A Convenient Entry to Asymmetrically Substituted Bis(iminophosphoranes) Derived from 1,2-Bis(diphenylphosphino)ethane.

Pilar Llamas-Lorente, Mateo Alajarín, Carmen López-Leonardo

Departamento de Química Orgánica, Facultad de Química,

Universidad de Murcia, Campus de Espinardo, E-30100 Murcia, Spain.

 Introduction

            The two main synthetic routes leading to iminophosphoranes are: 1) the imination of tertiary phosphines with organoazides followed by elimination of dinitrogen, the Staudinger reaction,^[1] and 2) the reaction of tertiary phosphane dihalides with primary amines in the presence of base, the Horner-Oediger method.^[2]

            We have recently reported a new two-step procedure for preparing iminophosphoranes starting with compounds bearing P-N single bonds, aminophosphanes.^[3] In the global process, the aminophosphanes can be considered as iminophosphoranyl synthons of the type [R¹₂P=NR²]^-.

            On the other hand, the electron-withdrawing characteristics of the diphenylphosphinoyl (Ph₂P=O) and diphenylthiophosphinoyl (Ph₂P=S) moieties, serve for activating an alkene group to nucleophilic (Michael-type) addition of phosphanes, alcohols or amines.^[4] The polarity of the P=N bond also makes the iminophosphoranyl unit an activating group of alkenes in Michael-type additions. This has been demostrated in the context of the synthesis of dendrimers,^[5] and we have also reported the addition of diphenylphosphane and amines to diphenylvinyl iminophosphoranes as a method for the synthesis of monoiminophosphoranes derived from 1,2-bis(diphenylphosphino)ethane^[6] as well for preparing N,N-ligands containing a diphenyliminophosphoranyl unit.^[7]

In this poster we describe the Michael-type addition of aminophosphanes, as iminophosphoranyl synthons, to P,P,P-diphenylvinyl iminophosphoranes as a potential source of asymmetrically substituted bis(iminophosphoranes) derived from 1,2-bis(diphenylphosphino)ethane.

 Results and discussion

We have explored the Michael-type addition of aminophosphanes to unsubstituted (1) and substituted (2 and 3) vinyl iminophosphoranes as Michael acceptors. Compounds 1 and 2 are readily prepared by Staudinger reaction of diphenylvinylphosphane or diphenyl(1-phenylvinyl)phosphane^[8] with a range of aryl, acyl and phosphoryl azides. Iminophosphorane 3 was obtained by isomerization of the corresponding P-allyl iminophosphorane.^{[3],[ 9]}

The reaction of diphenylvinyl iminophosphoranes 1 with P,P-diisopropyl aminophosphanes or P,P-diphenyl aminophosphanes in benzene at reflux temperature for 2 hours proceeded as a Michael-type addition reaction giving rise to the corresponding bis(iminophosphoranes) 5 in good yields (Scheme 1, Table 1). This process occurs with total P-regioselectivity of the P,N-ambident nucleophile 4.

Scheme 1

Table 1. Bis(iminophosphoranes) 5.

Attempts to carry out similar processes with the iminophosphoranes 2 and 3, bearing a substituent in the 1 or 2 position of the vinyl moiety, under the same reaction conditions (benzene at reflux) were unsuccessful. Only when one equivalent of a strong base (Bu^tOK) was added and refluxing THF used as solvent the iminophosphoranes 6 could be obtained although in poor yields (Scheme 2, Table 2).

Scheme 2

Table 2. Bis(iminophosphoranes) 6.

This methodology for preparing asymmetrically substituted bis(iminophosphoranes) derived from 1,2-bis(diphenylphosphino)ethane results synthetically useful since the sequential Staudinger reaction of this bis(phosphane) with azides is not straightforward because the selective monoimination of the bis(phosphane) is difficult to achieve. It has only been carried out efficiently by the utilization of the particularly unreactive trimethylsilylazide.^[10]

 Relevant spectroscopic data of compounds 5.

-The ¹H-NMR spectra of 5b,c,e-h show a multiplet in the range d 2.62-2.84 ppm as result of the overlapping of the signals corresponding to both methylene groups of the PCH₂CH₂P backbone, whereas compounds 5d,i show two separated multiplets centered near d 2.6 and d 2.9 ppm.

-The ¹³C-NMR spectra of 5a,b,d-i show two sets of signals, one for each different Ph₂P=N- group, in the usual region of chemical shifts, with the ¹³C-³¹P coupling constants commonly found in phenyl groups linked to P(V) atoms. The two methylene carbons appear in the range d 19.31-21.57 ppm with typical ¹J_CP 57.4-66.1 Hz and ²J_CP 0-3.2 Hz coupling constants.

- The ³¹P-NMR spectra of 5a,b,d-i show two doublets with ³J_PP values close to 50 Hz.

 Conclusion

We have developed a convenient route for the synthesis of asymmetrically substituted bis(iminophosphoranes) derived from 1,2-bis(diphenylphosphino)ethane via Michael-type addition of  aminophosphanes, as synthetic equivalents of iminophosphide anions,  to diphenylvinyl iminophosphoranes.

 Acknowledgments

This work was supported by MCYT and FEDER (Proyect BQU2001-0010) and Fundación Séneca-CARM (Proyect PI-1/00749/FS/01). P. L.-L. also thanks Fundación Séneca-CARM for a fellowship.

 References

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