Phosphaalkynes Diels—Alder reactions

Dienophiles like phosphaalkyne 44a or activated alkynes and triphosphadewarbenzene 45 undergo a [2+2+2] homo-Diels-Alder reaction that leads to C3P4 cage 46. Cyclic dienophiles yield pentacycles (Scheme 13) <1997JOM(529)215, 1999S1363>. 

Outstanding properties are the transformation to 1H- or 2/f-phosphirenes after carbene addition (9->- i0),12b 21 22 [3 + 2]-cycloadditions of 1,3-dipoles leading to a wide variety of heteroatom-substituted phospholes (9 — ll)18 and Diels-Alder reactions (9 - 12) that make not only the phosphinines but also their valence isomers accessible.23,24 In ene reactions phosphaalkynes... 

The hitherto unknown l-aza-3-phosphabenzenes are obtained via a regiospecific Diels-Alder reaction with inverse electron demand of 2-trifluormethyl-4-methyl-6//-l,3-oxazin-6-one with phosphaalkynes (81) or phosphaalkenes [Eq. (31)] (82). 

Phosphaalkynes also play a prominent role as dienophiles in Diels-Alder reactions hence the X -phosphinines 13 [23] are formed from cyclic 1,3-dienes such as a-pyrone or cyclopen-tadienones by way of extrusion of CO2 or CO, respectively reactions with anthracene provide an access to the phosphabarrelene series [24]. This type of reaction is also of significance for the construction of phosphorus-carbon cage compounds. The same is true for homo-Diels-Alder reactions which, with 2-phosphabicyclo[2.2.2]octa-2,5-diene as reaction partner, lead to the diphosphatetracyclodecenes 14 [25]. Last but not least, ene reactions with phosphaalkynes as enophiles [25] are also valuable for the construction of polycyclic phosphorus-carbon compounds. The reactions of 9 with 2,3-dimethyl-2-butene (- 15) [26, 27] emphasize this behavior. 

The Diels-Alder reaction of cyclohexa-1,3-diene (17) with the phosphaalkyne 9a only proceeds under drastic conditions in a pressure Schlenk tube but then does provide the 2-phos-phabicyclooctadiene 18 [23] in quantitative yield (Scheme 6-4). This compound represents the first member of the previously unknown class of polycyclic phosphaallrenes. From a thermal point of view, 18 is unexpectedly stable cycloreversion only occurs under flash vacuum pyrolysis conditions (520°C/10 mbar) but then it does not give rise to the starting materials 9a and 17 instead it proceeds by cleavage of ethylene to furnish the 2-terr-butyl-X -phosphinine [23]. 

When the cyclobutadienecarboxylate 21 [35] stabilized by the presence of bulky substituents, is allowed to react with the phosphaalkynes 9a-d, the 2-phospha-Dewar-benzenes are obtained in quantitative yields [36] (Scheme 6-5). For steric reasons the [4 + 2]-cycloaddition takes place at Cl and C4 and is highly selective with regard to the dienophile orientation. In the cases of the phosphaalkynes 9a-c, products 22a-c dominate over the isomers 24a-c by a ratio of at least 93 7. Within the detection limits of P-NMR spectroscopy, the Diels-Alder reaction 21 + 9d furnishes 22 d exclusively. The structure of 22 d was confirmed by X-ray crystallography [36]. The phosphaalkene moieties of the phospha-Dewar-benzenes 22a-d, which are utilized in the subsequent reactions, can be recognized, among others, by their low-field P-NMR signals (5 = 312-315). 

A rich cycloaddition chemistry with phosphaalkynes 9 may be expected from cross-conjugated carbocyclic compounds of the heptafulvene type (in the present case, tropone (41)). Thermal [rtf - and [jt a ]-cycloadditions are allowed by symmetry. Thus, the thermal reaction of tropone (41) with an excess of the phosphaalkynes 9 a, 9 e (without a solvent) furnishes the diphosphatetracyclic products 43a,b [45]. The initiating Diels-Alder reaction to 42 (there are P-NMR spectroscopic indications for its intermediate formation) is accordingly followed by a [2 -t- 2 + 2]-cycloaddition (ashomo-Diels-Alder reaction) with a second equivalent of the phosphaalkyne to yield 43 (Scheme 6-9). The observed peri- and regioselectivities of the two reaction steps can be confirmed by frontier orbital calculations [43],... 

Scheme 30. Homo-Diels-Alder reaction of 19 with phosphaalkyne la...

Compound 105 has been prepared by an independent protocol. Phosphaalkyne la undergoes quantitative cyclotrimerization under the influence of a Lewis acid such as AICI3 to furnish the spirocyclic diphosphirenium betaine 106 [91 ]. The Lewis acid can be removed by treatment with the weak Lewis base DMSO. The spirocycUc system 107 cannot be isolated since it rearranges rapidly to the 1,3,5-triphospha-Dewar-phosphinine 19 which - again - cannot be isolated under these reaction conditions but is, in turn, trapped by added phosphaalkyne in a homo-Diels-Alder reaction to afford the phosphaalkyne tetramer 105 [92]. 

Diels-Alder reactions have also been reported in which phosphaal-kynes function as the dienophile component to yield phosphaben-zenes. An AMI semiempirical study comparing aU nes and phosphaalkynes as dienophiles has been published recently. It has also been reported that ethyl cyanoformate and p-toluenesulfonylcy-anide undergo Diels-Alder reaction with some pyrones to yield pyrido[3,4- >]-indoles 14 and isoquinolines 15. This methodology was limited in scope, however, and it could not be extended to other nitriles (R = COPH, Me, Ph, Py, MejN) nor to imines. 

MO studies have shown that the Diels-Alder reaction of substituted selenocar-bonyl compounds with buta-1,3-diene or 2-methoxybuta-1,3-diene proceeds through a concerted, asynchronous transition state. Q ,jS-Unsaturated seleno ketones and seleno aldehydes readily undergo 4 - - 2-cycloaddition with alkenes and 4 - - 2-dimerization. The reaction of phosphaacetylene (138) with buta-1,3-diene produces triphosphatri-cyclooctenes (139) through a sequence involving Diels-Alder, ene, and intramolecular 4 -I- 2-cycloadditions (Scheme 53). The 4 + 2-cycloaddition of phosphaalkynes with 5,8-bis(trimethylsilyl)cycloocta-l,3,6-triene (140) readily yields the tricyclodecadiene (142) via the bicyclic intermediate (141) (Scheme 54). "... 

Erom a formal point of view the reaction of kinetically stabilized phosphaalkynes with 1,3-dienes should furnish 1-phosphacyclohexa-l,4-dienes. However, at 90°C with alkyl-substituted phosphaalkynes the primarly formed Diels-Alder adducts have not been detected, because the initial [4-f 2] cycloaddition was followed by an ene-reaction and a final intramolecular [4-1-2] cycloaddition to give diphosphatricyclooctenes. 

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