Aromatic Borataheterocycles: Surrogates for Cyclopentadienyl in

Jul 6, 2009 - Synopsis. This review describes the syntheses and ligand properties of a group of borataheterocycles, 2−6. These compounds are formall...
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Organometallics 2009, 28, 4236–4248 DOI: 10.1021/om900308e

Aromatic Borataheterocycles: Surrogates for Cyclopentadienyl in Transition-Metal Complexes Arthur J. Ashe III* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055 Received April 22, 2009

This review describes the syntheses and coordination chemistry of selected unsaturated borataheterocycles. These compounds are formally derived from common heteroaromatic rings by the isoelectronic substitution of an anionic BH- group for a neutral CH group. Specifically included are the five-membered rings 1,2- and 1,3-thiaborolyls, derived from thiophene, 1,2-oxaborolyl, derived from furan, and 1,2-azaborolyl, derived from pyrrole. Also included is the six-membered ring 1,2-azaboratabenzene, derived from pyridine. These anionic rings can serve as surrogates for cyclopentadienyl in organometallic compounds. Introduction The formal replacement of a CH group of an aromatic ring by the isoelectronic BH- group leads to the new family of anionic boron heterocycles, illustrated in Figure 1. In this manner, boratabenzene 1 is derived from benzene. Thiophene, furan, pyrrole, and pyridine are certainly the most important heterocyclic aromatic compounds. A similar formal substitution of BH- for CH converts these familiar neutral heterocycles to the anionic 1,2-thiaborolyl 2, 1,3thiaborolyl 3, 1,2-oxaborolyl 4, 1,2-azaborolyl 5, and 1,2azaboratabenzene 6. This review centers on the syntheses and ligand properties of borataheterocycles 2-6. The chemistry of boratabenzene has been extensively reviewed and is not a major focus of this article.1-4 However, for the sake of comparison with borataheterocycles 2-6, it is useful to recall some of its salient features. The first boratabenzene complex was reported in 1970 by Herberich and coworkers.5 The reaction of cobaltocene with boron halides leads to the unusual insertion of a BR group into the Cp rings to produce complexes 7 and 8. Subsequently it was found that these complexes could be degraded by treatment with cyanide to give alkali-metal salts of boratabenzenes 1.6 In 1971 Ashe and Shu reported a general synthesis of 1 using the exchange reaction of stannacyclohexadiene 9 with boron *To whom correspondence should be addressed. E-mail: ajashe@ umich.edu. (1) Herberich, G. E.; Ohst, H. Adv. Organomet. Chem. 1986, 25, 199. (2) Ashe, A. J., III; Al-Ahmad, S.; Fang, X. G. J. Organomet. Chem. 1999, 581, 92. (3) Fu, G. C. Adv. Organomet. Chem. 2001, 47, 101. (4) Norris, P. In Comprehensive Heterocyclic Chemistry III; Black, D. StC., Ed.; Elsevier: Oxford, U.K. 2008; Vol. 7, p 1049. (5) Herberich, G. E.; Greiss, G.; Heil, H. F. Angew. Chem., Int. Ed. Engl. 1970, 9, 805. (6) Herberich, G. E.; Becker, H. J.; Carsten, K.; Engelke, C.; Koch, W. Chem. Ber. 1976, 109, 3382. (7) Ashe, A. J., III; Shu, P. J. Am. Chem. Soc. 1971, 93, 1804. (8) (a) Ashe, A. J., III; Meyers, E.; Shu, P.; Von Lehman, T. M.; Bastide, J. J. Am. Chem. Soc. 1975, 97, 6865. (b) Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III; Al-Ahmad, S.; M€uller, C. J. Am. Chem. Soc. 1996, 118, 2291. (c) Ashe, A. J., III; Kampf, J. W.; M€uller, C.; Schneider, M. Organometallics 1996, 15, 387. pubs.acs.org/Organometallics

Published on Web 07/06/2009

halides followed by deprotonation of the intermediate boracyclohexadienes 10.7 Boratabenzenes have been converted to a large number of complexes of both late, e.g. 11,8a and early transition metals, e.g. 128b,8c (see Scheme 1). Although boratabenzenes are often compared to Cp as a ligand, they are obviously less symmetrical. Invariably the metal is slip-distorted away from boron so that the B-metal distance is longer than the C-metal distances (for 8a,9 Co-B = 2.28 A˚ and Co-C(av) = 2.17 A˚).10 In the case of bis(1-(diisopropylamino)boratabenzene)zirconium dichloride (12b) the slip distortion away from boron (Zr-B = 2.98 A˚) is so extreme that it appears to be η5-bound to zirconium.8a This distortion is considerably smaller for bis (1-phenylboratabenzene)zirconium dichloride (12c)9 (Zr-B = 2.81 A˚),11 which illustrates that the electronic properties of boratabenzene-transition-metal complexes can be controlled by exocyclic substituents on the boron atom. The effect has important consequences on reactivity. On activation by methylaluminoxane (MAO) 12b is a good catalyst for the polymerization of ethylene with an activity similar to that of Cp2ZrCl2.8b However, under identical conditions 12c affords only ethylene oligomers.11 These results show that boratabenzenes can serve as surrogates for Cp in important organometallic compounds, but the reactivities of these boron-based catalysts are distinct from those of their related Cp-based catalysts. One goal of examining complexes of borataheterocycles 2-6, which can also be viewed as Cp surrogates, is to find other compounds which can modulate the reactivity of Cpbased catalysts. It should be remembered that thiophene, furan, pyrrole, and pyridine are not particularly good π-ligands toward transition metals.12 The substitution of BH- for CH in the (9) Thorough this review the suffixes a-d indicate the substituents on boron. The substituents are Me for a, N(i-Pr)2 for b, Ph for c, and vinyl for d. (10) Huttner, G.; Kreig, B.; Gartzke, W. Chem. Ber. 1972, 105, 3424. (11) Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III; Al-Ahmad, S.; Kampf, J. W. Organometallics 1997, 16, 2492. (12) Elschenbroich, C. In Organometallics, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2005; p 560. r 2009 American Chemical Society

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Scheme 1. Syntheses of Boratabenzenes

Figure 1. Selected aromatic borataheterocycles.

heterocycles changes their charge type which should make 2-6 better π-donors. Since boron is more electropositive than carbon, there should be a smaller π-occupation at boron. This may have the consequence of making 2-6 better π-acceptors, particularly toward the late transition metals.

1,2- and 1,3-Thiaborolyls A larger variety of thiaborolyl ring systems are known than for any of the other borataheterocycles. Both the parent 1,2-thiaborolyl 213 and 1,3-thiaborolyl 314 and their benzofused-ring relatives (13 and 14) have been described, although the 2,1-benzothiaborolyl 15 remains unknown. The only B substituent is diisopropylamino, and the metal complexes are confined to those of Ru(II) and Zr(IV). This lack of diversity aids in the comparison of different compounds. The first example of a 1,3-thiaborolyl complex was the 1998 synthesis of the Cp*Ru complex of 3-(diisopropylamino)-1,3-benzothiaborolyl (18b), illustrated in Scheme 2.15 Dilithiation of thioanisole by BuLi/TMEDA followed by reaction with dibutyltin dichloride afforded 3,3-dibutyl-2,3dihydro-1,3-benzothiastannole (16) in 47% yield. The key step in construction of the boron-sulfur ring system was the boron/tin exchange reaction of 16 with BCl3 followed by immediate reaction with diisopropylamine, which gave the 2,3-dihydro-1,3-benzothiaborole 17b in 92% yield. Deprotonation of 17b with tert-butyllithium followed by reaction of the anion 14b with [Cp*RuCl]4 afforded a 64% yield of complex 18b. The Cp*Ru complex of 2-(diisopropylamino)-1,2-benzothiaborolyl (21b) has been prepared by a similar route (see Scheme 3).16 In this case the key step in construction of the boron-sulfur heterocycle was the reaction of the (13) Ashe, A. J., III. In Comprehensive Heterocyclic Chemistry III; Joule, J., Ed.; Elsevier: Oxford, U.K. 2008; Vol. 4, p 1189. (14) Varvounis, G. In Comprehensive Heterocyclic Chemistry III; Joule, J., Ed.; Elsevier: Oxford, U.K. 2008; Vol. 4, p 1225. (15) Ashe, A. J., III; Fang, X. G.; Kampf, J. W. Organometallics 1998, 17, 2379. (16) Ashe, A. J., III; Kampf, J. W.; Schiesher, M. W. Organometallics 2000, 19, 4681.

1,4-dilithio compound 19 with (i-Pr)2NBCl2, which afforded 20b. Deprotonation of 20b by tert-butyllithium gave the 1,2benzothiaborolyl 13b, which has been converted to Ru(II) complex 21b and Zr(IV) complex 22b. The synthesis of the monocyclic 1,3-thiaborolyl ligand is shown in Scheme 4.17 The sulfur and the three carbon atoms of the heterocycle are derived from the rather labile but easily prepared chloromethyl ethynyl sulfide (23).18 Treating 23 with Bu2SnH2/LDA at low temperature effected the nucleophilic substitution of chloride by tin to give intermediate 24, which on warming afforded 3,3-dibutyl-2,3-dihydro-1,3thiastannole (25) via an intramolecular hydrostannation of the ethynyl group. It was disappointing that the reaction of 25 with BCl3 gave only intractable products. However, the reaction of 25 with 2 equiv of BuLi gave the dilithio reagent 26, which was converted to the 2,3-dihydro-1,3-thiaborole 27b on reaction with (i-Pr)2NBCl2. Deprotonation of 27b followed by reaction with either [Cp*RuCl]4 or Cp*ZrCl3 gave the Cp*Ru complex 28b or Cp*ZrCl2 complex 29b, respectively. The monocyclic 1,2-thiaborolyl 2b has been prepared by a route involving ring-closing metathesis (RCM), as illustrated in Scheme 5.19 The appropriate (allylthio)vinylborane 31 needed for RCM could be prepared in three steps from commercially available tributylvinyltin. Upon treatment of 31 with 1 mol % Grubbs catalyst, (Cy3P)2(PhCH) RuCl2, in CH2Cl2 at 25 °C cyclization occurred smoothly to give 32b in 95% yield. Deprotonation of 32b with LDA in ether gave the lithium 1,2-thiaborolyl 2b, which could be isolated as a white solid. The reaction of 2b with [Cp*RuCl]4 generated the Cp*Ru complex 33b. 2b has also been converted to Cp*ZrCl2 complex 34b and to bridged Zr(IV) complex 35. The lithium salts of thiaborolyls and benzothiaborolyls have been isolated as white or yellow-white solids. The 1H, (17) Ashe, A. J., III; Fang, X. G.; Kampf, J. W. Organometallics 1999, 18, 1821. (18) Brandsma, L.; Verkruijsse, H. D. In Synthesis of Acetylenes, Allenes, and Cumulenes; Elsevier: Amsterdam, 1981; p 73. (19) Ashe, A. J., III; Fang, X. D.; Kampf, J. W. Organometallics 2000, 19, 4935.

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Scheme 2. Synthesis of 1,3-Benzothiaborolyl

Scheme 3. Synthesis of 1,2-Benzothiaborolyl

Scheme 4. Synthesis of 1,3-Thiaborolyl

11 B, and 13C NMR spectra have been recorded in THF-d8, in which these compounds are likely to be contact ion pairs.20 Comparison of the spectra of the parent 1,2-thiaborolyl 2b19 and 1,3-thiaborolyl 3b,17 illustrated in Figure 2, is particularly interesting, since it suggests differences in the degree of electron delocalization for the two isomers. The NMR spectra of the 1,2-thiaborolyl 2b suggest that the compound is essentially an allyl anion which is stabilized by the terminal heteroatoms. The 13C NMR signals for C(3) and C(5) at δ 84.0 and δ 80.6, respectively, are consistent with

(20) For CpLi see: Cox, R. H.; Terry, H. W., Jr. J .Magn. Reson. 1974, 14, 317.

carbanionic character for these atoms, while the signal for C(4) at δ 136.8 suggests little excess π-electron density at this position.21 Similarly, the 1H NMR chemical shifts of C(3)H (δ 4.69) and C(5)H (δ 3.93) are upfield from that of the presumably less negative C(4)H (δ 6.78). The 11B NMR shift of 2b (δ 39.5) is upfield from its conjugate acid 32b (δ 43.6), which is consistent with an enhanced electron density at boron.22 Apparently the negative charge in the π-system (21) (a) Olah, G. A.; Asensio, G.; Mayr, H.; Schleyer, P. v. R. J. Am. Chem. Soc. 1978, 100, 4347. (b) Jutzi, P.; Meyer, M.; Rasika Dias, H. V.; Power, P. P. J. Am. Chem. Soc. 1990, 112, 4841. (22) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1988, 20, 1.

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Scheme 5. Synthesis of 1,2-Thiaborolyl

is distributed largely to B(2), C(3), and C(5), as would be expected from consideration of the classical resonance structures. For 1,3-thiaborolyl 3b the 1H and 13C NMR signals for C(2)H and C(2) at δ 2.89 and δ 59.8, respectively, are consistent with an sp2 hybridization at C(2) which bears a high degree of negative charge.17,21 The 11B NMR shift of 3b (δ 35.1) shows an upfield shift relative to that of 27b (δ 43.9), which is consistent with an enhanced electron density at boron.22 The 1H NMR signals (δ 6.19, 6.95) and the 13C NMR signals (δ 126.8, 132.2) for the CH groups at C(4) and C(5), respectively, are in the normal aromatic/olefinic region. Apparently little negative charge is transferred to these atoms, which is consistent with consideration of the resonance structures. Unlike those of their lithium salts, the NMR spectra of the Cp*Ru complexes of 1,2-thiaborolyls and 1,3-thiaborolyls are quite similar. Thus, the 11B and 13C NMR signals of the heterocyclic ring atoms of 33b and 28b are shifted upfield relative to those of 2b and 3b, respectively. Although precise comparison is not warranted because of differences in solvents, similar upfield shifts are observed in the 1H NMR spectra for the signals for the CH ring atoms. These π-coordination shifts are consistent with a similar η5-coordination of the ring atoms to the ruthenium.23 In general the B-N bonds of aminoboranes have doublebond character.24-27 Typical rotational barriers are in the range of 15-23 kcal/mol, which has the consequence of making the N substituents nonequivalent in the 1H and 13C (23) Elschenbroich, C.; Salzer, A. In Organometallics, 2nd ed.; VCH: Weinheim, Germany, 1991; pp 296, 307. (24) Wiberg, E. Naturwissenschaften 1948, 35, 182. (25) (a) Greenwood, N. N.; Thomas, B. S. In Comprehensive Inorganic Chemistry; Bailor, J. C., Emeleus, H. J., Nyholm, R., Trotman-Dickenson, A. F., Eds.; Pergamon: Oxford, U.K., 1973; Vol. 1, p 916. (b) Pelter, A.; Smith, K. In Comprehensive Organic Chemistry; Barton, D., Ollis, W. D., Eds.; Pergamon: Oxford, U.K., 1979; Vol. 3, p 925. (26) (a) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 3402. (b) Budzelaar, P. H. M.; Kos, A. J.; Clark, T.; Schleyer, P. v. R. Organometallics 1985, 4, 429. (27) (a) Imbery, D.; Jaeschke, A.; Friebolin, H. Org. Magn. Reson. 1970, 2, 271. (b) Freibolin, H.; Rensch, R.; Wendel, H. Org. Magn. Reson. 1976, 8, 287. (c) Freibolin, H. Basic One- and Two-Dimensional NMR Spectroscopy, 3rd ed.; Wiley-VCH: Weinheim, Germany, 1998; p 316.

NMR spectra recorded at ambient temperatures.27 In this respect the dihydrothiaboroles (17b, 20b, 27b, 32b) are typical boramines, since their NMR spectra showed nonequivalent isopropyl signals. On the other hand, the lithium thiaborolyls and their Cp*Ru complexes showed equivalent NMR signals for the isopropyl groups, indicating rapid rotation about the B-N bonds. In the case of 18b the barrier has been evaluated as 10.2 kcal/mol.15 This indicates that incorporation of a boron atom into an aromatic ring greatly diminishes its ability to form an external π-bond to nitrogen. A similar effect has previously been noted for aminoboratabenzenes.8c X-ray structural data are available for Cp*Ru complexes of the four different thiaborolyls (18b, 21b, 28b, 33b). The structural data show that all the C3BS rings are π-coordinated aromatic rings. All structures are essentially diheteroruthenocenes in which the five thiaborolyl ring atoms are ηbound to Ru. In all cases the Ru atoms are closer to the Cp* plane than to the thiaborolyl ring. The Ru atom is slipdistorted away from boron so that the Ru-B distance is the longest. Similar slip distortions away from boron are common features of π-coordinated boron heterocycles. Comparison of the structures of the complexes of the monocyclic 1,2and 1,3-thiaborolyls (33b and 28b) shows that the metalring bonding is very similar for the two isomers (parts a and b of Figure 3). The only Zr(IV) thiaborolyl complex for which structual data are available is the Me2Si-bridged compound 35. Overall the structure is that of a typical ansa-zirconocene and resembles that of [Me2Si(C5H4)2]ZrCl2.28 However, the structure of 35 differs from that of 33b in that the Zr is quite unsymmetrically bound to 1,2-thiaborolyl ring. The Zr atom is strongly slip-distorted away from B so that the B-Zr distance (2.952(2) A˚) is too long for effective bonding, which leaves it η4 coordinated to the C3S unit. In addition the boron atom is strongly π-bonded to the sp2-hybridized nitrogen, as shown by the short B-N distance (1.395(21) A˚). These structural features are strongly reminiscent of the structure of the aminoboratabenzene zirconium complex 12b.8b In the (28) Bajgur, C. S.; Tikkanen, W. R.; Petersen, J. L. Inorg. Chem. 1985, 24, 2539.

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Figure 2. Comparison of the 1H NMR, 13C NMR (in parentheses) and 11B NMR [in brackets] chemical shift values of 2b, 3b, 33b, and 28b. The spectra of 2b and 3b were recorded in THF-d8, and spectra of 33b and 28b were recorded in benzene-d6. n.o. = not observed.

NMR spectra of 35 and those of 22b, 29b, and 34b nonequivalent isopropyl groups indicate slow rotation about the exocylic B-N bonds. Thus, it seems likely that the structures of all of these compounds would show a similar unsymmetrical metal-ring bonding. On activation by a 103 molar excess of MAO in a hydrocarbon solvent, thiaborolyl zirconium complexes 29b,17 34b,19 and 22b29 formed active catalysts for the polymerization of mixtures of ethylene and 1-octene. These results are compared with those of similar zirconium catalysts in Table 1. In all cases the products were polyethylene, with about 1% incorporation of 1-octene. The 1,2- and 1,3thiaborolyl complexes 29b and 34b showed essential identical activity, which was slightly less than that of boratabenzene complex 12b. The somewhat greater activity of 22b29 relative to that of 29b is consistent with the general observation that zirconium indenyl complexes are usually more active than their corresponding cyclopentadienyl relatives.

Figure 3. Molecular structures of (a) 28b and (b) 33b. Hydrogen atoms have been omitted. Table 1. Comparison of the Efficiency of Ethylene Polymerization of Selected Zr(IV) Complexes complex

efficiency ((kg of polymer)/(mol of Zr atm))a

ref

b

1,2-Oxaborolyl 2-Substituted 1,2-oxaborolyls have been prepared by two routes, which are outlined in Scheme 6.30 (Allyloxy)(diisopropylamino)vinylborane (36) was prepared in 87% yield from 30 by treatment with lithium allyloxide. Upon treatment with 2% Grubbs catalyst in methylene chloride 36 underwent RCM to give the ring-closed product 37b in 92% yield. Deprotonation of 37b with tert-butyllithium gave the lithium salt 4b as a pale yellow powder. The reaction of 4b with BrMn(CO)5 afforded the 3-substituted Mn(CO)5 complex 38, for which structural data have been obtained.31 The (29) Schiesher, M. W. Ph.D. Dissertation; University of Michigan, 2002; Chem. Abstr. 2002, 139, 22313. (30) Chen, J.; Fang, X. D.; Bajko, Z.; Kampf, J. W.; Ashe, A. J., , III. Organometallics 2004, 23, 5088. (31) Bajko, Z.; Kampf, J. W.; Ashe, A. J., III. Private communication to the Cambridge Crystallographic Database, deposition number CCDC 732430.

Cp2ZrCl2 [200] 8b 12b 204 8b, 17 22b 108 29 29b 72 17 34b 60 19 68 1200 47, 50 69 660 47, 50 63c 2340 45, 50 73 2730 47, 50 74 660 47, 50 75 1260 45, 50 a For the conditions of the polymerizations, see ref 45. b Cp2ZrCl2 and 12b have approximately the same efficiency under different conditions, as discussed in ref 9.

reaction of 4b with mixture, from which seems probable that responsible for the

Cp*ZrCl3 in THF gave a complex a trace of 39b has been isolated.32 It the high oxophilicity of Zr(IV) was low yield. Unfortunately the small

(32) Ashe, A. J., III; Bajko, Z.; Fang, X. D.; Kampf, J. W.; Yang, H. Phosphorus, Sulfur, Silicon Relat. Elem. 2004, 179, 711.

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Scheme 6. Syntheses of 1,2-Oxaborolyls

amount of 39b precluded any measurement of its polymerization activity. A second synthesis involved the boron-tin exchange of 2,2-dibutyl-2,5-dihydro-1,2-oxastannole (40) with phenylboron dichloride, which afforded an 83% yield of the 2,5-dihydro-1,2-oxaborole 41c. Since 40 is available in quantity from the reaction of Bu2SnH2 with propargyl alcohol,33 this preparation is quite facile. Deprotonation of 41c with KN(SiMe3)2 in ether gave K-4c as a pale yellow powder in 86% yield. The reaction of 4c with [Cp*RuCl]4 gave a 70% yield of the Cp*Ru complex 42c as bright amber crystals. The reaction of 4c with Mn(CO)3(CH3CN)3PF6 gave the η5-coodinated Mn(CO)3 complex 43c in 63% yield. The 1H, 11B, and 13C NMR spectra of the alkali-metal salts of 1,2-oxaborolyls 4b,c have been recorded in THF-d8. The chemical shift values are summarized in Figure 4. Comparison of the NMR spectra of 4b (Figure 4) and 2b (Figure 2) is interesting, since it suggests differences in the degree of π-electron delocalization between these congeneric compounds. For 1,2-oxaborolyl 4b the 1H and 13C NMR resonances of C(3)H and C(3) at δ 2.44 and 58 are consistent with sp2 hybridization at C(3), which bears a high degree of negative charge.21,30 The 11B NMR signal of 4b (δ 37.0) is an upfield signal in the same range as that of 2b (δ 39.5), which is consistent with a negatively charged boron atom.22 The 13C NMR signals for C(4) (δ 118.8) and C(5) (δ 122.0) and the 1H NMR signals for C(4)H (δ 6.04) and C(5)H (δ 6.2) are in the normal olefinic region. In contrast to the case for 2b, little negative charge seems to be transferred to the C(5) atom of 4b. It is also useful to compare the NMR spectra of 4c with those of 5c (also illustrated in Figure 4). The downfield 1H and 13C NMR signals for C(5)H and C(5) for both com(33) Massol, M; Satge, J.; Bouyssieres, B. Synth. Inorg. Met.-Org. Chem. 1973, 3, 1.

pounds are in the same region of the spectrum. Indeed, all of the NMR signals of 4c and 5c are very similar, which is consistent with a similarity of the electronic structures of the boron-oxygen and boron-nitrogen heterocycles. The 1H, 11B, and 13C NMR signals for all of the oxaborolyl ring atoms of 42c and 43c are shifted upfield relative to those of 4c. These shifts are consistent with the η5 coordination of the oxaborolyl rings to Ru and Mn, shown in their X-ray structures.23 See Figure 5 for the molecular structure of 42c. The ligand properties of 1,2-oxaborolyl can be compared with those of the isoelectronic (but uncharged) furan and those of cyclopentadienyl. Furan is an extremely poor πligand toward transition metals. The only complex containing an η5-furan is Chaudret’s [Cp*Ru(η-C4H4O)]Cl (44), which was reported to be stable only at low temperatures in noncoordinating solvents.34 The experimental evidence for the formation of 44 was limited to a low-temperature 1H NMR spectrum. In contrast, 42c is a robust crystalline compound which is stable in most common solvents at room temperature. The CO stretching frequencies in the IR spectrum of 43c in hexane are 2039, 1966, and 1948 cm-1, which are virtually identical with those of (1-phenylboratabenzene) Mn(CO)3.35 In contrast, the CO stretching frequencies of CpMn(CO)3 are 2028 and 1944 cm-1.35 These data suggest that boratabenzene and 1,2-oxaborolyl are weaker donors but better acceptors than Cp.

1,2-Azaborolyl 1,2-Azaborolyl chemistry has been much more thoroughly developed that that of the thiaborolyls and oxaborolyls. This is largely due to the extensive work by Schmid and (34) Chaudret, B.; Jalon, F. A. J. Chem. Soc., Chem. Commun. 1988, 711. (35) Herberich, G. E.; Becker, H. J. Angew. Chem. Int. Ed., Engl. 1973, 12, 764.

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Figure 5. Molecular structure of 42c. Hydrogen atoms have been omitted.

Figure 4. Comparison of the 1H NMR, the 13C NMR (in parentheses), and the 11B NMR [in brackets] chemical shift values of 4b,c, 5c, 42c, and 62c. The spectra of 4b,c and 5c were recorded in THF-d8, and the spectra of 42c and 62c were recorded in benzene-d8. n.o.=not observed.

co-workers in the 1980s.36 Since this work has been previously reviewed,13,36,37 we will only outline it here. The synthetic starting points for the Schmid work are N-(trimethylsilyl)- and N-tert-butylallylamines, which can be dilithiated to give 45.38,39 The reaction of 45 with (most commonly) MeBBr2 affords the 2,5-dihydro-1,2-azaborole 46, which on deprotonation by LTMP affords the corresponding (1,2-azaborolyl)lithium species 5a. Reaction of 5a with transition-metal halides gave metal complexes: e.g., 47 and 48. A large number of complexes of both early and late transition metals have been prepared. For the most part the 1,2-azaborolyl group is η5-bound to the metals (see Scheme 7). Although Schmid reported a large number of metal 1,2azaborolyl complexes, only a small number of different ring substituents were originally described. The nitrogen substituents were confined to t-Bu and Me3Si, while the boron substituent was usually methyl (sometimes phenyl). Subsequent work has considerably expanded this list. Fu and coworkers have found that the chloro group of 2,5-dihydro1,2-azaborole 50 can be readily exchanged for a variety of (36) (a) Schmid, G. In Comprehensive Heterocyclic Chemistry II; Shinkai, I., Ed.; Pergamon: Oxford, U.K., 1996; Vol. 3, p 739. (b) Schmid, G. Comments Inorg. Chem. 1985, 4, 17. (37) Bosdet, M. J. D.; Piers, W. E. Can. J. Chem. 2009, 87, 8. (38) H€ anssgen, D.; Odenhausen, E. Chem. Ber. 1979, 112, 2389. (39) Schulze, J.; Schmid, G. J. Organomet. Chem. 1980, 193, 83. (40) Liu, S.-Y.; Lo, M. M.-C.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 174. (41) Liu, S.-Y.; Hills, I. D.; Fu, G. C. Organometallics 2002, 21, 4323.

nucleophiles.40,41 Subsequent reaction of 51 with base followed by complexation with metals gave complexes 52. Alternatively, 50 could be directly converted to the Fe(II) complex 53. Substitution of the chlorine in 54 afforded 55. Electrochemical oxidation potentials of compounds 55 showed that the 1,2-azaborolyl is more electron-rich than Cp (see Scheme 8). Since polymethylcyclopentadienyls are important sterically demanding ligands in organometallic chemistry,42 the synthesis of polymethyl-1,2-azaborolyls should be useful. Fang has recently reported on a method for obtaining the polymethyl-1,2-azaborolyls shown in Scheme 9.43 Dilithiation of N-methylmethallylamine followed by reaction with Bu2SnCl2 gave the 2,5-dihydro-1,2-azastannole 56. The exchange reaction of 56 with BCl3 followed by reaction with methylmagnesium bromide afforded 57, which was easily converted to the (trimethyl-1,2-azaborolyl)lithium species 58. Methylation of 58 with methyl iodide took place at the 3-position, while subsequent deprotonation afforded the (tetramethyl-1,2-azaborolyl)lithium species 59. Ashe and co-workers have found that an RCM procedure similar to that discussed for the syntheses of 2 and 4 could be used to prepare 1,2-azaborolyls, as illustrated in Scheme 10.44 Thus, the reaction of B-vinyl-N-allylboramines 60 with 5% Grubbs catalyst in methylene chloride afforded an 82% yield of 2,5-dihydro-1,2-azaboroles 61, which were readily converted to the azaborolyls 5. The reaction of 5c with either [Cp*RuCl]4 or Cp*ZrCl3 gave the Cp*Ru complex 62c44 or Cp*ZrCl2 complex 63c,45 respectively. The RCM reaction has also been used to prepare fused-ring derivatives, as shown in Scheme 11.46,47 Thus, 64 underwent a double RCM, which gave 65 in 59% yield. Subsequent oxidation to 66 followed by deprotonation gave the boron-nitrogen analogue of the indenyl anion 67. Compounds 65 and 67 have been converted to the Zr(IV) derivatives 69 and 68, respectively. (42) (a) For Me5Cp: Manriquez, J. M.; Fagan, P. J.; Schertz, L. D., Marks, T. J. Inorg. Synth. 1990, 28, 317 and references cited therein. (b) For a recent example of Me4Cp: Chirik, P. J. Dalton Trans. 2007, 16. (43) Fang, X. D.; Assoud, J. Organometallics 2008, 27, 2408. (44) Ashe, A. J., III.; Fang, X.-D. Org. Lett. 2000, 2, 2089. (45) Yang, H.; Fang, X. D.; Kampf, J. W.; Ashe, A. J., III. Polyhedron 2005, 24, 1280. (46) Ashe, A. J., III; Yang, H.; Fang, X. D.; Kampf, J. W. Organometallics 2002, 21, 4578. (47) Fang, X. D.; Yang, H.; Kampf, J. W.; Banaszak Holl, M. M.; Ashe, A. J., III. Organometallics 2006, 25, 513.

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Organometallics, Vol. 28, No. 15, 2009 Scheme 7. Schmid Synthesis of 1,2-Azaborolyls

Scheme 8. Functionalization of 1,2-Azaborolyls at Boron

Scheme 9. Syntheses of Polymethyl 1,2-Azaborolyls

Scheme 10. RCM Synthesis of 1,2-Azaborolyls

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Scheme 11. RCM Synthesis of Fused -Ring 1,2-Azaborolyls

Structural data are available for a large number of 1,2azaborolyl transition-metal complexes. The molecular structure of 63c, a fairly typical early-transition-metal complex, is illustrated in Figure 6.45,46 Overall, the structure resembles that of the corresponding cyclopentadienyl complex, zirconocene dichloride. The largely coplanar 1,2-azaborolyl ligand binds to the metal in an unsymmetrical η5 manner. The B-Zr distance (2.68 A˚) is somewhat longer than the C-Zr and N-Zr distances (2.44-2.54 A˚). The longer boronmetal bond distance is consistent with the pattern shown by most metal π-coordinated complexes of boron heterocycles. The patent literature records the use of several zirconium(IV) 1,2-azaborolyl complexes as Ziegler-Natta catalysts for the polymerization of olefins. These include 71 and 72 and their bis(1,2-azaborolyl) relatives (no stereochemistry noted).48,49 Since there are no standard polymerization conditions, it is always difficult to compare data from different laboratories. However, it was noted that activated 71 and its derivatives were less active catalysts than Cp2ZrCl2.

The ethylene-1-octene polymerization data given in Table 1 were all recorded under the same conditions in the Midland Michigan Laboratories of the Dow Chemical Co. The derivatives of the 1-ethyl-2-phenyl-1,2-azaborolyl zirconium(IV) complexes 63c and 73-75 were found to be significantly more active toward ethylene polymerization than Cp2ZrCl2. In the cases of 63c and 73 the activities of these compounds approach those of commercial catalysts.45,50 (48) Nagy, S.; Krishnamatti, R.; Etherton, B. P. U.S. Patent 6,228,959, 2001; Chem. Abstr. 1997, 126, 19432j. (49) Wang, Q.; Zoricak, P.; Gao, X. Can. Pat. Appl. 2,225,014, 1999; Chem. Abstr. 1999, 142, 219701. (50) Ashe, A. J., III; Yang, H.; Timmers, F. J. U.S. Patent 7074865 B2, 2006; Chem. Abstr. 2003, 139, 323949.

We note that the substitution pattern of the system has yet to be optimized for polymerization activity. Although ansazirconocene catalysts often have higher activities than the corresponding nonbridged zirconocenes,51 the activities of the dimethylsilyl-bridged azaborolyl zirconium complexes 74 and 75 are somewhat lower than those of 63c and 74. However, the molecular structures of the dimethylsilylbridged complexes closely resemble those of the corresponding ansa-zirconocenes. It may be that the lack of conformational mobility enforced by the bridge holds the 1-ethyl and 2-phenyl substituents in positions which interfere with the propagation steps. Activated zirconium(IV) indenyl complexes are usually more active than the corresponding zirconium(IV) Cp catalysts.51 Since 68 is approximately twice as active as 69, this trend also applies to zirconium azaborolyl complexes.47 The first yttrium azaborolyl complex 77 has recently been reported, as illustrated in Scheme 12.52 The reaction of 5c with YCl3(THF)3 followed by treatment with ((trimethylsilyl)methyl)lithium afforded a mixture of rac and meso isomers, from which pure rac-77 can be obtained by recrystallization. The structure showed that the yttrium is only weakly bound to the boron, so that the coordination approaches η4. It was found that 77 could initiate the living polymerization of methyl methacrylate, which gave syndiotactic-rich polymethyl methacrylate. This result is consistent with a site-controlled polymerization mechanism. Fu and co-workers have used enantiopure iron(II) azaborolyl complexes to effect highly stereoselective reactions.53 Complex 78 was prepared from 53, as illustrated in Scheme 13. The racemic 78 was then resolved using chiral HPLC. Optically active 78 was converted to the tosylate, which was applied to the Mukaiyama aldol reaction. Hydrolysis of the product 80 gave the azaborolyl-free β-hydroxy ester with high ee. Alternatively, 78 was converted to the triflate 81. Reaction of 81 with an imine activates it so that it could be stereoselectively alkylated (e.g., 81 to 82).54 Hydrolysis furnished the azaborolyl-free alkylated amine with high ee. These reactions demonstrate that iron 1,2-azaborolyl complexes can efficiently transfer chirality to boron-bound organic substrates. (51) Gladysz, J. A., Ed. Chem. Rev. 2000, 100, 1167-1682. (52) Fang, X. D.; Deng, Y.; Xie, Q.; Moingeon, F. Organometallics 2008, 27, 2892. (53) Liu, S.-Y.; Hills, I. D.; Fu, G. C. J. Am. Chem. Soc. 2005, 127, 15352. (54) Liu, S.-Y; Lo, M. M.-C.; Fu, G. C. Tetrahedron 2006, 62, 11343.

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Figure 7. Comparison of the 1H NMR, 13C NMR (in parentheses), and 11B NMR [in brackets] chemical shift values of 6c and 1c. The spectra were recorded in THF-d8.

Figure 6. Molecular structure of 63c. Hydrogen atoms have been omitted. Scheme 12. Preparation of an Yttrium 1,2-Azaborolyl

Figure 8. Molecular structure of 88. Hydrogen atoms have been omitted.

1,2-Azaboratabenzenes 1,2-Azaboratabenzenes are generally available by the NH deprotonation of their conjugate acids, the 1,2-dihydro-1,2azaborines 83. Although there are a number of syntheses of derivatives of 83,37,55 we will only be concerned here with those which lead to 1,2-azaboratabenzenes. Dewar and coworkers reported the synthesis of a 1,2-benzazaboratabenzene in 1959, using the route outlined in Scheme 14.56 The boron-nitrogen heterocyclic ring system was constructed efficiently using the reaction of BCl3 with 2-aminostyrene, which gave 84. Alkylation of 84 followed by deprotonation afforded the benzazaboratabenzene 86, as indicated by trapping with methyl iodide. The groups of Paetzold57 and Ashe58 have subsequently investigated the coordination chemistry of this ring system. Pan and Ashe have prepared the B-phenyl and B-vinyl derivatives of the monocyclic 1,2-azaboratabenzene, as illustrated in Scheme 15.59 The key step in the construction of (55) Reviews: (a) Dewar, M. J. S. Prog. Boron Chem. 1964, 1, 235. (b) Fritsch, A. J. Chem. Heterocycl. Compd. 1977, 30, 381. (56) Dewar, M. J. S.; Dietz, R. J. Chem. Soc. 1959, 2728. (57) Paetzold, P.; Stanescu, C.; Stubenrauch, J. R.; Bienm€ uller, M.; Englert, U. Z. Anorg. Allg. Chem. 2004, 630, 2632. (58) Pan, J.; Kampf, J. W.; Ashe, A. J., III. Organometallics 2009, 28, 506. (59) Pan, J.; Kampf, J. W.; Ashe, A. J., III. Organometallics 2004, 23, 5626.

Figure 9. Molecular structure of 89. Hydrogen atoms have been omitted.

the six-membered boron-nitrogen ring involved the ring expansion of 1,2-azaborolyl 5e using CH2Cl2 and base to give 87.60 Removal of the SiMe3 group in the usual manner gave 83. Deprotonation of 83 by KN(SiMe3)2 in toluene gave K-6c as a white powder. The 1H, 11B, and 13C NMR chemical shift values are very similar to those of Li-1c, as illustrated in Figure 7. For both anions the high-field 11B chemical shift values indicate strong stabilization by π-bonding to boron.22 The 1H and 13C NMR signals for the CH groups R and γ to boron are shifted upfield relative to those which are β to boron. These shifts are consistent with a greater carbanionic character of the R- and γ-carbon atoms.21 From an electronic standpoint 6c appears to be a perturbed boratabenzene. (60) Ashe, A. J., III; Fang, X. D.; Fang, X. G.; Kampf, J. W. Organometallics 2001, 20, 5413.

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Scheme 13. Use of 1,2-Azaborolyl Iron Complexes in Stereoselective Syntheses

Scheme 14. Dewar’s Synthesis of 1,2-Benzazaboratabenzene

The reaction of 6c with the electrophilic Cp2ZrCl2 gave the bis(2-phenyl-1,2-azaboratabenzene)zirconocene 88, in which both ring nitrogen atoms are σ-bonded to zirconium61 (see Figure 8). Both C4BN rings are planar ((0.02 A˚), and the intra-ring distances are very close to those calculated for the parent 1,2-dihydro-1,2-azaborine. It appears that the σ-bonded nitrogen does not significantly affect the electron delocalization in the ring. On the other hand, the reaction of 6c with [Cp*RuCl]4 gave complex 89, in which the ruthenium is η6-bound to the ring59 (see Figure 9). The structure of 89 also shows that the C4BN ring is planar. Therefore, the coordination chemistry of 1,2-azaboratabenzene resembles that of pyrrolyl, which also forms σ- or π-complexes depending on the conditions. Thus, pyrrolyl reacted with Cp2ZrCl2 to form (η1-C4H4N)2ZrCp2,62 which is analogous to 88 and was converted to (η5-C4H4N)RuCp*,63 which is analogous to 89. Compound 6d, which has a B-pendant vinyl group, showed a more complicated reaction with excess [Cp*RuCl]4.64 The X-ray structure of the product 91 showed (61) Pan, J.; Kampf, J. W. Organometallics 2008, 27, 1345. (62) Bynum, R. V.; Hunter, W. E.; Rogers, R. D.; Atwood, J. L. Inorg. Chem. 1980, 19, 2368. (63) Garrett, C. E.; Fu, G. C. J. Am. Chem. Soc. 1998, 120, 7479. (64) Pan, J.; Kampf, J. W.; Ashe, A. J., III. J. Orgmet. Chem. 2009, 694, 1036.

Scheme 15. Synthesis of 1,2-Azaboratabenzene

that two Cp*Ru units were bound to the 2-vinyl-1,2-azaboratabenzene moiety. The first Cp*Ru was π-bound to the 1,2azaboratabenzene unit in the same manner as in 89, while the second was bound to the nitrogen, to the pendant B-vinyl group, and to chlorine. Compound 83 is too weakly acidic to be measurably deprotonated in aqueous alcohols. However, it can be reversibly deprotonated by strong bases in THF or DMSO. Bracketing experiments in DMSO indicated that its acidity is comparable with that of pentamethylcyclopentadiene

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Scheme 16. Transition-Metal Complexes of 1,2-Azaboratabenzenes

Scheme 17. Haptotropic Migrations of a Cr(CO)3 Group to an Anionic Ring

Scheme 18

(pKa ≈ 26).65 On coordination the acidity markedly increases, since the pKa of 90 is 9.21 ( 0.10 in 85% MeOH/H2O. The basicity of 89 is comparable with that of the nucleophilic catalyst 4-(dimethylamino)pyridine (DMAP). Although 89 is a nucleophilic catalyst for the acylation of benzyl alcohol by phenylethylketene, its activity is less than that of DMAP. Apparently the steric hindrance by the B-phenyl group of 89 limits its nucleophilicity.66

Haptotropic Rearrangememts of Derivatives of 5 and 6 Haptotropic rearrangements in which a π-coordinated metal migrates between different rings of polycyclic aro(65) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456. (66) Sammakia, T.; Hurley, T. B. J. Org. Chem. 1999, 64, 4652.

matics have been extensively studied.67 Among the best investigated systems have been those which involve a Cr(CO)3 group migration from a neutral arene ring to an anionic Cp ring, e.g. 92 to 9368 and 94 to 95,69 illustrated in Scheme 17. However, haptotropic migrations involving heterocyclic rings are relatively rare,70 perhaps because most heterocyles are poor π-ligands. Recently it has been found (67) For reviews, see: (a) D€ otz, K. H.; Wenzel, B.; Jahr, H. C. Top. Curr. Chem. 2004, 248, 63. (b) D€otz, K. H.; Jahr, H. C. Chem. Recl. 2004, 4, 61. (c) Oprunenko, Y. F. Russ. Chem. Rev. 2000, 69, 683. (d) Morris, M. J. In Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F. G. A.,Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 5, p 501. (68) Ceccon, A.; Gambaro, A.; Gottardi, F.; Santi, S.; Venzo, A.; Lucchini, V. J. Organomet. Chem. 1989, 379, 67. (69) Ceccon, A.; Gambaro, A.; Gottardi, F.; Santi, S. J.; Venzo, A. J. Organomet. Chem. 1991, 412, 85. (70) However, see: Zhu, G.; Tanski, J. M.; Churchill, D. G.; Janak, K. E.; Parkin, G. J. Am. Chem. Soc. 2002, 124, 13658.

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Ashe Scheme 19

that metal complexes of boron-nitrogen heteroaromatics undergo similar haptotropic rearrangements, as discussed below. When 83 was heated to 50 °C with (MeCN)3Cr(CO)3,71 it formed complex 96, in which the Cr(CO)3 group was π-coordinated to the heterocyclic ring. When it was heated to 150 °C, 96 formed isomer 97, which suggests that phenyl is a better π-ligand toward Cr(CO)3. Deprotonation of 97 formed 98. Heating 98 converted it to 99, suggesting that 1,2-azaboratabenzene is a better ligand than phenyl. Protonation of 99 re-formed 96. This unique cycle of haptotropic migrations and acid/base steps allowed the Cr(CO)3 group to be switched between either the phenyl or the heterocyclic ring (see Scheme 18). An analogous haptotropic migration has been found for the ring-fused benzazaboratabenzene complex 101, as illustrated in Scheme 18.58 The reaction of 85 with Cr(MeCN)3(CO)3 gave complex 100, in which the Cr(CO)3 group was coordinated to the arene ring. Treatment with base afforded azaboratabenzene 101, which underwent a Cr(CO)3 migration to afford 102. Therefore, the azaboratabenzene ring is again demonstrated to be a better ligand than the arene ring. When the Cr(CO)3 complex of azaborindene 103 was deprotonated at -60 °C, it formed complex 104, in which the 1,2-dihydro-1,2-azaborine ring is coordinated. When the temperature was raised to -30 °C, a smooth unimolecular rearrangement converted it to 105, in which the 1,2-azaborolyl ring was coordinated to Cr(CO)3. Clearly the anionic 1,2-azaborolyl ring of 105 is a better ligand than the neutral 1,2-dihydro-1,2-azaborine ring of 104. Density function theory computational work has suggested that this rearrangement involves a stepwise decomplexation of the π-bonds of one ring followed by or concomitant with the complexation of the π-bonds of the other ring. The metal was proposed to follow a trajectory along the periphery of the fused-ring system which brought it closer to boron than to nitrogen (see Scheme 19).17

Summary and Concluding Remarks Over the last 10 years the 1,2-thiaborolyl, 1,3-thiaborolyl, 1,2-oxaborolyl, and 1,2-azaboratabenzene ring systems have been prepared for the first time. New and efficient syntheses of the 1,2-azaborolyl ring system have been developed. The five-membered-ring borataheterocycles (2-5) form Cp-like (71) Pan, J.; Kampf, J. W.; Ashe, A. J., III. Organometallics 2006, 25, 197. (72) Pan, J.; Wang, J.; Banaszak Holl, M. M.; Kampf, J. W.; Ashe, A. J., III. Organometallics 2006, 25, 3463.

complexes with transition metals which are usually η5-bound to the rings. However, several early-transition-metal complexes, particularly those with π-donor substituents at boron, show a large slip distortion away from boron so that the metal-ring coordination approaches η4. The 1,2-azaboratabenzenes have been found to act as ambident ligands. They form η1-metal complexes with coordination through the ring nitrogen atom or η6-metal complexes with π-coordination through the ring. The boron-nitrogen rings continue to attract the most attention. Several early-transition-metal complexes of 1,2azaborolyls form highly active catalysts for the polymerization of olefins. It is noteworthy that these complexes have a higher polymerization activity than the corresponding Cp compounds. Optically active 1,2-azaborolyl complexes have been found to be excellent chirality transfer agents. The chemistry of 1,2-azaboratabenzenes is closely related to that of their conjugate acids, the 1,2-dihydro-1,2-azaborines, which are interesting for their bearing on the concept of aromaticity71,73,74 and their potential use as electronic materials37,75 (which is not a subject of this review). In spite of the progress in development of the chemistry of the borataheterocyles 2-6, several related ring systems have yet to be prepared. There have been no syntheses of 1,3oxaborolyls and 1,3-azaborolyls. In view of the fact that there are few important differences between the structures of complexes of 1,2- and 1,3-thiaborolyls, these compounds will probably be found to be stable. Similarly, there have been no reports on the syntheses of 1,3- and 1,4-azaboratabenzenes. Clearly significant synthetic challenges remain.

Acknowledgment. I wish to thank my co-workers and collaborators, whose names are in the references. Without them, none of these results would have been possible. Particular thanks are due to Dr. Jeff W. Kampf of the University of Michigan for solving many X-ray crystal structures and to Dr. Francis X. Timmers and colleagues at the Dow Chemical Co. for running polymerizations. The financial support of the National Science Foundation and the Dow Chemical Co. is gratefully acknowledged. (73) (a) Marwitz, A. J. V.; Hatus, M. H.; Zakharov, L. N.; Dixon, D. A.; Liu, S.-Y. Angew. Chem., Int. Ed. 2009, 48, 973. (b) Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250. (c) Marwitz, A. J. V.; Abbey, E. R.; Jenkins, J. T.; Zakharov, L. V.; Liu, S.-Y. Org. Lett. 2007, 9, 4905. (74) Pan, J.; Kampf, J. W.; Ashe, A. J., III. Org. Lett. 2007, 9, 679. (75) Liu, Z.; Marder, T. B. Angew. Chem., Int. Ed. 2008, 47, 242 and references cited therein.