Thermal Reactions of 5‐Alkylidene‐4,5‐dihydro‐3H‐1,2,4(λ³)‐diazaphospholes (4‐Phosphapyrazolines) – A Route to Various PHeterocycles and to 2‐Phosphabutadienes

Abstract

The 5-alkylidene-4,5-dihydro-3H-1,2,4(h3)-diazaphospholes (4-phosphapyrazolines) are thermally much more stable than related compounds without the exocyclic double bond. Thermolysis reactions typically occur in the range 110-150°C in toluene solution, and different, mostly competing, reaction pathways are observed. Thermal extrusion of nitrogen from 6a-g gives rise to b-phosphanylsiloxyalkenes 10, benzo[c]-phosphole derivatives 11, 14, and 15, (P-siloxyalky1idene)-phosphiranes 12, and dihydro-1,3-oxaphospholes 13. The thermolysis of 5-alkylidene-4,5-dihydro-4-trimethylsilyl-3-trimethylsilyloxy-3H-1,2,4-diazaphospholes 17 afforded three products, including the highly substituted and stable 2-phosphabutadienes 18 formed by nitrogen extrusion and rearrangement. Finally, the 4-chloro-3-trimethylsilyloxy-substituted heterocycle 21 was transformed at 170°C into 4H-1,2,4-diazaphosphole 23. The structures of 13c and 18a were determined by single-crystal X-ray diffraction.A convenient route to bis(alky1idene)phosphoranes 2 and phosphiranes 3 is provided by the thermal or, less frequently, the photochemical. extrusion of dinitrogen from 4,5-dihydr0-3H-l,2,4(3~~)-diazaphospholes 1 [']. So far, the heterocycles 1 have been prepared exclusively by [3 + 21 cycloaddition of diazo compounds to phosphaalkenes ['-5]; quite often, the nitrogen elimination already occurs during the cycloaddition piocedure at or below room temperature["-q. The resulting bis(alky1idene)phosphoranes 2 (as well as those prepared by other methods and which are substituted differentlyL5I) typically undergo a thermally induced, conrotatory 4n-electrocyclic isomerization to the thermodynamically more stable phosphiranes 3. Depending on the substituent pattern, this rearrangement occurs in a temperature range between -SO and +120"C, i.e. compounds 2 can have a fleeting existence (e.g. 2, R3 = Cl[l'.fl) and can be detected spectroscopically at room temperature (e.g. R' = R2 = %Mei, R' = R4 = R' = Ph[ld]), or a stable enough to be isolated (SiMe? groups at one or both C atoms, NR2 at P ['d,b,51). Thcrmal reactions of 2 which do not lead to phosphiranes are rare [j].Several 4,5-dihydro-3 H-diazaphospholes do not lend themselves as precursors to 2 and 3. As in other cases (see above), the dediazonation of P-trimethylsilyl- We have found recently that 5-alkylidene-4,5-dihydro-3H-1,2,4(h')-diazaphospholes can be synthesized from phosphaalkenes, and 1 -diazo-2-siloxyethenes generated in situ from a-diazo-a-silyl ketone^ [^,^]. These novel phoshole derivatives are thermally much more stable than their related compounds lacking the exocyclic C-C double bond, and can therefore be isolated and handled con~eniently [~]. Nevertheless. we expected them to lose nitrogen at higher

Photochemical Isomerizations of 5-Alkylidene-4,5-dihydro-3H-1,2,4(λ3)-diazaphospholes (4-Phosphapyrazolines): (5→4) Ring Contraction Generates Azomethineimine Dipoles

“…[34] The temperature dependence of the efficiency of N 2 elimination indicates a significant activation barrier to photochemical N 2 elimination in some cases, [1,31,33] and other modes of deactivation of the photochemically exited state [35] may become competitive due to this barrier. In the case of 4-phosphapyrazolines 1, it should be noted that products of the thermal decomposition mode by N 2 elimination [8] were observed either not at all (1 c) or only in trace amounts (1 a) even when the irradiation was performed at 60 8C (see the experiments designed to trap the azomethineimine dipoles, see Scheme 6). Quast et al have pointed out that a parallel between the barriers to photochemical and thermal loss of molecular nitrogen may exist not only for the so-called reluctant cyclic azoalkanes of the 2,3-diazabicyclo[2.2.2]oct-2-ene type, as suggested by Engel et al, [36] but also for 4-methylene-, 4-oxo-, and 4-imino-1-pyrazolines.…”

“…300 nm leads not to extrusion of molecular nitrogen, but to a (5 34) ring contraction caused by a 1,2(C 3N) shift of the phosphorus atom. This behavior contrasts with the smooth elimination of N 2 under thermal conditions [8] and is the more remarkable as most D 1 -pyrazolines efficiently lose molecular nitrogen under both thermal and photochemical conditions. [1] 5,5-Diphenyl-2-diphenylmethylene-1,3,4-oxadiazoline (15), structurally closely related to the title compounds, is a notable exception, since it is cleaved photochemically into diphenylketene and diphenyldiazomethane.…”

“…Nevertheless, extrusion of molecular nitrogen from 1 (R 1 tBu, 1-adamantyl, Me, 4-anisyl, 4-nitrophenyl) can be achieved in boiling toluene. [7,8] Depending on the substituents, vinylphosphanes, benzo[c]phosphole derivatives, alkylidenephosphiranes, or 1,3-oxaphospholes are formed, and we have proposed that the products are derived from intermediate 1,3-diradicals IV and methylene(vinylidene)phosphoranes V.…”

“…The direction of the lone pairs of electrons at P1(lp P ) and N1 (lp N ) is represented by narrow, open rods; for calculation of torsion angles, lp P and lp N were mimicked by dummy hydrogen atoms placed in calculated positions. Torsion angles [8]: P1-N1-C1-C2 À 179.8(2), C14-P1-N1-C1 À 63.1 (3), lp N -N1-P1-lp P À 111, C1-N1-P1-lp P 69, N1-P1-C23-N2 159.8(2), lp P -P1-C23-N2 À 76, lp P -P1-C23-C24 101, N2-C23-C24-O1 À 83.3(4). some noteworthy spectroscopic features are seen in the 13 C NMR spectra.…”

Cycloaddition Behavior of 1,2‐Thiaphospholes: Reactions with Diazocumulenes and with Cyclopentadiene

“…[27] and references cited therein) converts intermediates 10 into bicyclic alkylidenephosphiranes 6. The same diazo dipoles 1 or 2 react with acyclic phosphaalkenes [23,27,28] and with 2-acyl-1,2,3-diazaphospholes [16] to give cycloaddition products analogous to 9 that are stable enough to be isolated and are dediazoniated only at elevated temperature. In the diazaphosphole case, the thermal decomposition of the cycloadducts leads to tricyclic compounds related to 4.…”