Ionic Ga-Complexes of Alkylidene- and Arylmethylidenemalonates

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Ionic Ga-Complexes of Alkylidene- and Arylmethylidenemalonates, and Their Reactions with Acetylenes. An In-depth Sight into the Mechanism of the Occurring Gallium Chemistry. Roman A. Novikov, Dmitry A. Denisov, Konstantin V. Potapov, Yaroslav V. Tkachev, Evgeny V. Shulishov, and Yury V. Tomilov J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08913 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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Journal of the American Chemical Society

Ionic Ga-Complexes of Alkylidene- and Arylmethylidenemalonates, and Their Reactions with Acetylenes. An In-depth Sight into the Mechanism of the Occurring Gallium Chemistry. Roman A. Novikov,*,§,†,‡ Dmitry A. Denisov,§,† Konstantin V. Potapov,† Yaroslav V. Tkachev,‡ Evgeny V. Shulishov,† and Yury V. Tomilov*,† † N.

D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991 Moscow, Russian Federation ‡ Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, 32 Vavilov st., 119991 Moscow, Russian Federation KEYWORDS. Gallium, Methylidenemalonates, Ionic Complexes, Acetylenes, Trans-Addition, Vinyl Halides, 71Ga NMR.

ABSTRACT: A new synthetic concept was suggested in the chemistry of substituted methylidenemalonates, which enables their utilization as 1,2-zwitterionic synthons. This strategy is to generate liquid ionic Ga complexes from methylidenemalonates and GaHal3 with a strict 3/4 composition, and then use them in further synthesis. A number of complexes with different metal halides have been synthesized and studied in detail. The unique properties of gallium among all metals have been demonstrated and explained. Based on the discovered new class of gallium complexes of methylidenemalonates, a number of novel reactions with acetylenes has been elaborated, which are unknown in conventional chemistry of methylidenemalonates. The main demonstrated process is a three-component addition to a triple bond involving halide anions and leading to the formation of polyfunctional vinyl halides with high E-selectivity. The mechanism has been studied experimentally in fine detail. Application of specially optimized 71Ga NMR spectroscopy makes it possible to take an in-depth look into the gallium chemistry in a new light. In particular, the key participation of GaHal 4– anions in the occurring transformations has been established.

INTRODUCTION Donor-acceptor cyclopropanes (DACs)1 are a broad class of substituted three-membered carbocycles.2 They are known for their capability to undergo small ring opening to act as 1,3-zwitterionic synthones.3 Owing to this feature, they are currently popular in organic synthesis4 and have become quite ordinary building blocks. However, current challenges of organic synthesis require that ever more sophisticated and nontrivial methods be developed for synthesizing complex polyfunctional structures from simple starting compounds in a minimum number of stages. This is required because of the increasing demands for the creation of new biologically active compounds, pharmaceutical agents, functional materials, catalysts, etc. It might seem that DACs have been studied so well1–4 that their chemistry is already exhausted in the sense of creating something fundamentally new. However, this is by no means true, and new studies currently continue to appear where quite nontrivial ways of their application are considered.5 One of these cutting-edge approaches6 that appeared and was actively developed in the past few years involves the use of DACs as sources of 1,2-zwitterions.7–10 The method is based on the use of gallium compounds and allows the reaction pathways of DACs 1 with substrates of

many types to be altered completely. Using common compounds such as alkenes,7 acetylenes,8 aldehydes,9 diazo ethers,10 etc. in the reactions, diverse carbo- and heterocyclic structures can be assembled on the basis of 1,2zwitterions 2 generated (Scheme 1). No doubt, the 1,2zwitterionic chemistry of DAC is already a very important and significant concept in synthesis. Scheme 1. Utility of the gallium 1,2-zwitterions in D–A cyclopropane chemistry.

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Taking into consideration the above facts and the obvious structural limitations for generation of 1,2-zwitterions from DACs, an obvious question arises: can such 1,2zwitterions be generated from the corresponding unsaturated precursors? The answer seems self-evident: substituted methylidenemalonates would be very convenient sources since 1,2-zwitterions are in fact their complexes with a strongly polarized double bond (Scheme 2). Everything appears very simple, but no developments of this kind have been reported in the literature. This concept is certainly rather close to the classical Michael chemistry of alkylidenemalonates, but would these reactions occur in a similar way? Moreover, yet another question arises: would gallium halides manifest their unique properties here as well, and if yes, to what extent? Scheme 2. Concept of this work.

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1/1 intermediate complex at –70 °С by means of spectroscopy). If the reagents are mixed in a smaller ratio than 3/4, then the NMR spectra contain a double set of signals that belong to the individual complex 5 and the free ligand 3. It is interesting to note that pure complex 5 is a liquid; apparently it is some analogue of ionic liquids.15,16 Scheme 3. Formation of the different type of complexes with metal halides from DACs and methylidenemalonates.

RESULTS AND DISCUSSION At the first step, it was necessary to study the possible formation of methylidenemalonate complexes with various metal halides. It was found that methylidenemalonates 3 are rather good bidentate ligands that readily coordinate metal atoms at the two carbonyls of the ester groups. This causes a polarization of the C=C bond, and its strength can be monitored by the downfield shifts of CH in the 1H and 13C NMR spectra.6b In most cases, 1/1 covalent complexes 4 are formed, e.g., with SnCl4, TiCl4, ZnCl2, etc. (Scheme 3). The coordination of the metal in these complexes is not very strong, as the gradual change in the chemical shifts upon varying the reagent ratio confirms. ZnCl2 is coordinated much more weakly. Quite a different picture is observed in the complexation with GaCl3. In this case, ionic complex 5 with a strict 3/4 composition is formed, in which three methylidenemalonate ligands are coordinated to one gallium atom (Scheme 3). They replace three chloride ions at that gallium atom, which then form three GaCl4– anions. The structure of the complex was thoroughly studied using NMR spectroscopy (Figures 1 and 2), including that on 71Ga atoms,11,14 and 2D experiments (see also Supp. Inf.). Complexes with similar structures have been reported in only a few cases,12,13 and we studied them just recently using model ligands, diethyl malonate and cyclopropanedicarboxylate, as an example.14 It was found that complexes 5 had a surprisingly stable and beneficial structure. They were readily formed upon mixing the reagents even at low temperatures and did not decompose on heating up to 100 °C (we failed to detect the

Figure 1. Characteristic NMR spectra for ionic Ga complex 5b (X=Br) in CD2Cl2 (left). 1H NMR spectra of neat 5b and its solution in CD2Cl2 at +30 °C. (right). 71Ga NMR spectrum (after deconvolution) of neat 5b demonstrated two types of Ga atoms in the complex (middle). Samples of complexes 5a–c in NMR tubes in CD2Cl2; at the bottom of the tubes neat liquid Ga complexes can be seen that are analogs of ionic liquids.


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Figure 2. 71Ga NMR spectra of complexes 2a (top, ref. [6b]) and 5a (bottom) demonstrating a fundamental difference in the environment of Ga atoms. Small and highly symmetrical GaCl4– anions have a much narrower signal with a characteristic chemical shift of ca. +250 ppm.

The C=C double bond in the malonate moiety of complex 5 is strongly polarized, much more strongly than upon coordination of malonate 2 with SnCl4 or TiCl4 (complexes 4), despite the fact that three malonate ligands at once are bound to one gallium atom in complex 5. The strength of polarization matches that in complex 2 formed from a DAC,6b though the structure of this complex differs from that of the GaCl3 complex with methylidenemalonates (5a– e). The complexation of methylidenemalonates 3 with AlCl3 or EtAlCl2 gives rise to a mixture of a few hardly identifiable complexes and is apparently of intermediate nature. In this case, partial formation of AlCl4– anions within the complexes is also observed. There is a very interesting situation with the stereochemistry of complexes 5. The fact is that the ligand 3 is not symmetrical in terms of R-substituent. As a result, from the theoretical point of view complexes 5 can exist as two stereoisomers A and B with different relative positions of the C=CH(R) fragments (Figure 3). From statistical considerations the ratio of stereoisomers A and B should be 1/3. These facts are excellent confirmed by detailed studies of the complexes 5 by means of NMR spectroscopy at various temperatures (Figure 3). At temperatures below –20 °C, 4 signals of each type are observed in the spectra with equal intensities (for both CH fragments and for each of the MeOgroups). From the considerations of symmetry, isomer B should give three signals, and isomer A — a single signal with tripled intensity. Taking into account the ratio of the isomers of A and B as 1/3, this gives the observed picture (Figure 3). When heated to +30 °C, all 4 signals collapse into one for all types of groups. It is very important fact, because it is an evidence of the strong polarization of C=C double bond and its high semi-double character, which allows it to rotate at temperatures above –20 °C.

Figure 3. 1H NMR spectra for complexes 5a,b (Hal = Cl, Br) at different temperatures in the range from –50 °C to +30 °C (CH+ and CO2Me regions are shown); these spectra show the presence of two stereoisomers A and B in solution at low temperatures, while at temperatures above –20 °С they are averaged due to rotation around the polarized C-C double bond. At the top, isomers A and B are presented, as well as their simplified projection in an orange frame.

We have also carried out detailed studies of complex 5a by mass spectrometry method (ESI-HRMS) (Scheme 4). Unfortunately, it turned out that this method is poorly suited for the study of such complexes, since they are unstable under the conditions of the extremely dilution necessary for the recording of mass spectra. Thus, using ESIHRMS we were able to detect the GaCl4-anions (in negative masses), as well as the free ligand 3a and its dimer in minor amounts (Scheme 4). The cation part of any gallium complexes could not be detected (using any solvents including common MeCN and CH2Cl2). EI-MS was failed to detect any species except the ligand 3a. In order to study the nature of the destruction of complexes 5, we have carried out the reaction of 5a with a large excess of acetonitrile. 71Ga NMR studies show the ligand exchange in the cationic part of 5a with preservation of GaCl4-anions (Scheme 4). Herewith, an octahedral cation [Ga3+(MeCN)6] is formed.



Scheme 4. Mass spectrometric studies of complex 5a, and its partial decomposition by MeCN.

Benzylmethylidenemalonates (1´) noticeably differ from other substituted methylidenemalonates 3 in their behavior upon reaction with Ga halides, and are similar in this to D–A cyclopropanes 1 (Scheme 5). They can form two types of complexes 2 and 5 depending on the conditions, one of the main of which is the used excess of Ga halide. Herewith, the initially formed complexes 2 transform into ionic complexes 5 under certain conditions. The latter precipitate as a thick oil and are not very stable in this state. They are easily dimerized and oligomerized by analogy with D-A cyclopropanes.3g The processes of formation of complexes 2/5 have been studied in detail by NMR spectroscopy on the example of reaction of 1a and 1a´ with GaCl3 (Scheme 5). And the previous data6,7b were refined and supplemented. NMR spectra for both complexes are similar to those shown in Figure 2. Benzylmethylidenemalonates 1´ with other aryl substituents react similarly.6,7b Also, we have synthesized the stable ionic complex 5g based on sterically hindered ortho,ortho-Me2-substituted malonate 1b´ and GaBr3 (Scheme 5), used for additional structural assignments. Obtained structural data allow to look deeper at the "gallium" reactions of D–A cyclopropanes, which have a significant synthetic utility.7–10 It should be noted that the 1/1 gallium complexes like 2 from other methylidenemalonates are highly unstable and were not detected.

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Scheme 5. Formation of complexes with Ga halides from DACs and isomeric benzylmethylidenemalonates.

The formation of ionic complexes 5 from methylidenemalonates allows their usual reactivity to be changed completely. The next stage involved a study of their new interesting chemistry. In this paper, we focused our attention on the carbocationic processes characteristic of the gallium chemistry of DACs7,8 — mainly reactions with acetylenes that allow a number of new processes to be performed (Scheme 6). Apparently, it is just the first chapter of their new interesting gallium chemistry. Scheme 6. Some reactions of ionic Ga complexes 5.

The reaction with acetylenes 6 was found to be a convenient model process for studying and demonstrating the synthetic potential of ionic gallium complexes 5. In this case, it is not necessary to generate this complex beforehand: it is sufficient to mix all the reagents at once, which simplifies the synthetic protocol. Apart from GaCl3, it was interesting to study the behavior of other Lewis acids and metal halides in this reaction (Table 1). It should be noted that, despite its simplicity, the combination of these substrates was not reported in literature. However, a number of similar examples was described, especially the intramolecular version of the reactions or the use of specific types of malonates but without the participation of gallium(III).17 In the present work, the main process that we designed is a three-component addition to a triple bond involving halide anions and leading to formation of polyfunctional vinyl


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halides. Vinyl halides are very convenient building blocks in organic synthesis and promising compounds for further modifications.18 Table 1. Screening of different Metal Chlorides. a

Table 2. Optimization of the reaction conditions. a

Entry

Metal Chloride (1 equiv.)

T, (°C)

t, (h)

Yield, (%) b

Entry

Metal Chloride

Yield, (%) b,c

1

EtAlCl2

–78

1

0

1d

GaCl3

70

2

EtAlCl2

0

2

19

2

AlCl3

25

3

EtAlCl2

rt

0.33

21

3

EtAlCl2

37

4

EtAlCl2

rt

2

37

4

Et2AlCl

26

5

GaCl3

rt

2

45

5

FeCl3

39

6

GaCl3

0

0.5

60

6

SnCl4

I-2 –> I-3 –> I-4, the latter being a complex of three product molecules in enolate form. The shift of 1H NMR signals of the CH moiety in the polarized malonate double bond gradually changes upfield from 9.13 to 8.88 ppm in the series I-1 –> I-2 –> I-3, indicating that the positive charge on the central gallium cation gradually decreases. However, the process does not yet end in complex I-4. When C–Hal bonds are formed, a lot of free reactive GaX3 is released (intermediate I-2). It decomposes the I-4 complex due to a cascade of trans-ligandation processes, to eventually give the gallium enolate of the I-5 product. It is already rather stable under the reaction conditions and is only decomposed later upon hydrolysis of the reaction mixture. It should be noted that complexes I-1–I-4, enolate I-5 and the final product 7 strongly differ in molecular mass, which allows one to distinguish them easily and reliably by diffusion NMR spectroscopy (DOSY).22 Using the DOSY technique, it is possible to measure the diffusion coefficients of the dissolved molecules easily and directly from the NMR experiment. Diffusion coefficients are related to the speed of molecular motions in solution and depend on the size of dissolved compounds. In the first approximation diffusion of molecules only depends on their molecular masses, hence one can estimate the solute molecular masses rather precisely by measuring the diffusion coefficient.22 Other reactions of complexes 5 with acetylenes follow similar scenarios. For example, indenes 6 are formed upon an intramolecular electrophilic attack on the aromatic ring in intermediate C/F with a certain combination of substituents. Lactones 11 are formed upon intramolecular cyclization on the carbonyl group (intermediate G), showing that intramolecular coordination of the ester group to the vinyl carbocation actually takes place. Diastereoselectivity of the formation of lactones 11 is excellent. Corresponding stereochemical model is presented on Scheme 13. Cis-selectivity is set in the hydrolysis stage of pyran H, which is formed after cyclization of the intermediate G. Apparently, hydrolysis occurs intramolecularly by bound H2O molecule in the gallium complex I. Steric hindrance of the R-substituent blocks the access of water from one side, and the intramolecular attack provides high diastereoselectivity. Scheme 13. Stereochemical model for the formation of lactones 11.

and in the transfer of halogen atoms. This partially explains the exceptional features of gallium compounds in processes of this kind. These results will undoubtedly be useful later in further studies on the organic chemistry of gallium and will perhaps expand this field of chemistry considerably. For example, GaCl3 is a unique reagent in the DAC chemistry that modifies the direction of reaction.6–10 It is possible that they are also complex and follow scenarios similar to those described above. As an elegant completion of this work, we have performed a number of additional mechanistic experiments (Table 3). The latter had also the mission of a preliminary search for an approach to the catalytic implementation of the above described gallium chemistry, as well as the addition of other nucleophiles besides the halide anions. All obtained results are in excellent consistency with the proposed mechanistic scheme, and carry a lot of additional confirming information. Two key aspects should be noted: 1) the necessity of using the gallium(III) salts; and 2) the key and essential role of GaHal4– anions in the transport of nucleophiles. Also, the process can be carried out in a "semi-catalytic" version using Ga(OTf)3 as a catalyst, and Bu4N+ GaI4– as a nucleophile source (as an example) (Entries 12, 13; and Scheme 14). It should be noted that the replacing gallium with another metal (Sc, Sn) leads to the impossibility of realization of the process (Entries 2, 3, 14, 15). Similarly acts the replacement of Bu4N+ GaI4– by Bu4N+ I– (Entries 10, 12). As a consequence, the addition of sources of other anions (NaN3, KCN, KSCN, ZnCl2) doesn't lead to their insertion in the product even in trace amounts in model reaction of 3k with 6b and Ga(III) salts (Entries 4, 5, 7, 8, 18). The creation of tetrahedral GaNu4– anions is required for the successful introduction in the reactions of other nucleophiles (Nu) besides halide anions. This approach was perfectly implemented in a preliminary version for the SCN– anions (Entries 19, 20; and Scheme 14). In this case as sources of the latter we used Ga(SCN)3 and Bu4N+ Ga(SCN)4–, which are similar in properties to its halogen analogs. Scheme 14. “Semi-catalytic” version of the main process, and approach to transfer of different nucleophiles.

In total, six intermediate complexes (B & I-1 – I-5) were detected quite reliably by means of NMR (Scheme 12). These complexes are very useful for understanding the essence of the occurring processes. The GaX4– anions play the key role, both in the formation of reactive complexes



Table 3. Additional mechanistic experiments, and approach toward catalytic version and transfer of different nucleophiles.

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lonates as 1,2-zwitterionic synthons. The developed strategy is to generate ionic Ga complexes from methylidenemalonates and gallium halides with a strict 3/4 composition. These complexes are ionic liquids and they are used in further syntheses. We synthesized a number of complexes with different metal halides and showed that gallium possessed unique properties among all metals. Based on the discovered new class of ionic gallium complexes of methylidenemalonates, a number of novel reactions with acetylenes have been elaborated. The main process is a three-component addition to a triple bond involving halide anions and leading to the formation of polyfunctional vinyl halides with high E-selectivity. The mechanism has been studied in fine detail, including the use of specially optimized 71Ga NMR spectroscopy to take a look into the gallium chemistry, which involves the key participation of GaHal4– anions.

Entry MXn (mol.%) a

Nu–-sourse Conditions Nu (equiv.) a

Product (Yield) b

1

GaCl3

–

rt, 2h

Cl

7m (97%)

2

SnCl4

–

rt, 2 h

–

no product

3

SnCl4

TMSCl

rt, 2 h

–

no product

4

GaCl3

KCN (5 equiv.)

rt, 1.5 h

Cl

7m (26%) conv. 56%

5

GaCl3

NaN3 (5 equiv.)

rt, 1 h

Cl

7m (90%)

6

Ga(OTf)3 (20%)

–

40 °C, 6 h

–

n/r c

7

Ga(OTf)3 (10%)

ZnCl2 (2 equiv.)

40 °C, 2 h

–

n/r

* E-mail: [email protected]. * E-mail: [email protected].

8

Ga(OTf)3 (10%)

NaN3

40 °C, 2 h

–

n/r

ORCID

9

GaI3

–

rt, 2h

I

7u (95%)

Roman A. Novikov: 0000-0002-3740-7424 Yury V. Tomilov: 0000-0002-3433-7571

10

Ga(OTf)3 (10%)

Bu4N+ I– (2 equiv.)

40 °C, 2 h

–

n/r

11

GaCl3 (10%)

Bu4N+ I–

40 °C, 2 h

–

n/r

12

Ga(OTf)3 (10%)

Bu4N+ GaI4– 40 °C, 2 h

I

7u (15%) conv. 17%

13

–//–

–//–

40 °C, 12 h I

7u (75%)

ACKNOWLEDGMENT

14

Sc(OTf)3 (10%)

Bu4N+ GaI4– 40 °C, 2 h

–

n/r

15

SnCl4

Bu4N+ GaI4– rt, 2h

–

n/r

16

GaCl3

Bu4N+ GaI4– 40 °C, 2 h

I + Cl 7u (47%) 1.5/1 7m (31%)

17

GaCl3

Bu4N+ I–

40 °C, 2 h

Cl

7m (80%)

18

GaCl3

KSCN

rt, 2 h

Cl

7m (60%)

19

Ga(SCN)3

–

40 °C, 4 h, SCN 4 Å MS

15a (45%)

This work was supported by the Russian Science Foundation (grant no. 14-13-01054-P). High resolution mass spectra were recorded in the Department of Structural Studies of N. D. Zelinsky Institute of Organic Chemistry RAS, Moscow. Dr. Roman A. Novikov and Dr. Yaroslav V. Tkachev thank the Program of fundamental research for state academies for 2013–2020 years (№ 01201363817) for support of NMR studies (Engelhardt Institute of Molecular Biology, Russian Academy of Sciences).

20

Ga(OTf)3 (20%)

Bu4N+ 40 °C, 8 h, SCN Ga(SCN)4– 4 Å MS

15a (40%)

a

(2 equiv.)

1 equiv. by default. b NMR yields. c n/r = no reaction.

CONCLUSIONS In conclusion, we suggest a new synthetic protocol, which makes it possible to use substituted methylidenema-

ASSOCIATED CONTENT Supporting Information. 1H, 13C, 71Ga, 119Sn and 2D NMR spectra for metal complexes and pure products (PDF) This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

Author Contributions § Roman

A. Novikov and Dmitry A. Denisov contributed equal-

ly.

Notes The authors declare no competing financial interest.

ABBREVIATIONS DAC, Donor-Acceptor Cyclopropane.

REFERENCES (1) Fundamental review on the DACs: Reissig, H. U.; Zimmer, R. Chem. Rev. 2003, 103, 1151. (2) Synthesis of DACs: Tomilov, Y. V.; Menchikov, L. G.; Novikov, R. A.; Ivanova, O. A.; Trushkov, I. V. Russ. Chem. Rev. 2018, 87, 201.


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(10) Reactions with diazo esters: Novikov, R. A.; Borisov, D. D.; Tomilov, Y. V. Russ. Chem. Bull. 2018, 67, 265. (11) For reviews of 71Ga NMR spectroscopy, see: (a) Hinton, J. F.; Briggs, R. W. in Group III — Aluminium, Gallium, Indium, and Thallium // NMR and Periodic Table; Mann, B. E.; Harris, R. K., Ed.; Acad. Press: New York, 1978, pp. 279–308. (b) Akitt, J. W.; Greenwood, N. N.; Storr, A. J. Chem. Soc. 1965, 4410–4416. (c) Bock, S.; Noth, H.; Wietelman, A. Z. Naturforsh 1990, 45b, 979–984. (12) For some Ga-complexes with 1,3-dicarbonyl compounds, see: (a) Coates, G. E.; Hayter, R. G. J. Chem. Soc. 1953, 2519–2524. (b) Sievers, R. E.; Connolly, J. W. Inorg. Synth. 1970, 12, 72–77. (c) Beachley, Jr. O. T.; Gardinier, J. R.; Churchill, M. R.; Toomey, L. M. Organometallics 1998, 17, 1101–1108. (d) Beachley, Jr. O. T.; Gardinier, J. R.; Churchill, M. R.; Churchill, D. G.; Keil, K. M. Organometallics 2002, 21, 946–951. (e) Hellwig, M.; Xu, K.; Barreca, D.; Gasparotto, A.; Winter, M. Eur. J. Inorg. Chem. 2009, 1110–1117. (f) Pugh, D.; Bloor, L. G. Eur. J. Inorg. Chem. 2011, 1953–1960. (13) For some examples of representative cationic Gacomplexes, see: (a) Dagorne, S.; Atwood, D. A. Chem. Rev., 2008, 108, 4037–4071. (b) Dagorne, S.; Bellemin-Laponnaz, S.; MaisseFrançois, A.; Rager, M.-N.; Jugé, L.; Welter, R. Eur. J. Inorg. Chem., 2005, 4206–4214. (c) Lichtenberg, C.; Spaniol, T. P.; Okuda, J. Inorg. Chem., 2012, 51, 2254–2262. (14) Ionic Ga-complexes of malonates and strategy for its analysis: Novikov, R. A.; Potapov, K. V.; Chistikov, D. N.; Tarasova, A. V.; Grigoriev, M. S.; Timofeev, V. P.; Tomilov, Y. V. Organometallics 2015, 34, 4238. (15) Ionic liquids: (a) Abbott, A. P.; Frisch, G.; Ryder, K. S. Annu. Rep. Prog. Chem. Sect. A 2008, 104, 21. (b) Schaltin, S.; Brooks, N. R.; Stappers, L.; Van Hecke, K.; Van Meervelt, L.; Binnemans, K.; Fransaer, J. Phys. Chem. Chem. Phys. 2012, 14, 1706. (16) Liquid coordination complexes: Coleman, F.; Srinivasan, G.; Swadzba-Kwasny, M. Angew. Chem. Int. Ed. 2013, 52, 12582– 12586. (17) (a) Yamazaki, S.; Ohmitsu, K.; Ohi, K.; Otsubo, T.; Moriyama, K. Org. Lett. 2004, 7, 759–762. (b) Yamazaki, S.; Fujinami, K.; Maitoko, Y.; Ueda, K.; Kakiuchi, K. J. Org. Chem. 2013, 78, 8405– 8416. (c) Yamazaki, S.; Niina, M.; Kakiuchi, K. Synthesis, 2016, 48, 2889–2895. (d) Yamazaki, S.; Yamada, K.; Yamamoto, K. Org. Biomol. Chem. 2004, 2, 257–264. (e) Yamazaki, S.; Yamamoto. K.; Mikata, Y. Tetrahedron 2009, 65, 1988–1994. (f) Duan, S.; Jana, R.; Tunge, J. A. J. Org. Chem. 2009, 74, 4612–4614. (g) Nickerson, D. M.; Mattson, A. E. Chem. Eur. J. 2012, 18, 8310–8314. (h) Yamazaki, S.; Yamada, K.; Yamabe, S.; Yamamoto, K. J. Org. Chem. 2002, 67, 2889–2901. (18) Utility of vinyl halides: (a) Evano, G.; Blanchard, N.; Toumi, M. Chem. Rev. 2008, 108, 3054. (b) Ma, D.; Cai, Q. Acc. Chem. Res. 2008, 41, 1450. (c) Cahiez, G.; Moyeux, A. Chem. Rev. 2010, 110, 1435. (d) Jana, R.; Pathak, T. P.; Sigman, M. S. Chem. Rev. 2011, 111, 1417. (e) Xia, Y.; Zhang, Y.; Wang, J. ACS Catal. 2013, 3, 2586. (f) Lee, C.-F.; Liu, Y.-C.; Badsara, S. S. Chem. Asian J. 2014, 9, 706. (g) Fustero, S.; Simon-Fuentes, A.; Barrio, P.; Haufe, G. Chem. Rev. 2015, 115, 871. (19) Some calculations of Ga(III) & In(III)-mediated reactions: (a) Michelet, B.; Thiery, G.; Bour, C.; Gandon, V. J. Org. Chem. 2015, 80, 10925–10938. (b) Zhuo, L.-G.; Zhang, J.-J.; Yu, Z.-X. J. Org. Chem. 2012, 77, 8527–8540. (c) Zhuo, L.-G.; Shi, Y.-C.; Yu, Z.X. Asian J. Org. Chem. 2014, 3, 842–846. (20) Tu, P.-K.; Zhou, L.; Kirillov, A. M.; Fang, R.; Yang, L. Org. Chem. Front. 2018, 5, 1702–1717. (21) El-Hellani, A.; Monot, J.; Guillot, R.; Bour, C.; Gandon, V. Inorg. Chem. 2013, 52, 506–514. (22) Diffusion Ordered Spectroscopy (DOSY): (a) Johnson, C. S. Jr. Prog. Nucl. Magn. Reson. Spectrosc. 1999, 34, 203. (b) Pregosin, P. S.; Kumar, P. G. A.; Fernandez, I. Chem. Rev. 2005, 105, 2977. (c) Macchioni, A.; Ciancaleoni, G.; Zuccaccia, C.; Zuccaccia, D. Chem. Soc. Rev. 2008, 37, 479.



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Ionic Ga-Complexes of Alkylidene- and Arylmethylidenemalonates

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