9-Borabicyclo[3.3.1]nonane Dimer1

[70658-61-6]  · C16H30B2  · 9-Borabicyclo[3.3.1]nonane  · (MW 244.03)

(highly selective, stable hydroborating agent;1,3 anti-Markovnikov hydration of alkenes and alkynes;1d effective ligation for alkyl-, aryl-, allyl-, allenyl-, alkenyl- and alkynylboranes;1a,4,5 forms stable dialkylboryl derivatives, borinate esters, and haloboranes;1f organoboranes from hydroboration and organometallic reagents;1,5 precursor to boracycles;1,11 can selectively reduce conjugated enones to allylic alcohols;1a,30 organoborane derivatives for a-alkylation and arylation of a-halo ketones, nitriles, and esters;1b vinylation and alkynylation of carbonyl compounds;1a,46 conjugate addition to enones;1a,47 homologation; asymmetric reduction;1,8 Diels-Alder reactions;1a,18,50 enolboranes for crossed aldol condensations;1a,20,52 Suzuki-Miyaura coupling1a,54-57)

Alternate Name: 9-BBN-H.

Physical Data: mp 153-155 °C (sealed capillary); bp 195 °C/12 mmHg.1,3

Solubility: sparingly sol cyclohexane, dimethoxyethane, diglyme, dioxane (<0.1 M at 25 °C); sol THF, ether, hexane, benzene, toluene, CCl4, CHCl3, CH2Cl2, SMe2 (ca. 0.2-0.6 M at 25 °C); reacts with alcohols, acetals, aldehydes, and ketones.1,3

Form Supplied in: colorless, stable crystalline solid; 0.5 M solution in THF or hexanes.

Analysis of Reagent Purity: the melting point of 9-BBN-H dimer is very sensitive to trace amounts of impurities. Recrystallization from dimethoxyethane is recommended for samples melting below 146 °C. The dimer exhibits a single 11B NMR (C6D6) resonance at d 28 ppm and 13C NMR signals at 20.2 (br), 24.3 (t), and 33.6 (t) ppm.1,3

Handling, Storage, and Precautions: the crystalline 9-BBN-H dimer can be handled in the atmosphere for brief periods without significant decomposition. However, the reagent should be stored under an inert atmosphere, preferably below 0 °C. Under these conditions the reagent is indefinitely stable. In solution, 9-BBN is more susceptible both to hydrolysis and oxidation, and contact with the open atmosphere should be rigorously avoided. Many 9-BBN derivatives are pyrophoric and/or susceptible to hydrolysis so that individuals planning to use 9-BBN-H dimer should thoroughly familiarize themselves with the special techniques required for the safe handling of such reagents prior to their use.1b The reagent should be used in a well-ventilated hood.

Organoboranes from 9-BBN-H.

First identified by Köster,2 9-BBN-H dimer is prepared from the cyclic hydroboration of 1,5-cyclooctadiene (eq 1).3 As a dialkylborane, 9-BBN-H exhibits extraordinary steric- and electronic-based regioselectivities which distinguish these derivatives from the less useful polyhydridic reagents (Table 1).4

However, in contrast to other dialkylborane reagents (e.g. Disiamylborane or Dicyclohexylborane) which must be freshly prepared immediately prior to their use, 9-BBN-H dimer is a stable crystalline reagent1,3 which is commercially available in high purity. This feature of the reagent facilitates the control of reaction stoichiometry at a level unattainable with most borane reagents. The remarkable thermal stability of 9-BBN derivatives permits hydroborations to be conducted over a broad range of temperatures (from 0 °C to above 100 °C) either neat or in a variety of solvents.4 The B-alkyl-9-BBN products can frequently be isolated by distillation without decomposition and fully characterized spectroscopically.1,4,5 The integrity of the 9-BBN ring is retained even at elevated temperatures (200 °C), but positional isomerization in the B-alkyl portion can take place at ca. 160 °C.6

Like most other dialkylboranes, 9-BBN-H exists as a dimer, but hydroborates as a monomer (eq 2).7 In general, the rates of hydroboration follow the order R2C=CH2 > RHC=CH2 > cis-RHC=CHR > trans-RHC=CHR > RHC=CR2 > R2C=CR2.1,4 For relatively unsubstituted alkenes the dissociation of the 9-BBN-H dimer is rate-limiting (T1/2 at 25 °C &AApprox; 20 min) so that the hydroborations of typical 1-alkenes are normally complete in less than 3 h at room temperature. Competitive rate studies have revealed that electron-donating groups enhance the rates within these groups, e.g. for p-XC6H4CH=CH2, krel = 1 (X = CF3), 5 (X = H), 70 (X =  OMe).4c

Hydroborations of more substituted alkenes such as a-pinene8 or 2,3-dimethyl-2-butene4b with 9-BBN-H are slower (k2 is rate-limiting) and require heating at reflux temperature in THF for 2 and 8 h, respectively, for complete reaction to occur. However, the enantioselective reducing agent8a Alpine-borane® (see B-3-Pinanyl-9-borabicyclo[3.3.1]nonane) is formed quantitatively as a single enantiomer, the process taking place with complete Markovnikov regiochemistry, exclusively through syn addition from the least hindered face of the alkene (eq 3).

While the monohydroboration of symmetrical nonconjugated dienes with 9-BBN-H is thwarted by competing dihydroboration because these remote functionalities act as essentially independent entities, with nonequivalent sites the chemoselectivity of the reagent can be excellent (eq 4).4 Also, whereas the monohydroboration of conjugated dienes is not always a useful process because of competitive dihydroboration, highly substituted dienes and 1,3-cyclohexadiene produce allylborane products efficiently. In contrast to the monohydroboration of allene itself, which gives a 1,3-diboryl adduct, 9-BBN-H is an effective reagent for the preparation of allylboranes from substituted allenes.4g For example, excellent selectivity has been observed for silylated allenes where hydroboration occurs anti to the silyl group on the allene and at the terminal position (eq 5).9 It is also important to note that the diastereofacial selectivities of 9-BBN-H can be complementary to those obtained with Rh-catalyzed hydroborations (eq 6).10

Medium-ring boracycles are efficiently prepared by the dihydroboration of a,o-dienes with 9-BBN-H followed by exchange with borane.11 In this process 9-BBN-H is particularly useful because it not only fixes the key 1,5-diboryl relationship, but also the 9-BBN ligands do not participate in the exchange process (eq 7).

Unlike most dialkylboranes, 9-BBN-H hydroborates alkenes faster than its does the corresponding alkynes, a feature which leads to the competitive formation of 1,1-diboryl adducts in the hydroboration of 1-alkynes with 9-BBN-H employing a 1:1 stoichiometry (eq 8).1,12,13 In some cases, the (E)-1-alkenyl-9-BBN derivative can be efficiently prepared by employing either a large excess of the alkyne12,13 or through the use of 1-trimethylsilyl derivatives.6 However, these vinylboranes are now perhaps best prepared through the dehydroborylation of their 1,1-diboryl adducts with aromatic aldehydes (eq 8).12

By contrast, 9-BBN-H effectively monohydroborates internal alkynes to produce the corresponding vinylboranes in >90% yields.1,13 Compared to 1-alkynes, their 1-silyl counterparts also produce better yields of vinylboranes but, in contrast to normal internal alkynes, produce vicinal rather than geminal diboryl adducts with dihydroboration.6,13,14 Larger silyl groups can effectively be used to redirect the boron to the internal position producing the silyl-Markovnikov vinylborane, exclusively, without competitive dihydroboration.6 For 1-halo-1-alkynes, hydroboration with 9-BBN is slow, but the (Z)-1-halovinylboranes (eq 9) are produced cleanly and these are protonolyzed to provide cis-vinyl halides.15 The isomeric (Z)-2-bromovinyl-9-BBN derivatives are available from the bromoboration of 1-alkynes with B-Br-9-BBN16 (see 9-Bromo-9-borabicyclo[3.3.1]nonane). It is important to point out that the preference of 9-BBN-H to hydroborate alkenes in the presence of alkynes can have useful synthetic applications (eq 10).17

Organometallic reagents can provide very useful entries to many B-substituted 9-BBN derivatives. These are particularly important for organoboranes which cannot be prepared by hydroboration.5 Both B-alkoxy and B-halo derivatives serve as useful precursors to B-alkyl, -allyl, -aryl, -vinyl or -alkynyl-9-BBN derivatives (eqs 11-16). B-Halo-9-BBN derivatives are effectively vinylated with organotin reagents.18 However, B-MeO-9-BBN is superior to its B-halo counterparts for secondary and tertiary alkyllithium reagents where the latter undergo some reduction to 9-BBN-H through b-hydride transfer from the organolithium.

Hydroboration of the byproduct alkene gives isomeric B-alkyl-9-BBN products.5b Generally, hydrocarbon solvents are preferable to ether or THF for this process because the greater stability of the intermediate methoxyborate complexes (i.e. Li[R(MeO)-9-BBN]) at -78 °C in these solvents prevents the product from being formed and competing with B-MeO-9-BBN for the alkyllithium reagent prior to its complete consumption.5b The complex is stable for alkenyl and alkynyl derivatives which require BF3.Et2O to remove the methoxy moiety. The procedure has also been used for the preparation of cis-vinyl-9-BBN derivatives19 since the normal route to such derivatives based upon the hydroboration of 1-haloalkynes, followed by hydride-induced rearrangement gives ring expansion products competitively with (Z)-1-halovinyl-9-BBNs.20 Similar behavior has been observed for the reaction of a-methoxyvinyllithium with B-alkyl-9-BBNs (see 1-Methoxyvinyllithium).21

Derivatives of 9-BBN.

Like other boron hydrides, a variety of proton sources (ROH, RCO2H, RSO3H, HX (X = Cl, Br, OH, SH, O2P(OH)2, NHR)) as well as boron halides can be effectively employed to prepare useful derivatives from 9-BBN-H.1d,5b,22 The synthetic value of B-MeO-9-BBN lies principally in the preparation of B-alkyl derivatives through organometallic reagents as described above. As a byproduct in other processes, it is also easily converted to 9-BBN-H with BMS (eq 17).22

Efficient procedures have been developed for the preparation of B-Cl-9-BBN from 9-BBN-H (HCl in Et2O)5b and B-Br-9-BBN (BBr3 in CH2Cl2),23 the latter being a useful reagent for ether cleavage, the bromoboration of 1-alkynes, and for conjugative additions to enones (see 9-Bromo-9-borabicyclo[3.3.1]nonane). 9-BBN triflate is highly useful in formation of enolboranes for stereoselective crossed aldol reactions24 (see 9-Borabicyclononyl Trifluoromethanesulfonate). The B-acyloxy-9-BBN derivatives have been employed in conjuction with borohydrides for the reduction of carboxylic acids to aldehydes25 (see Lithium 9-boratabicyclo[3.3.1]nonane). Amine complexes of 9-BBN-H and borohydride derivatives are easily prepared from the addition of amines or metal hydrides to 9-BBN-H.26 B-Alkyl-9-BBNs and their borohydrides are very selective reducing agents and, with chiral terpenoid or sugar appendages, can also effectively function as enantioselective reagents (eq 18)8,27 (see B-3-Pinanyl-9-borabicyclo[3.3.1]nonane and Potassium 9-Siamyl-9-boratabicyclo[3.3.1]nonane). 9,9-Dialkylborate derivatives of 9-BBN are also highly selective reducing agents28 (see Lithium 9,9-Dibutyl-9-borabicyclo[3.3.1]nonanate), transferring a bridgehead hydride with rearrangement to bicyclo[3.3.0]octylboranes. This process is best accomplished with Acetyl Chloride and provides a highly versatile entry to these organoboranes for subsequent conversions (eq 19).29

Functional Group Conversions with 9-BBN-H.

9-BBN-H selectively reduces acid chlorides, aldehydes, ketones, lactones, and sulfoxides at 25 °C.1e Alcohol rather than amine products are produced as the major products from tertiary amides, while primary derivatives are not reduced effectively. Reduction is slow with esters, carboxylic acids, nitriles, and epoxides, and does not occur with nitro compounds, nor with alkyl or aryl halides. At 65 °C, carboxylic acids and esters are cleanly reduced to alcohols, the former being significantly slower (18 vs. 4 h). Moreover, 9-BBN-H is a highly selective reducing agent for the reduction of enones to allylic alcohols (eq 20).30

As noted earlier, B-substituted-9-BBN derivatives are available from a variety of sources and organoboranes serve as a versatile entry to other functionalities. Their oxidative conversion to alcohols with alkaline Hydrogen Peroxide or Sodium Perborate31 is quantitative and occurs with complete retention of configuration, making the process highly useful.1 The 9-BBN moiety is oxidized to cis-1,5-cyclooctanediol (eq 21), a compound which can be removed from less polar products through extraction with water, selective crystallization, or by chromatography.1 The monooxidation of 9-BBN derivatives with anhydrous Trimethylamine N-Oxide (TMANO) produces 9-oxa-10-borabicyclo[3.3.2]decanes (eq 22),14 many of which are air-stable and undergo useful coupling reactions.

Anomalous oxidation products are observed from the oxidation of tetraalkylborate salts (i.e. Li(R2-9-BBN)), which produces bicyclo[3.3.0]octan-l-ol as a co-product through a skeletal rearrangement which occurs during the oxidation process.32 Moreover, the alkaline hydrogen peroxide oxidation of 1,1-di-9-BBN derivatives gives primary alcohols rather than aldehydes because of their solvolysis prior to oxidation.6,13

While the protonolysis of B-alkyl-9-BBNs, like other trialkylboranes, with carboxylic acids takes place only at temperatures above 100 °C, B-vinyl derivatives are readily cleaved by HOAc at 0 °C with complete retention of configuration.1,13 This can be combined with other 9-BBN processes (e.g. thermal isomerization or dehydroborylation) to give remarkable overall conversions (eq 23).6 The hydrolysis of allylic and alkynic 9-BBN derivatives is more facile, occurring even with water or alcohols.5c,32

In the absence of light, molecular bromine readily cleaves B-(s-alkyl)-9-BBN derivatives through a hydrogen abstraction process, to give excellent yields of the corresponding alkyl bromides, the 9-BBN moiety being converted to 9-Br-9-BBN (eq 24).33 However, bicyclo[3.3.0]octylborinic and -boronic acids are produced from B-Me- and B-MeO-9-BBN through this radical bromination under hydrolytic conditions where the facile 1,2-alkyl migration of a ring B-C bond occurs (eq 25).1b,34 This latter compound serves as a convenient source of 9-oxabicyclo[3.3.1]nonane through base-induced iodination via an SE2-type inversion.35

Mechanistically similar to the oxidation of 9-BBN derivatives with TMANO, the amination of B-alkyl-9-BBN proceeds through ring B-C migration rather than through B-alkyl migration (eq 26).36 Similar behavior is observed for the thermal reaction of organic azides with these derivatives. Dichloroboryl derivatives have proved to be more versatile and general for the synthesis of amines, including optically active derivatives.37

Carbon-Carbon Bond Formation via 9-BBN Derivatives.

Consistent with the versatility of organoboranes in synthetically useful chemical transformations, most conversions with 9-BBN derivatives are very efficient and occur with strict stereochemical control.1 Of particular importance are those which involve the formation of new carbon-carbon bonds because valuable R groups can often be selectively transferred to the substrates without competition from the 9-BBN ring. For example, whereas only one of the alkyl, vinyl or aryl groups can be transferred from BR3 to the anions derived from a-halo ketones, esters and nitriles, these reactions are ideally suited to B-R-9-BBN derivatives which transfer the B-R group selectively (eq 27).1a,38 The vinylogous g-alkylation of g-bromo-a,b-unsaturated esters efficiently leads to g-substituted-b,g-unsaturated esters, the double bond transposition being commonly observed in the kinetic protonation of enolates with extended conjugation. Both sulfur ylides and a-bromo sulfones undergo related alkylations.1a

Base-induced eliminations of g-haloalkyl-9-BBN derivatives give cycloalkanes (C3 to C6)1b with inversion of configuration at both carbon centers.1,39 1,1-Diboryl adducts from the dihydroboration of 1-alkynes with 9-BBN-H serve as useful precursors to B-cyclopropyl-9-BBN derivatives by a similar process (eq 28).

The carbonylation of B-alkyl-9-BBNs at 70 atm, 150 °C in the presence of ethylene glycol produces intermediate boronate esters which are oxidized with alkaline hydrogen peroxide to give high yields of the corresponding carbinols (Scheme 1).40 In the presence of hydride-reducing agents (e.g. LiHAl(OMe)3 or Potassium Triisopropoxyborohydride), the carbonylation of B-R-9-BBN derivatives can be carried out at atmospheric pressures at 0 °C, producing an intermediate a-alkoxyalkylborane which can be further reduced with Lithium Aluminum Hydride. This results in homologated organoboranes and, after oxidation, alcohols. Alternatively, the intermediate a-alkoxyalkylborane can be directly oxidized to produce aldehydes (Scheme 1).1a,38c,41

As a useful alternative to carbonylation, the Brown dichloromethyl methyl ether (DCME) process has been effectively used for the synthesis of 9-alkylbicyclo[3.3.1]nonan-9-ols.42 The ketone bicyclo[3.3.1]nonan-9-one (eq 29)42b has also been prepared from a hindered B-aryloxy-9-BBN derivative, with simple B-alkoxy-9-BBN derivatives failing to undergo this process. However, most borinate esters are smoothly converted to ketones through this process, including germa- and silaborinanes (eq 30).11e,f In these cases, 9-BBN-H provides the essential 1,5-diboryl relationship which allows the formation of borinane by the exchange reaction described earlier.

Allylboration with 9-BBN derivatives (see B-Allyl-9-borabicyclo[3.3.1]nonane) is an efficient process, resulting in the smooth formation of homoallylic alcohols (eq 31).43 Alkynylboranes also undergo 1,2-addition to both aldehydes and ketones.44 As with other reactions producing B-alkoxy-9-BBN byproducts, the conversion of these to alcohols with Ethanolamine also results in the formation of an alkane-insoluble 9-BBN complex which is conveniently removed, thereby greatly simplifying the workup procedure.

Vinyl derivatives of 9-BBN uniquely undergo Grignard-like 1,2-additions to aldehydes to produce stereodefined allylic alcohols (eq 32).12,45 The thermal stability of the 9-BBN ring system, as well as its resistance to serve as a b-hydride source, facilitates this highly effective process. These vinylboranes are also the borane reagents of choice for the conjugate additions to enones which can adopt a cisoid conformation, providing a convenient entry to g,d-unsaturated ketones from enones (eq 33).46 Alkynylboranes undergo a related addition-elimination process with b-methoxyenones, giving enynones (eq 34).47

Vinyl(methoxy)-9-BBN ate complexes undergo an unusual homocoupling reaction when treated with Zinc Chloride (0.5 equiv) (eq 35).48 Related intermediates, formed through the CuI-catalyzed addition of stannylborate complexes to 1-alkynes, can be coupled either through catalytic palladium or stoichiometric copper chemistry to produce stereodefined vinylstannanes (eq 36).49

Both vinyl- and alkynyl-9-BBN derivatives are effective dienophiles in Diels-Alder cycloadditions, leading to boron-functionalized cyclohexenes in a selective manner (eqs 37 and 38).18,50 Silylated allenylboranes add selectively as allylboranes to aldehydes, a reaction which has been effectively used to prepare the steroid nucleus through a Hudrlik elimination followed by a Bergman rearrangement (eq 39).51

Stereodefined 9-BBN enolboranes which contain a directing chiral auxiliary undergo highly selective crossed aldol condensations as do other dialkylboryl systems (eq 40).20,52 The conjugate addition of B-Br-9-BBN also produces enolboranes which condense with aldehydes to produce, after the elimination of the elements of B-HO-9-BBN, a-bromomethyl enones stereoselectively (eq 41).53

While catechol- and disiamylborane derivatives were originally employed in the Pd-catalyzed cross coupling of organoboranes to unsaturated organic halides under basic conditions (Suzuki-Miyaura coupling), 9-BBN has recently found an important place in this process. Initially, B-(primary alkyl)-9-BBNs, with added bases (NaOH, TlOH, NaOMe, or K3PO4), were found to undergo efficient coupling with iodobenzene using Dichloro[1,1-bis(diphenylphosphino)ferrocene]palladium(II) as the catalyst.54 However, while secondary alkylboranes may require this catalyst, Tetrakis(triphenylphosphine)palladium(0) is perfectly satisfactory for the coupling of n- or i-alkyl-9-BBN derivatives to unsaturated bromides, iodides, or triflates under basic conditions (eqs 42-45).

Several recent applications include the syntheses of pharmaceuticals, pheromones, and prostaglandins, with complete retention of configuration being observed with alkenyl substrates.54b,55 Either carbon monoxide or t-Butyl Isocyanide can be used to prepare ketones through the sequential formation of two new carbon-carbon bonds with this reaction.56 Moreover, vinyl-9-BBNs (eqs 46-48) are also smoothly cross-coupled with retention of configuration to these substrates and, with these now being readily available, their expanded use in this process should flourish.12,57 It is important to mention that vinyl vs. primary alkyl group transfer is favored by oxygenated ligation on the borane.52b,58

Related Reagents.

Borane-Dimethyl Sulfide; Borane-Tetrahydrofuran.


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5. (a) Brown, H. C.; Rogić, M. M. JACS 1969, 91, 4304. (b) Kramer, G. W.; Brown, H. C. JOM 1974, 73, 1. (c) ibid., JOM 1977, 132, 9. (d) Soderquist, J. A.; Brown, H. C. JOC 1980, 45, 3571. (e) Soderquist, J. A.; Rivera, I.; Negron, A. JOC 1989, 54, 4051.
6. (a) Soderquist, J. A.; Colberg, J. C.; Del Valle, L. JACS 1989, 111, 4873. See also: (b) Negishi, E.-I. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol 7, p 265.
7. (a) Brown, H. C.; Scouten, C. G.; Wang, K. K. JOC 1979, 44, 2589. (b) Brown, H. C.; Wang, K. K.; Scouten, C. G. PNA 1980, 77, 698. (c) Wang, K. K.; Brown, H. C. JOC 1980, 45, 5303. (d) Nelson, D. J.; Cooper, P. J. TL 1986, 27, 4693. (e) Brown, H. C.; Chandrasekharan, J.; Nelson, D. J. JACS 1984, 106, 3768. (f) Chandrasekharan, J.; Brown, H. C. JOC 1985, 50, 518.
8. (a) Midland, M. M. CR 1989, 89, 1553. (b) Brown, H. C.; Ramachandran, P. V. PAC 1991, 63, 307; ibid., ACR 1992, 25, 16. (c) Srebnik, M.; Ramachandran, P. V. Aldrichim. Acta 1987, 20, 9. (d) Brown, H. C.; Srebnik, M.; Ramachandran, P. V. JOC 1989, 54, 1577.
9. Liu, C.; Wang, K. K. JOC 1986, 51, 4733.
10. (a) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. JACS 1988, 110, 6917. (b) Burgess, K.; van der Donk, W. A.; Jarstfer, M. B.; Ohlmeyer, M. JACS 1991, 113, 6139.
11. (a) Negishi, E.-I.; Burke, P. L.; Brown, H. C. JACS 1972, 94, 7431. (b) Burke, P. L.; Negishi, E.-I.; Brown, H. C. JACS 1973, 95, 3654. (c) Brown, H. C.; Pai, G. G. H 1982, 17, 77. (d) ibid., JOM 1983, 250, 13. (e) Soderquist, J. A.; Shiau, F.-Y.; Lemesh, R. A. JOC 1984, 49, 2565. (f) Soderquist, J. A.; Negron, A. JOC 1989, 54, 2462. However, for the unusual behavior of the 9-BBN systems with alkynyltins, see: (g) Bihlmayer, C.; Kerschl, S.; Wrackmeyer, B. ZN(B) 1987, 42, 715. (h) Wrackmeyer, B.; Abu-Orabi, S. T. CB 1987, 120, 1603. (i) Bihlmayer, C.; Abu-Orabi, S. T.; Wrackmeyer, B. JOM 1987, 322, 25.
12. Colberg, J. C.; Rane, A.; Vaquer, J.; Soderquist, J. A. JACS 1993, 115, 6065.
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15. Nelson, D. J.; Blue, C. D.; Brown, H. C. JACS 1982, 104, 4913.
16. Hara, S.; Dojo, H.; Takinami, S.; Suzuki, A. TL 1983, 24, 731.
17. Brown, C. A.; Coleman, R. A. JOC 1979, 44, 2328.
18. Singleton, D. A.; Martinez, J. P. JACS 1990, 112, 7423.
19. Brown, H. C.; Bhat, N. G.; Rajagopalan, S. OM 1986, 5, 816.
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21. Soderquist, J. A.; Rivera, I. TL 1989, 30, 3919.
22. Soderquist, J. A.; Negron, A. JOC 1987, 52, 3441.
23. (a) Bhatt, M. V. JOM 1978, 156, 221. See also: (b) Köster, R.; Grassberger, M. A. LA 1968, 719, 169. (c) Brown, H. C.; Kulkarni, S. U. JOC 1979, 44, 281. (d) ibid., JOC 1979, 44, 2422.
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John A. Soderquist

University of Puerto Rico, Rio Piedras, Puerto Rico



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