Spectral Assignment of Phenanthrene Derivatives Based on 6H-Dibenzo[C,E][1,2] Oxaphosphinine 6-Oxide by NMR and Quantum Chemical Calculations

Abstract

Organophosphorus compounds such as 6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DOPO, 1) and its derivatives are important and versatile compounds for a broad field H coupling patterns are discussed and compared with the derivatives possessing different chemical environments around the phosphorus atom. In addition, we compared our results with density functional theory calculations. Even though the prediction of NMR data of organophosphorus compounds via molecular modeling is limited, this study presents a method that yields good results for this class of heterocycles. This knowledge should help to quickly assign NMR spectroscopic data of other DOPO (1) derivatives and can be extrapolated to organophosphorus compounds in general.

Multinuclear NMR Studies and Spin System Analyses on Dibenzo[c.e][1,2]Oxaphosphorine 2‐Oxides

“…More information cannot be extracted from solely proton‐based NMR experiments, hence further presumptions are required to resolve this ambiguity. The interpretation of 1 H chemical shifts may involve, for example, speculating on inductive effects of oxygen and phosphoryl substituents ; observing δ H trends in related structures; chemical shift prediction by suitable computer programs or analyzing the experimental long‐range phosphorus‐proton coupling constants ( n J P,H ) . The mentioned approaches are commonly applied for 1 H NMR assignment of organophosphorus compounds, but may occasionally lead to mistakes .…”

“…The interpretation of 1 H chemical shifts may involve, for example, speculating on inductive effects of oxygen and phosphoryl substituents ; observing δ H trends in related structures; chemical shift prediction by suitable computer programs or analyzing the experimental long‐range phosphorus‐proton coupling constants ( n J P,H ) . The mentioned approaches are commonly applied for 1 H NMR assignment of organophosphorus compounds, but may occasionally lead to mistakes . We thus followed a more general and robust strategy by turning our attention to the experimental 13 C chemical shifts and their one‐bond (HSQC) and multiple‐bond (HMBC) connectivities to protons .…”

“…The assignment of 2a published by Wagner et al. is also listed in the last four columns of Table for comparison. It is obvious that the 13 C chemical shifts at positions 11 to 14 are reversed with respect to our assignment.…”

“…1 H NMR studies often revealed crowded, overlapping spectral patterns for the aromatic protons, which quite frequently were described as “complex multiplets,” without attempting a more detailed spectral analysis. Systematic determination and interpretation of long‐range homo‐ and heteronuclear coupling constants ( n J H,H , n J P,H , and/or n J P,C ) were undertaken in a few cases only , revealing that magnitudes of heteronuclear couplings do not necessarily decrease with increasing distance from the phosphorus atom, hence their utilization for assignment purposes should be exercised with caution. A closer inspection of the published 1 H/ 13 C NMR data of 2 H ‐dibenzo[c.e] oxaphosphorine 2‐oxide 2a (HDOPO; see Scheme ) by Wagner et al.…”

“…A closer inspection of the published 1 H/ 13 C NMR data of 2 H ‐dibenzo[c.e] oxaphosphorine 2‐oxide 2a (HDOPO; see Scheme ) by Wagner et al. indeed revealed some inconsistencies and mistakes. This inspired us to reinvestigate three compounds of this type ( 2a , 2b , and 2c in Scheme ) at higher magnetic field strengths in order to determine possibly all long‐range coupling constants, to identify the pitfalls of previous assignment strategies, and to substitute them with more robust approaches.…”