Dr. Charles D. Schaeffer, Jr.
A.C. Baugher Professor of Chemistry
A. Abstract
Trivalent silylium ions (R3Si+) have been pursued for more than twenty years. Unlike their carbon analogs, their existence is controversial, particularly in solution. It is the intent of this proposal to use intramolecular charge-dipole interactions, similar to those in crown ether adducts of cations, to stabilize the silylium ion without introducing additional covalent bonding. These conditions will be met with groups of the type -N(CH2CH2OCH3)2, which are linked to the silicon at the nitrogen (which in itself should lend some stability to the ion through -interactions) and which contain two polar side chains that have sufficient conformational flexibility to allow the alkoxy dipole to be directed toward the positively charged silicon in the silylium ion. These groups are similar to the polyether backbone of crown ethers. As a result of their structure and the organization provided by the charge-dipole interactions, the groups can surround both sides of the trigonal planar silicon center. This encapsulation should hinder close approach of both counterion and solvent and mitigate the need for esoteric counterions and solvents of extremely low nucleophilicity.
We have already demonstrated the synthetic viability of one approach to the immediate precursor to the ion by preparing compounds of the type: RSiH[N(CH2CH2OCH3)2]2.
These compounds will be treated with trityl salts such as the perchlorate in the final step to the preparation of ions of the type: RSi[N(CH2CH2OCH3)2]2+. Products will be identified by 1H, 13C, 17O, and 29Si NMR. The 29Si chemical shift will help in determining the electronic nature of the silicon center in these ions.
The use of groups containing longer side chains and greater numbers of alkoxy groups will help to establish the structure necessary for stabilization of the ion. We believe that intramolecular charge-dipole encapsulation can become a general method for the stabilization of positively or negatively charged organometallic centers.
The results of the study should provide: (a) evidence for the specifics
of nucleophilic substitution reactions at Group 14 elements; (b) a route
to novel intramolecularly stabilized compounds such as siliconium ions;
and (c) a method for determining relative intramolecular basicities of
groups toward the Group 14 elements.
B. Narrative
Introduction. In contrast to the rich and well-characterized chemistry of carbenium ions (R3C+) which has evolved during more than one hundred years of study,1 the chemistry of silylium ions (R3Si+) is meager, controversial and poorly understood.2 In fact, at least two different workers3,4 regard questions concerning the solution-state nature of R3Si+ as "among the most controversial in organosilicon chemistry."
Silicon has properties such as larger size, greater polarizability, and lower electronegativity that should make the formation of silylium ions more favorable, and, in fact, hydride affinities in the gas phase indicate that silylium ions are indeed more stable than the corresponding carbenium ions with hydride as a reference base.5 This order is reversed with fluoride as a reference base, however, and in solution heats of solvation probably favor the smaller ions.5Mass spectral evidence6 provides support for the relative stabilities of carbenium and silylium ions. Moreover, silylium ions appear to play a role as intermediates in a variety of reactions.7
The attempts to stabilize silylium ions in condensed phases have included the use of: intermolecular dative interactions, substituent effects, counterions, and solvents that minimize interactions with the highly electrophilic species. The first report of an isolable silylium ion resulted from the coordination of 2,2'-dipyridyl with triphenylsilyl iodide and -bromide.8 Nevertheless, this and the salt [SiH3(2,2'-bipy)]Co(CO)49 are five-coordinate siliconium ions.
Evidence from theoretical quarters10,11 indicates that -donors such as NH2 and OH should be much less effective in stabilizing silylium ions than in stabilizing carbenium ions, presumably because of the different sizes of the interacting orbitals. For the same reason, groups such as phenyl should produce little stabilization through conjugation.12 Lambert and Schultz13 employed the strategy of using a substituent which should stabilize the silylium ion through the charge induced-dipole interaction. The isopropylthio substituent was selected because of the relatively large polarizability of the sulfur. Reaction with trityl perchlorate (eq. (1)) produced a perchlorate product which was described as the first silylium ion in solution.13
(i-C3H7S)3SiH + (C6H5)3C+ClO4- Æ(i-C3H7S)3Si+ClO4- + (C6H5)3CH (1)
A poorly-resolved 29Si NMR signal, deshielded by only 17 ppm relative to the silane precursor, appears to cast considerable doubt on the existence of a trivalent ion.14
In spite of the stabilization that might be expected from (pÆd) interactions in the Si-N bond, attempted production of the [(Me2N)3Si]+ ion from the reaction of (Me2N)3SiCl with Al2Cl6 led instead to a complex product as determined by multinuclear NMR and X-ray crystallographic analysis.15
Many efforts to isolate the silylium ion or to produce them as transient species involve hydride abstraction from silicon by trityl perchlorate.16 Lambert et al.17 have prepared triphenylsilyl perchlorate from this reaction (eq. (2)), citing evidence from molar conductances for the ionic nature of the compound.
(C6H5)3SiH + (C6H5)3C+ClO4- Æ(C6H5)3Si+ClO4- + (C6H5)3CH (2)
Olah et al.18 note that this claim was in direct conflict with Lambert's own earlier work12 in which he concluded that the perchlorate is a covalent ester or tight ion pair, and that the conductance data can be understood in terms of small quantities of hydrolysis products. Olah et al.18 also note several key points regarding NMR evidence. The 1H and 13C shifts observed for Lambert's compound are very similar to most known covalent triphenyl derivatives. The 29Si NMR chemical shift for trivalent Ph3Si+ is anticipated to fall at 125 ± 25 ppm. Both Lambert and Olah identify weak 29Si signals at 3 ppm, a value compatible only with a species of a largely covalent nature. A more recent Lambert study19 of 35Cl NMR chemical shifts and line widths, originally interpreted in terms of rapid, two-site exchange equilibrium between associated and fully ionized forms of silyl perchlorate, is explained by Olah et al.20 in terms of hydrolysis by residual low-level water impurities (hexamethyl-disiloxane was detected). Lambert has responded to the hydrolysis question21 by repeating his studies under rigorously anhydrous conditions, and concludes that hydrolysis has no significant effect on the "fundamental problem of the nature of these silyl perchlorates."
More recently, Lambert has reported the X-ray crystal structure for Et3Si+[(C6F6)4B-] which shows no coordination to anion and only distant coordination to solvent.22 A toluene solvent molecule is apparently close enough to cause minor deviations from planarity at silicon, but the silicon-toluene distance is greater than the sum of the silicon and carbon covalent radii. Nevertheless, several groups23-25 have criticized these claims of pure three-coordination at silicon on the basis of the observed 114° C-Si-C bond angle and an estimated transfer of more than 35% of the positive charge of the cation to the toluene molecule.
The most recent work on the problem involves the use of large counterions for stabilization. Jutzi and Bunte26 have reported the preparation of [(-Me5C5)2SiH]+ which is stabilized with a catechol dimer anion. This is the first example of an R3Si+ ion in which ligands contribute to the stabilization of the cation center. Bahr and Boudjouk27 have used trityl tetrakis[bis(3,5-trifluoromethyl)phenyl]borate (anion: TFPB) and found that when a strongly-coordinating solvent such as butyronitrile is used, a nitrile-stabilized silylium ion (silylnitrilium ion) appears to form: R3Si(NCC3H7)+[TFPB]-. Nevertheless, Kira and Sakurai28 report that use of this anion, in some attempts to form [R3Si][TFPB] in methylene chloride, lead to fluoride abstraction from the anion and chloride abstraction from the solvent. Reed et al.29 have developed a new large, weakly coordinating anion for the purpose of stabilizing R3Si+ ions. The authors claim that closo-6,7,8,9,10-Br5CB9H5- develops "more cationic character in a trialkyl silicon moiety than has been conclusively demonstrated to date." These claims are support by 29Si NMR chemical shifts in toluene which are as high as 97.9 ppm for (i-C3H7)3Si(Br5-CB9H5) (the predicted 29Si NMR chemical shift for (CH3)3Si+ is 250 ± 25 ppm).7
Stabilization Through Intramolecular Charge-Dipole Interactions. Thus, at present, several silylium ions with varying degrees of interaction with counterion or solvent have been characterized in the solid state, but no species with an appropriately large 29Si chemical shift to be classified as a trivalent species has been characterized in solution. The effects that can be used to stabilize silylium ions can be categorized according to whether they are associated with substituents attached to silicon, the counterion, or the solvent used in the preparation. Most researchers have concentrated on using a counterion and solvent of low nucleophilicity, but some work has been done on the effect of substituents (vide supra). We are concerned primarily with the effect of substituents because we believe that an appropriate choice of substituent makes the difficult choice of counterion and solvent less important; that is, sufficient stabilization may be obtained with certain substituents that even counterions known to have some nucleophilic character will not be able to attack the electrophilic silicon.
The substituent effects that can be employed with the silylium ion fall into the three major categories of resonance effects, inductive effects, and steric effects. Generally, delocalization of the positive charge on the silicon by interactions and electron release through the inductive effect should stabilize the silylium ion. A theoretical study of the effects of -donor substituents showed that the NH2 group was more stabilizing than PH2, OH, SH, Cl, and F.11 The phenyl group is almost certainly more stabilizing than alkyl groups, primarily as a result of its high polarizability which leads to strong charge-induced dipole interactions.
Inductive effects include through-bonds release of electrons to the positive silicon site as well as electrostatic field effects such as charge-dipole interactions and charge-induced dipole interactions. It is the charge-dipole interaction that forms the basis for this proposal. When the dipole is oriented toward the charge the magnitude of this interaction is given approximately by the equation qm/er2, where q is the charge on the cation, m is the dipole moment, e is the dielectric constant of the medium, and r is the distance between the center of the charge and the center of the dipole. That this interaction can result in considerable stabilization of the positive charge can be seen from the lowering of potential energy of 18 kJ/mol that results when a positive charge is located 4 Å from a dipole with a moment of 1.0 D (assuming a dielectric constant of 1.0, an appropriate value for the gaseous state).
It is the objective of the present proposal to explore the stabilization of the silylium ion through internal charge-dipole interactions similar to those found in host-guest adducts of cations. These interactions can be incorporated into the silylium ion by using substituents that contain several polar groups. These groups should be of relatively low nucleophilicity. The substituent should also have considerable conformational flexibility so that the polar groups can be oriented toward the positive site.
A second important feature of this proposal is that the substituents have stereochemical characteristics that permit them to deploy their dipoles to encapsulate the positive site in the center of a host-guest like adduct. This encapsulation not only results in favorable orientations of the dipoles but also produces greater attractive dispersion forces and a steric "coating" that hinders attack by nucleophilic species.
These two features, as well as the stabilizing effect of the amino function, are incorporated into our target molecules with the substituent
CH3OCH2CH2NCH2CH2OCH3
Typical of these molecules is the ion
R-Si[-N(CH2CH2OCH3)2]2+
which contains four polar alkoxy groups that can be oriented toward the silicon. The (CH3OCH2CH2)2N- group has sufficient conformational flexibility to wrap around both sides of the trigonal planar silylium ion. This encapsulation is shown schematically below
Molecular mechanics calculations show conformations of this species with distances between
the oxygens and the silicon of between 3.0 and 3.7 Å. For four charge-dipole interactions at an average distance of 4 Å, using a dipole moment of 1.2 D for the alkoxy oxygen (the moment for methyl ethyl ether is 1.2 D), the total stabilization is 86 kJ/mol. Although this is only an approximation that does not take into account charge delocalization, conformational flexibility, etc., these interactions are likely to contribute appreciably to the stability of the cation. Moreover, this conformation, maintained by the charge-dipole interactions, effectively protects the ion from attack by counterion or solvent. This is particularly important for a trigonal planar species that is vulnerable to nucleophilic attack from both sides of the plane.
Although the substituent could produce onium-type ions
we believe that these species are not likely to be of lower energy than the encapsulated ion. It could also be argued that the more closely the silicon-oxygen distance approaches the sum of the covalent radii (1.9 Å), the more likely that the interaction will have covalent character. The extent of this character will be ascertained from 29Si and 17O NMR chemical shifts (vide infra).
Specifically, we propose to make three series of silylium precursors:
a. RnSiX[N(CH2CH2OCH3)2]3-n, where X = H, Cl; R = CH3, C6H5; n = 0-2
b. R-SiX[N(CH2CH2OCH3)2]2, where X = H, Cl; R = CH3, t-C4H9, C6H5
c. C6H5SiH[NR2]2, where R = CH2CH2CH2OCH3, CH2CH2OCH2CH2OCH3
In the first series, the number of (CH3OCH2CH2)2N- groups is varied to determine how many charge-dipole interactions (and steric interactions) are necessary for stabilization and encapsulation of the ion. In the second series, the non-alkoxy substituent will be varied to determine the relative amount of stabilization provided by alkyl groups of different size and polarizabilities and by the phenyl group. In the third series, the number of polar groups, as well as the length of the side chain of the amino substituent, is varied (several appropriate potential ligands are known, including HN(CH2CH2CH2OCH3)230 and HN(CH2CH2CH2OCH2CH3)231).
The precursors will then be converted to the silylium ion by several procedures. The chlorides will be treated with silver perchlorate and silver tetrakis(perfluorophenyl) borate in order to incorporate these counterions (eq. (3)).
RSi[N(CH2CH2OCH3)2]2Cl + AgClO4 Æ RSi[N(CH2CH2OCH3)2]2ClO4 + AgCl (3)
The hydrides will be treated with trityl perchlorate and trityl perfluorophenyl borate (eq. (4)).
RSi[N(CH2CH2OCH3)2]2H + (C6H5)3CClO4 ÆRSi[N(CH2CH2OCH3)2]2ClO4 + (C6H5)3CH (4)
Both are standard literature procedures.32,33 The chlorides will also be treated with dipyridyl in order to obtain the analogous chelated silylium ions.7
Because the 29Si chemical shift has been shown to the best indicator of cationic character in solution,18,34 this method will be employed, along with 1H, 13C, 17O, and 35Cl NMR spectroscopy, to provide evidence about the structure of the compounds in solution. Solid-state 29Si NMR as well as X-ray analysis (in collaboration with A. Rheingold, University of Delaware) will be used to characterize solids. Of course, these methods will be supplemented by IR, molecular weight, and conductance measurements.
Preliminary Work. We have reacted bis(2-methoxyethyl) amine ((CH3OCH2CH2)2NH) with dichlorophenylsilane (C6H5SiCl2H) in the presence of triethylamine and have obtained
compound I in good yield. In similar fashion we have obtained the compounds (bmea)2SiHCH3, bmeaSiCl(C6H5)2, and (bmea)2Si(C6H5)2, where bmea = bis(2-methoxyethyl)amino. The compounds have been characterized by proton and 13C NMR. For example, the proton spectrum of I contains a singlet at 3.22 ppm for the methoxy protons, triplets at 3.05 and 3.38 ppm for the methylene protons, and a singlet at 4.80 ppm for the Si-H. We have also reacted I with trityl perchlorate, observed the disappearance of the Si-H proton resonance and the appearance of the C-H resonance. In a single preliminary experiment we have observed a weak signal at approximately 90 ppm in the 29Si spectrum of the product in the solid state.
We have prepared by amination or transsilylation a series of N-chlorodimethylsilylmethyl amides of the type RCONR'(CH2Si(CH3)2Cl), where R = H, R' = CH3; R = CH3, R' = CH3; R = H, R' = C6H5; and R = CH3, R' = p-XC6H4 (X = H, OCH3, Cl, Br, CF3, NO2)35. Hydrogen, carbon-13, nitrogen-14, oxygen-17, and silicon-29 NMR spectra were obtained and used to determine the structure of the compounds. Nitrogen-14 and oxygen-17 chemical shifts are indicative of the amide structure reported previously. Oxygen-17 and silicon-29 chemical shifts can be related to the extent of dative interaction between the carbonyl oxygen and the silicon and correlate with the carbonyl stretching frequencies36. The hydrogen spectra of most of the acetanilides in CDCl3 contain a broad, less intense peak for the Si(CH3)2 protons. At lower temperatures, this peak separates into two sharp peaks of unequal intensities. The two peaks can be attributed to rotamers, with the large peak at higher frequencies arising from the rotamer with the CH2Si(CH3)2Cl group cis to the carbonyl oxygen. In toluene, CH2Cl2 and THF, the Si(CH3)2 protons appear as a sharp peak which does not broaden upon heating to the boiling point of the solvent. The free energies of activation for rotation about the carbon-nitrogen bond were determined by the approximate method of Shanan-Atidi and Bar-Eli and show a rough correlation with the substituent's sigma constant.
A second series of amides containing the group Si(CH3)2Si(CH3)2Cl is currently being prepared from the reaction between the N-lithio salt of the appropriate para-substituted acetanilide and ClSi(CH3)2Si(CH3)2Cl (available as a distillable liquid in good yield from the reaction of hexamethyldisilane, aluminum chloride, and acetyl chloride). These compounds, which also appear to be solids, are currently being characterized by Fourier transform infrared and multinuclear NMR spectroscopy.
Significance. This project should allow us to assess the role of the charge-dipole interaction and encapsulation in stabilizing charged centers. If the CH3OCH2CH2NCH2CH2OCH3 group does confer stability on the silylium ion we will be able to explore the relative roles of the charge-dipole interaction as opposed to encapsulation through the use of groups such as [CH3O(CH2)n]2N, which should provide excellent encapsulation with alkyl bridges of varying length, and varying numbers of polar groups such as (CH3OCH2CH2OCH2CH2)2NH, which should provide for encapsulation and additional charge-dipole interactions.
The use of different numbers of CH3OCH2CH2NCH2CH2OCH3 groups will permit an assessment of the number of charge-dipole interactions needed to stabilize the silylium ion.
If sufficient stabilization is provided by these groups, traditional counterions such as perchlorate may function as true ionic, distant counterions. If this is not the case, less nucleophilic ions such as tetrakis(perfluoroborate) ions may be required. The same assessment can be made for solvents.
Trivalent triorganostannyl cations are as yet unknown,37 but can perhaps be stabilized in the same way. Indeed, if the stabilization by intramolecular charge-dipole encapsulation is successful it can become a general method for the stabilization of charged reactive centers.
Our short-term (two-year) goal in this work will be product characterization by physical and spectroscopic techniques such as 13C and 29Si NMR spectroscopy. For each incipiently hypervalent species the rate will be compared to the rate of reaction at a simple model four-coordinate species. For example, in the nucleophilic displacement reaction at silicon in ClSi(CH3)2CH2N(R)COR', the comparison will be made with the rate of reaction at ClSi(CH3)2CH2Cl because of the ready availability of this compound and the similar steric and electronic effects in the amide.
Student Involvement. Sophomore, junior and senior undergraduate chemistry students will perform the synthetic and instrumental work associated with all aspects of this project. Students will work for ten-week periods during the summer sessions covered by this proposal. Juniors or seniors who elect research for academic credit will conduct the remainder of the work. Aspects of this project include small-scale synthetic organometallic chemistry of air- and moisture-sensitive materials, computer data fitting, literature searching and retrieval, hands-on fourier transform infrared and multinuclear NMR experience, and spectral interpretation. Exposure to these facets of the project will enable students to receive an intensive, balanced introduction to fundamental research in main-group organometallic chemistry, an area often neglected in many undergraduate curricula. We have found that after reading the appropriate sections in several of the excellent introductions to the manipulation of air- and moisture-sensitive compounds, most students are adequately prepared to begin the synthetic work.
At Elizabethtown College there is a long tradition of scholarly activityin chemistry, and recent graduates continue to maintain a long tradition of performance excellence in graduate school, medical school, and in the work force. Although modest in its numbers, the Elizabethtown program traditionally has also made a significant contribution to the number of women entering graduate and professional schools from this institution: during the years 1981-2002, the department graduated 207 chemistry students, of whom 139 (67%) are women. The Elizabethtown College chemistry department is one of 212 (Tier 2) nondoctoral granting departments listed in the sixth edition (1995) of Directory of Research in Chemistry at Primarily Undergraduate Institutions, published by the Council on Undergraduate Research (Rabindra N. Roy, Ed.), and the department is included in all five previous editions as well.
Dissemination of research results from the projects described in this proposal will take several forms. Student present seminars to their colleagues and faculty members at neighboring institutions at several periodic meetings. These include the Middle Atlantic Regional Meeting of the American Chemical Society and the annual Intercollegiate Student Chemists (ISC) convention, a regional ACS-type meeting attended annually by students and faculty from 30-40 institutions (two prizes are awarded in each of five divisions each year). During the period 1985-2005, there have been five 1st or 2nd place winners from among all Elizabethtown speakers at the ISC. Three examples include: (1) M.F. Lehman '94 and C.D. Schaeffer, Jr., "Reactivity of Chloromethyldimethylchlorosilane with N-Trimethylsilyl-Substituted Amides," 58th Annual Convention of the Intercollegiate Student Chemists, West Chester University, West Chester, PA, April 16, 1994 (First Prize, Inorganic Division); (2) K.J. Sullivan '94 and C.D. Schaeffer, Jr., "Preparation and Structure of Incipiently Hypervalent Silicon Compounds," 58th Annual Convention of the Intercollegiate Student Chemists, West Chester University, West Chester, PA, April 16, 1994; and (3) B.J. Frost '95 and C.D. Schaeffer, Jr., "Reactivity of N-(Chlorodimethylsilylmethyl) Amides," 59th Annual Convention of the Intercollegiate Student Chemists, Muhlenberg College, April 8, 1995 (Second Prize, Inorganic Division 1). Mr. Frost is completing his Ph.D. in chemistry at Texas A&M University (College Station, TX); Ms. Sullivan has finished her Ph.D. in medicinal chemistry at the University of Georgia (Athens, GA) and is employed at Emisphere, Inc. Two other recent students, Heidi E. Gasswint '98 and Stephanie D. Kerstetter '98, presented public seminars on this project and graduated with departmental honors in chemistry. Copies of the two honors dissertations are on file in chemistry library and with Professor Schaeffer. Several Elizabethtown chemistry students and faculty attended the 207th National Meeting of the American Chemical Society, held in San Diego, CA, March 12-15, 1994; two of my students presented posters: (1) M.F. Lehman and C.D. Schaeffer, Jr., "Reactivity of Chloromethyldimethylchlorosilane with N-Trimethylsilyl-Substituted Amides," Division of Chemical Education/Undergraduate Research Poster No. 184; (2) K.J. Sullivan and C.D. Schaeffer, Jr., "Preparation and Structure of Incipiently Hypervalent Silicon Compounds," Division of Chemical Education/Undergraduate Research Poster No. 183.
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