Diels-Alder cycloaddition reactions of 1,1-dichloro-2,3,4,5-tetraethylgermole and 1-chloro-2,3,4,5-tetraethylphosphole with maleic anhydride and maleimide

The Diels-Alder reaction of 1-chloro-2,3,4,5-tetraethylphosphole with maleic anhydride and with maleimide yields 4-chloro-1,7,8,9-tetraethyl-4-oxa-10-phosphatricyclo[5.2.1.0 2,6 ]deca-8-ene-3,5-dione ( 1 ) and 4-chloro-1,7,8,9-tetraethyl-4-aza-10-phosphatricyclo[5.2.1.0 2,6 ]deca-8-ene-3,5-dione ( 2 ). The molecular structure of 1 confirms the formation of the endo-cyclo-adduct with the chlorine atom in an anti-position to the C=C double bond. In contrast to this chlorophosphole, 1,1-dichloro-2,3,4,5-tetraethylgermole reacts with two equivalents of the maleic derivatives to 1,4,7,8-tetraethylbicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acid anhydride ( 3 ) as well as the aza-analogue 4 . The molecular structure of 3 shows the formation of an exo,exo-product. The 1,1-dichloro-2,5-bis(trimethylsilyl)-3,4-diorganylgermoles with R = Me ( 5 ), n Pr ( 6 ) and Ph ( 7 ) show no reactivity toward maleic acid derivatives.


Introduction
Germoles and phospholes (germa-and phosphacyclopentadienes) belong to a substance class of unsaturated heterocycles which gained tremendous interest as such as well as their anions during the last decades.Due to the diagonal relationship the similarity of the CH and P units seemed to suggest a comparison of the cyclopentadienes with the phospholes regarding the structures and chemical and physical properties.Furthermore, comparisons are undertaken for cyclopentadienes with their heavier homologous metalloles.The first preparative procedures include the metathesis reaction of the halides of phosphorus and germanium with 1,4-dilithiobutadienes. 1,2 Later a procedure starting from 1,1-cyclopentadienyl-1-zirconacyclopenta-2,4-dienes was developed. 3,4 ][7][8][9][10][11][12] Synthesis and chemical behaviour of germoles 13,14 and phospholes [15][16][17] are summarized in several review articles.
Germoles and phospholes readily undergo [4+2] cycloaddition Diels-Alder reactions with a wide variety of multiple bonds such as alkenes, alkynes or butadienes.Therefore, dimerization of phospholes can also be observed [18][19][20][21] and was investigated by quantum chemical methods. 22, 231-Arylphospholes react with maleic anhydride as well as with N-phenylmaleimide to the corresponding cycloaddition products. 24Furthermore, also 1-aryl-thioxophospholes undergo Diels-Alder reactions. 25A reduction of these phosphole sulfides with nickelocene and a [4+2] cycloaddition with N-phenylmaleimide also lead to the formation of the appropriate product. 26urther examples are summarized in review articles [15][16][17] as well as in the literature cited therein.
The investigations on germoles are far less extensive.[33]

Results and Discussion
Synthesis 1-Chloro-2,3,4,5-tetraethylphosphole reacted at room temperature in a solvent mixture of 1,2dimethoxyethane (DME) and THF with maleic anhydride (X = O) as well as with maleimide (X = NH) according to equation (1) to the cycloaddition products 1 and 2, respectively.These compounds precipitated at -20°C in the shape of orange crystals.In contrast to the starting chlorophosphole, no reaction of bis(2,3,4,5-tetraethylphospholyl) (Et 4 C 4 P-PC 4 Et 4 ) with maleic anhydride or with maleimide was observed.
In a similar procedure, 1,1-dichloro-2,3,4,5-tetraethylgermole was reacted with an equimolar amount of maleic anhydride (X = O) and maleimide (X = NH).Already the NMR spectroscopic characterization of the reaction showed that the cycloaddition reactions followed a 1:2 stoichiometry.Whereas half of the amount of the dichlorogermole was still present in the reaction mixture, the maleic acid substrates were already converted into the cycloaddition products (Scheme 1).After cooling of the concentrated solutions colorless single crystals of 3 or oily 4 were isolated.

Scheme 1
Surprisingly, the "simple" 1:1 Diels-Alder cycloaddition product was not detected as for the reaction of the chlorophosphole with the maleic acid derivatives.Instead, a second maleic anhydride attacked the germanorbornene, a mechanistic proposal is represented in equation (2).The first Diels-Alder cycloaddition reaction step is very slow for the chlorophosphole and for the dichlorogermole.Therefore, a complete reaction was achieved after approximately one day (X = O) and two days (X = NH).The second reaction has to be much faster and therefore, we were unable to see the germanorbornene intermediate.The eliminated germanium(II) chloride was not stable under these reaction conditions and underwent subsequent decomposition reactions.These rather complex decomposition reactions were already studied during the warm-up procedure of matrix isolated GeCl 2 .In contrast to our observations, the 1,1-dimethylgermoles form the Diels-Alder products with maleic anhydride and with maleimide, regardless whether the carbon atoms are substituted by hydrogen or phenyl groups. 30Therefore, we repeated the reaction of maleic anhydride with the less bulky 1,1,-dichloro-2,3,4,5-tetramethylgermole.However, the same reaction mechanism was observed and the already well-known 34 1,4,7,8-tetramethyl-bicyclo[2.2.2]oct-7-ene-2,3,5,6tetracarboxylic acid anhydride was obtained.Criegee et al. 34 isolated this compound after the reaction of 3,4-diiodo-1,2,3,4-tetramethylcyclobutene with two equivalents of maleic anhydride in the presence of mercury.
The molecular structures of 5, 6 and 7 are very similar and therefore, only the molecular structures of the n-propyl (6, Fig. 1) and phenyl derivatives (7, Fig. 2) are shown.Selected structural parameters are summarized in table 1.The molecule of 5 lies on a mirror plane, whereas the molecule of 7 shows crystallographic C 2 symmetry.As expected there is no delocalization of the double bond character within the GeC 4 ring.Therefore, these compounds could react as dienes in the Diels-Alder reaction.However, the trimethylsilyl groups reduce the electron density at the α-carbon atom by hyperconjugation (back donation) of electron density from the p z (C) orbital into a σ*(Si-C) bond of the trimethylsilyl group.This so-called α-silyl effect seems to reduce the reactivity of the 2,5-bis(trimethylsilyl)-substituted 1,1-dichlorogermoles and no Diels-Alder cycloaddition reactions occurred with maleic acid derivatives.Figure 2. Molecular structure and numbering scheme of 1,1-dichloro-2,5-bis(trimethylsilyl)-3,4diphenylgermole (7).The ellipsoids represent a probability of 40%, H atoms are not shown due to clarity reasons.Symmetry-related atoms (-x, y, -z+1.5) are marked with apostrophes.

Molecular Structures of 1 and 3
The Diels-Alder product 1 as well as its numbering scheme is shown in Figure 3.During the [4+2] cycloaddition reaction the endo-isomer is formed.The phosphorus atom is in a pyramidal environment and the chlorine substituent at the phosphorus atom is in an anti-position to the C=C double bond.A similar stereochemistry was observed for example for the cycloaddition reactions of 1-phenyl-3,4-dimethylphosphole with N-phenylmaleimide. 261-(2,4,6-trialkylphenyl)-3methylphospholes yielded with maleic acid derivatives a mixture of endo-and exo-products, however, the P-bound group was always in a trans-position to the C=C fragment. 24The reaction of 3,4-dimethyl-2H-phosphole with maleic anhydride gave the endo-Diels-Alder cyclo-adduct but the phosphorus atom is the bridge head atom of the heterocycle of 3,4-dimethyl-1-phospha-2norbornene-5,6-dicarboxylic acid anhydride.The structural parameters are in accordance to the characteristic values.The deviations of the carbon atoms C2 and C3 of the C=C double bond from a trigonal planar environment are negligible, even though pyramidalization of these carbon atoms due to a heteroatom substitution has attracted considerable interest. 36However, usually no deviation from planarity of the C=C fragment of norbornenes was observed for the substitution of the methylene moiety by heavier main group elements. 30ompound 3 crystallized with two crystallographically independent molecules A and B in the asymmetric unit.In Figure 4 molecule A is represented with its numbering scheme.The bond lengths correspond very well to those of the organic backbone of 1.This structure shows that the stereochemistry of the first [4+2] cycloaddition step follows the same principle as observed for 1 and that the endo-isomer was formed.The cycloaddition under loss of the GeCl 2 fragment leads to an isomer with the second carboxylic acid anhydride in a syn position to the C=C double bond as shown in equation (3).4), C2A-C3A 1.349(4), C3A-C4A 1.535(4), C1A-C14A 1.555(4), C13A-C14A 1.528(4), C4A-C13A 1.567(4), C1A-C18A 1.562(4), C17A-C18A 1.526(4), C4A-C17A 1.555(4).

4-Chloro
The structures were solved by direct methods with the program SIR-97 38 and refined with software package SHELXL-97. 39Neutral scattering factors were taken from Cromer and Mann 40 and for the hydrogen atoms from Stewart et al. 41 The non-hydrogen atoms were refined anisotropically.The H atoms were considered with a riding model under restriction of ideal symmetry at the corresponding carbon atoms, however, the N-bound hydrogen atoms were refined isotropically.

Figure 1 .
Figure 1.Molecular structure and numbering scheme of 6.The ellipsoids represent a probability of 40%, H atoms are neglected for clarity reasons.

Table 2 .
Crystallographic parameters and details of data collection and refinement details