The effect of ligands on the change of diastereoselectivity dimerization of 2-(naphthyl-1)cyclopropanedicarboxylate in the presence of GaCl 3

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Introduction
During the rather long history of studies on donor-acceptor cyclopropanes (DAC), they found an adequate and quite important place in modern organic chemistry and are now widely used in its various branches.  Dimization reactions are one of considerable achievements of the chemistry of DAC, primarily 2-arylcyclopropane-1,1-dicarboxylates.This is an interesting field where intense studies are currently performed, that allows substituted aliphatic [22][23][24] or cyclic 22,23,[25][26][27] structures with various regioselectivity to be assembled in one experimental stage from rather simple substrates.Special attention in DAC dimerization processes belongs to reactions involving annelation of the original 2-arylcyclopropane-1,1-dicarboxylates with a benzene ring.In these reactions, DAC can act not only as 1,3-dipoles [28][29][30] but also as sources of formal 1,2-zwitter-ions formed due to positive charge "migration" induced by anhydrous GaCl3. 31The regio-and diastereoselectivity of these reactions were generally very high.However, the stereochemical result of these reactions just confirmed the observed stereoselectivity but did not allow one to significantly control the isomeric composition of the resulting compounds.Perhaps there is only one example of process stereoselectivity change, namely, dimerization of 2-indol-3-yl- 28 or 1-naphthylcyclopropane diesters 29 1a,b which involves electrophilic ipso-attack on aromatic ring that induces a cascade of transformations affording complex polycyclic systems 2 and 3 (Scheme 1).The most efficient process was found to be dimerization of 3-indolyl derivatives 1a that occurs in the presence of SnCl4 with high diastereoselectivity and yields pentaleno [1,6-a,b]indoles 2. 28 For 1-naphthyl derivative 1b, the reaction occurs with the opposite diastereoselectivity under catalysis with GaCl3 together with an organocatalyst to give pentaleno[6a,1-a]naphthalene 3. 29 However, it should be mentioned that not only the reaction conditions, including the nature of the Lewis acid used, but also the type of the aromatic substituent were changed in these reactions.

Results and Discussion
In continuation of this research, we studied the effect of various ligands and additives on the type of transformations of 2-arylcyclopropane-1,1-dicarboxylates induced by Lewis acids, and primarily on the behavior of 1,2-zwitterion intermediates formed upon opening of the threemembered ring in the presence of anhydrous gallium trichloride and migration of the carbocationic center. 31,32In this paper, we demonstrate for the first time the possibility to use ligands for stereo-controlled synthesis of two different diastereomers using the same substrate.
We demonstrated this effect for [4+2]-cyclodimerization of 2-(1-naphthyl)cyclopropanedicarboxylate (1b) to give polysubstituted tetrahydrophenanthrene 4a as an example.In the standard procedure, 1 equivalent of anhydrous GaCl3 was first added at 0-5°С, then 1 equivalent of tetrahydrofuran was added and the reaction mixture was stirred for 6 h at 20°С.The use of tetrahydrofuran that decreased the Lewis acidity of GaCl3 affected the regioselectivity of the process as a whole and ensured a considerably higher yield (from 30 to 67%) of the target producttetrahydrophenanthrene 4a, 31 which was formed exclusively as the trans,trans-isomer under these conditions (Scheme 2).
However, if one equivalent of pyrene-1-carboxaldehyde rather than tetrahydrofuran is used as the ligand, then the overall reaction direction remains the same and the yield of the target product is rather high, but its isomeric composition is different, i.e., the diastereoselectivity is changed almost completely in this case and tetrahydrophenanthrene containing 92% of trans,cisisomer 4b is obtained (Scheme 2).This effect of the ligand on the reaction diastereoselectivity appears quite remarkable and is undoubtedly of considerable interest in synthesis as it opens a way to control the diastereomeric composition of products in directed synthesis of compounds with a particular isomeric composition.
2][3][4][5][6][7][8][9] However, in all cases, the reaction diastereoselectivity was determined by the type of substituents, reaction type, etc., and it could not be altered at will.Therefore, the approach that we discovered appears quite important and valuable for subsequent development of DAC chemistry.
It is rather difficult to answer at this point why it is aromatic aldehydes that work successfully.From experimental point of view, it is a result of long and thorough studies on various ligands, additives, and substrates in reactions involving 1,2-zwitter-ions formation.Since this domain is quite extensive and has vague boundaries, we now focused only on a study of aromatic aldehydes and detailed tuning of experimental conditions.It should be noted that processes involving 1,2-zwitter-ion gallium complexes are very complex, 8,[23][24][25]29,31,32 and in general, various ligands and substituents in the starting cyclopropanes can considerably affect their chemical reactions.
Optimization of conditions of [4+2]-cyclodimerization of cyclopropane 1b and data on the effect of the nature of aromatic aldehydes on the diastereoselectivity of formation of trans,transtetrahydrophenanthrene 4a and trans,cis-isomer 4b are presented in Table 1.At first, a relatively stable 1,2-dipolar intermediate 5 is generated from cyclopropane 1b upon treatment with gallium trichloride at 0-5°С. 32It is then entered into the reaction with a second portion of cyclopropane 1b at 40°С in the presence of the corresponding ligand (method A).In an alternative version of this process, instead of an additional portion of cyclopropane 1b, intermediate 5 reacts with preliminary obtained isomeric 2-(naphth-1-yl)vinylmalonate 6 25,33 (method B).Both methods are quite similar in the reaction result and stereochemistry of products.However, method B was found to be more convenient because it was found to be less sensitive to the process conditions and gave somewhat higher product yields due to suppression of the side reaction of [3+2]cycloaddition of the cyclopropane to the aldehyde.
As noted above, diastereomer 4a was formed exclusively in the presence of THF (Table 1, Entries 2 and 3).It should be noted that diastereomer 4a was also the main product in the absence of THF, but its yield was much lower (Table 1, Entry 1) due to oligomerization side processes. 31It is interesting to note that the use of a sterically substituted tetrahydrofuran, namely dimethyl 2,5-diphenyltetrahydrofuran-3,3-dicarboxylate 34 (Table 1, Entry 4), had nearly no effect on the diastereomeric composition of the target products.At the same time, the use of aromatic aldehydes as the ligands changed the diastereoselectivity of this reaction considerably (Table 1, Entries 5-11).The formation of tetrahydrophenanthrenes 4a,b in the presence of benzaldehyde or 1-formylnaphthalene was not selective at all (Table 1, Entries 5, 6 and 8).On the other hand, aldehydes with more bulky aryl substituents, such as anthracene-9carboxaldehyde and pyrene-1-carboxaldehyde, showed much better diastereoselectivity and the reaction resulted in a considerable prevalence of another diastereomer, namely, trans,cis-isomer 4b (Table 1, Entries 9-11).In the case of pyrene-1-carboxaldehyde, the ratio of diastereomers 4b and 4a was found to be 92:8, which appears to be rather a high value.It should however be noted that in the latter two cases, more drastic conditions (80 °С, 1 h) had to be used due to a strong decrease in the reactivity of the intermediate gallium complexes because of the considerable steric volume of the aromatic substituents in the aldehydes specified.Naphthylcyclopropanedicarboxylate 1b was not chosen at random, either.5][36][37][38][39] However, naphthylsubstituted cyclopropane 1b stands apart and slowly reacts with aldehydes.This fact allowed us to use it as an example in order to develop a diastereoselective control method to synthesize substituted tetrahydrophenanthrenes 4a,b in the course of 1b dimerization.The composition of the target products 4a and 4b was established by means of elementary analyses or HRMS.The structure and stereochemistry of the compounds obtained were determined by 1 H and 13 C NMR spectroscopy using 1D and 2D DEPT, COSY, TOCSY, NOESY, HSQC, and HMBC. Figure 1 shows the key differences between both diastereomers in the 2D 1 H, 1 H-NOESY NMR spectra.Based on the data obtained here and known previously, the following reaction mechanism and reasons of diastereoselectivity change by various ligands can be suggested (Scheme 3).The first stage of the process that involves generation of a 1,2-zwitter-ion gallium complex from a cyclopropane-1,1-dicarboxylate was described in detail in our studies. 31,32After that, the ligand comes into action.It is coordinated to the gallium atom of complex 5 and gives intermediates 7 with retention of the 1,2-zwitter-ion structure.Such coordination of ligands with 1,2-zwitterions of type 5 was studied by us in considerable detail previously; 32 what is more, a 1,2-zwitterion complex formed from 2-phenylcyclopropanedicarboxylate with THF (similar to 7a) was detected and its structure was determined. 32Therefore, there is no doubt that intermediates 7 are formed in this process.
Intermediates 7a and 7b only differ in the nature of the ligand coordinated to the gallium ligand, therefore the subsequent fate of these intermediates is similar in a certain range of conditions: they undergo dimerization by [4+2]-annelation pathway to give the tetrahydrophenanthrene skeleton by coupling of 1,2-zwitter-ion intermediates 7a or 7b with naphthylvinylmalonate 6 formed upon isomerization of the second portion of cyclopropane 1b, or by using a previously prepared sample.The reaction of 1,2-zwitter-ion 7 with naphthylvinylmalonate 6 where the double bond is polarized with partial negative charge on the benzyl carbon atom gives a new 1,2-zwitter-ion intermediate 8, in which the induced positive charge is stabilized due to intramolecular re-coordination of the malonyl moieties.In this case, the difference in the ligand structures dramatically affects the stereochemical outcome of the key step of С-С coupling.

Scheme 3. Proposed mechanism and stereoselectivity of [4+2]-dimerization of cyclopropane 1b.
When tetrahydrofuran ligands are used or if they are absent, the naphtyl and coordinated malonyl substituents are arranged in the trans-orientation (Scheme 3, intermediate 8a), since considerable steric hindrance takes place in the cis-orientation due to their large size, and the intermediate with the cis-arrangement of substituents is nearly not formed.In this case, stereochemical control is exclusively caused by steric effects of substituents.
On the contrary, when aromatic aldehydes are used as the ligands, the reaction occurs in violation of steric control, via the sterically unfavorable cis-orientation of the naphthyl and coordinated malonyl substituents in the transition state (Scheme 3, intermediate 8b), which actually manifests itself as the necessity of more drastic reaction conditions.In this case, the cisconfiguration of these substituents apparently arises from --stacking between the aryl substituents due to interaction between the -systems of the naphthyl substituent in the substituted malonate and the aryl substituent in the aldehyde, which strongly favors their approach.Furthermore, the aromatic aldehyde molecule is polarized positively due to coordination with gallium, whereas the naphthyl substituent is polarized negatively, and the opposite polarization of both aromatic rings favors their stronger interaction.The version about --stacking was confirmed very well by varying a number of aromatic aldehydes (Table 1).The best diastereoselectivity values in the formation of trans,cis-isomer 4b are observed for aldehydes with a large -system of a few fused benzene rings (Table 1, Entries 9-11).As the same time, a mixture of diastereomers is formed in the presence of "small" aromatic aldehydes (Table 1, Entries 5-8), i.e., the force of --stacking appears insufficient and the reaction is controlled to a considerable extent by steric factors.The version about the effect of the considerable steric volume, which also grows strongly with enlargement of the -system, on diastereoselectivity change turns out to be invalid.In fact, a check experiment using a strongly loaded tetrahydrofuran ligand (dimethyl 2,5-diphenyltetrahydrofuran-3,3-dicarboxylate) (Table 1, Entry 4) without a large -system did not give noticeable amounts of trans,cis-isomer 4b.Hence, the observed change in the process diastereoselectivity by aromatic aldehydes used as the ligands is due to the effect of --stacking of aryl substituents in the key transition state.
The assumed mechanism was partially studied and confirmed by monitoring the reactions in an NMR spectrometer at reduced temperatures.These experiments were found to be very difficult to perform due to the low stability of intermediate complexes, as well as their not-toogood solubility, strong lability to traces of moisture, and sensitivity to reaction conditions.Nevertheless, we succeeded in detection of a few intermediate dipolar gallium complexes at the first process stages (5, 7a, 7b with benzaldehyde, 7b with pyrene-1-carboxaldehyde) (Figure 2, Table 2).
In fact, we confirmed ligand coordination to the gallium atom without breakdown of the 1,2zwitter-ion structure to give the corresponding complexes (see Scheme 3).The primary complex 5 is relatively stable and exists for dozens of minutes in solution at 0-10°C.Second generation complexes 7a,b are much less stable and decompose in a few minutes even at reduced temperatures.Decomposition occurs by a number of pathways that in many respects differ from reactions in a flask (since the conditions cannot be accurately reproduced in an NMR tube), which considerably complicates their study.a Chemical shifts 13 C were determined from 2D HSQC spectra; b Data for 13 C were not obtained due to a low stability of the complex; c ArCHObenzaldehyde; d ArCHOpyrene-1carboxaldehyde.
The structures of the complexes were studied and confirmed by 2D COSY, NOESY, DOSY, HSQC and HMBC NMR experiments at various temperatures with recording in "high-speed" mode.Coordination of the aldehyde to the gallium atom and formation of complexes 7 have been additionally confirmed by diffusion-ordered NMR experiments (DOSY).In this case, the aldehyde is coordinated quite weakly and the signals in the 1 H and 13 C NMR spectra almost do not shift.The complex is unstable and apparently exists in equilibrium with the free aldehyde.

Conclusions
Thus, we have discovered a new method to regulate the diastereoselectivity of dimerization of donor-acceptor cyclopropanes by means of ligand control.The method has been demonstrated for [4+2]-cyclodimerization of 2-(1-naphthyl)cyclopropanedicarboxylate to give polysubstituted tetrahydrophenanthrenes in the presence of gallium trichloride as an example.It has been shown that trans,trans-isomer 2a is formed exclusively if tetrahydrofuran is used as the ligand, whereas replacement of the ligand by an aromatic aldehyde results in predominant formation of the trans,cis-isomer.The best diastereoselectivity values are attained using pyrene-1carboxaldehyde.A possible mechanism of diastereoselectivity change has been suggested.This effect of the ligand on the reaction diastereoselectivity is of considerable interest in synthesis as it opens rather a simple way to control the diastereomeric composition of the products without changing the overall process regioselectivity.

Experimental Section
General.All reagents and solvents were used commercial grade chemicals without additional purification.All operations with GaCl3 were carried out under dry argon atmosphere.TLC analysis was performed on Silufol chromatographic plates.For preparative chromatography, silica gel 60 (0.040-0.063 mm) was used. 1 H and 13 C NMR spectra were recorded on a 400 MHz (400.1 and 100.6 MHz, respectively) and 300 MHz (300.1 75.5 MHz, respectively) spectrometers in CDCl3 containing 0.05% Me4Si as the internal standard.Determinations of structures and stereochemistry of obtained compounds and assignments of 1 H and 13 C signals were made by 2D COSY, NOESY, DOSY, HSQC and HMBC NMR experiments.IR spectra were obtained on a FT-IR spectrometer in CHCl3 solution (0.5-2%).High resolution mass spectra were obtained using simultaneous electospray (ESI).

Table 1 .
Ligand effect on the diastereoselectivity of the model reaction of dimerization of 1b