311, which undergoes thermal cyclization to the product pyridin-2(1H)-one (eq 272). 249 Trimerization of isocyanates is catalyzed by the aminosubstituted N-heterocyclic carbene 312, in which enhancement of the catalytic activity of the carbene is attributed to a synergistic effect by the amine (eq 273).…”

Synthesis and reactivity of N-heterocyclic carbene (NHC)-supported heavier nitrile ylides

“…This finding is noteworthy, particularly in the context of 1,3-dipolar additions involving main group ylides with heavier elements, which are barely observed. 34–36,53–55…”

“…This nding is noteworthy, particularly in the context of 1,3-dipolar additions involving main group ylides with heavier elements, which are barely observed. [34][35][36][53][54][55] The HOMO−1 and HOMO in 3 possess lone pair orbitals located on phosphorus, whereas the LUMO shows the vacant p porbital on the carbene carbon (Fig. S29 †).…”

“…Notably, among E 14 –P multiple bonded compounds, the homo or hetero-allenic heavier analogues of nitrile ylides (R 2 E 14 E 15 E 14 R, E 14 = Si–Pb, E 15 = P, Scheme 1c) remains elusive, despite the known constitutional isomers of “heavy heteroallenes” (R 2 E 14 E 14 =E 15 R) that behave as 1,3-dipoles. 34–37 The persistent elusiveness of these compounds underscores the intricate challenges associated with their synthesis, highlighting uncharted territories within the expansive field of heavy main group chemistry. 38,39…”

“…Notably, among E 14 -P multiple bonded compounds, the homo or hetero-allenic heavier analogues of nitrile ylides (R 2 E 14 E 15 E 14 R, E 14 = Si-Pb, E 15 = P, Scheme 1c) remains elusive, despite the known constitutional isomers of "heavy heteroallenes" (R 2 E 14 ]E 14 =E 15 R) that behave as 1,3dipoles. [34][35][36][37] The persistent elusiveness of these compounds underscores the intricate challenges associated with their synthesis, highlighting uncharted territories within the expansive eld of heavy main group chemistry. 38,39 In this study, we introduce N-heterocyclic carbene (NHC)coordinated bent nitrile ylides 1 and 2, offering preliminary insights into the structural, bonding, and reactive characteristics of these intriguing heavier nitrile ylide analogues.…”