Archive for the 'benzynes' Category

Indolyne regioselectivity explained

The nature of reactions of indolynes is the subject of two recent computational/experimental studies. There are three isomeric indolynes 1a-c which are analogues of the more famous benzyne (which I discuss in significant detail in Chapter 4.4 of my book).

One might anticipate that the indolynes undergo comparable reactions as benzyne, like Diels-Alder reactions and nucleophilic attack. In fact the indolynes do undergo these reactions, with unusual regiospecificity. For example, the reaction of the substituted 6,7-indolyne undergoes regioselective Diels-Alder cycloaddition with substituted furans (Scheme 1), but the reaction with the other indolynes gives no regioselection. 1 Note that the preferred product is the more sterically congested adduct.

Scheme 1

In the case of nucleophilic addition, the nucleophiles add specifically to C6 with substituted 6,7-indolynes (Scheme 2), while addition to 4,5-indolynes preferentially gives the C5-adduct (greater than 3:1) while addition to the 5,6-indolynes preferentially gives the C5-adduct), but with small selectivity (less than 3:1).2

Scheme 2

The authors of both papers – Chris Cramer studied the Diels-Alder chemistry and Ken Houk studied the nucleophilic reactions – employed DFT computations to examine the activation barriers leading to the two regioisomeric products. So for example, Figure 1 shows the two transition states for the reaction of 2c with 2-iso-propyl furan computed at MO6-2X/6-311+G(2df,p).

ΔG = 9.7

ΔG = 7.6

Figure 1. MO6-21/6-311+G(2df,p) optimized TSs for the reaction of 2-iso-propylfuran with 2c. Activation energy (kcal mol-1) listed below each structure.1

The computational results are completely consistent with the experiments. For the Diels-Alder reaction of 2-t-butylfuran with the three indolynes 2a-c, the lower computed TS always corresponds with the experimentally observed major product. The difference in the energy of the TSs leading to the two regioisomers for reaction with 2a and 2b is small (less than 1 kcal mol-1), consistent with the small selectivity. On the other hand, no barrier could be found for the reaction of 2-t-butylfuran with 2c that leads to the major product. Similar results are also obtained for the nucleophilic addition – in all cases, the experimentally observed major product corresponds with the lower computed activation barrier.

So what accounts for the regioselectivity? Both papers make the same argument, though couched in slightly different terms. Houk argues in terms of distortion energy – the energy needed to distort reactants to their geometries in the TS. As seen in Figure 2, the benzyne fragment of 2a is distorted, with the C-C-C angle at C4 of 125° and at C5 of 129°. In the transition states, the angle at the point of nucleophilic attack widens. Since the angle starts out wider at C5, attack there is preferred, since less distortion is needed to achieve the geometry of the TS.

2a

TS at C4
ΔG = 12.9

TS
at C5

ΔG = 9.9

Figure 2. B3LYP/6-31G(d) optimized structures of 2a and the TSs for the reaction of aniline with 2a. Activation energy in kcal mol-1.2

Cramer argues in terms of the indolyne acting as an electrophile. Increasing substitution at the furan 2-position makes is better at stabilizing incipient positive charge that will build up there during a (very) asymmetric Diels-Alder transition state. This explains the increasing selectivity of the furan with increasing substitution. The indolyne acting as an electrophile means that the attack will lead from the center will lesser charge. In 2c, the C-C-C angle at C6 is 135.3°, while that at C7 is 117.2°. This makes C7 more carbanionic and C6 more carbocationic; therefore, the first bond made is to C6, leading to the more sterically congested product. Note that Houk’s argument applies equally well, as C6 is predistorted to the TS geometry.

References

(1) Garr, A. N.; Luo, D.; Brown, N.; Cramer, C. J.; Buszek, K. R.; VanderVelde, D., "Experimental and Theoretical Investigations into the Unusual Regioselectivity of 4,5-, 5,6-, and 6,7-Indole Aryne Cycloadditions," Org. Lett., 2010, 12, 96-99, DOI: 10.1021/ol902415s

(2) Cheong, P. H. Y.; Paton, R. S.; Bronner, S. M.; Im, G. Y. J.; Garg, N. K.; Houk, K. N., "Indolyne and Aryne Distortions and Nucleophilic Regioselectivites," J. Am. Chem. Soc., 2010, 132, 1267-1269, DOI: 10.1021/ja9098643

InChIs

1a: InChI=1/C8H5N/c1-2-4-8-7(3-1)5-6-9-8/h2,4-6,9H
InChIKey=RNDHGGYOIRREHC-UHFFFAOYAU

1b: InChI=1/C8H5N/c1-2-4-8-7(3-1)5-6-9-8/h3-6,9H
InChIKey=WWZQFJXNXMIWCD-UHFFFAOYAO

1c: InChI=1/C8H5N/c1-2-4-8-7(3-1)5-6-9-8/h1,3,5-6,9H
InChIKey=UHIRLIIPIXHWLT-UHFFFAOYAH

2a: InChI=1/C9H7N/c1-10-7-6-8-4-2-3-5-9(8)10/h3,5-7H,1H3
InChIKey=VTVUPAJGRVFCKI-UHFFFAOYAJ

2b: InChI=1/C9H7N/c1-10-7-6-8-4-2-3-5-9(8)10/h4-7H,1H3
InChIKey=KKPOWDDYMOXTFW-UHFFFAOYAN

2c: InChI=1/C9H7N/c1-10-7-6-8-4-2-3-5-9(8)10/h2,4,6-7H,1H3
InChIKey=MDAHOGWZOBLIEX-UHFFFAOYAZ

Aromaticity &benzynes &Cramer &Houk Steven Bachrach 29 Mar 2010 3 Comments

Tridehydrobenzene

In section 4.4 of the book, I discuss in great detail the computational (and some experimental) studies of the benzynes, the formal diradicals created by loss of two hydrogen atoms from benzene. Now comes a very nice experimental study on a molecule that takes the next step: 1,3,5-tridehydrobenzene 1, benzene that lacks three hydrogen atoms. Sander reports the preparation and characterization of trifluoro-1,3,5-tridehydrobenzene 2.1 The characterization of this novel molecule is made through comparison with computed IR spectra.

2 is prepared by flash vapor pyrolysis of 1,3,5-triiodo-2,4,6-trifluorobenezene
and then trapping the products in a low temperature matrix. Sander identifies five IR peaks of a product he believes is 2. These IR frequencies are listed in Table 1.

Table 1. Experimental and computeda IR frequencies (cm-1) and relative intensities of 2.

Expt

2a

2bb

ν

I

ν

I

ν

I

954

60

921.7

57

976.2

57

1030

30

997.6

54

1016.0

55

1266

40

1221.8

35

1291.3

33

 

 

1310.6

16

1325.4

30

1560

70

1530.0

73

1572.6

100

1738

100

1726.6

100

1690.6

88

aUBLYP/cc-pVTZ. bTransition state.

In order to confirm that this IR spectra comes from 2, Sander computed the structure and IR frequencies of both 1 and 2. The 2A1 structure of 1 had been studied previously2, but what had gone unnoticed is that another structure is possible, the 2B2 state. These two states differ in the separation between C1 and C3. When the distance is short, the SOMO is of a1 symmetry and this orbital has bonding character between these two carbon centers, giving rise to the 2A1 state (1a). As the distance gets longer between C1 and C3, a b2 orbital, having antibonding character between C1and C3, becomes lower in energy than the a1 orbital, so that the structure is 2B2 (1b). The UBLYP/cc-pVTZ optimized structures are shown in Figure 1. 1a is 2-3 kcal mol-1 lower in energy than 1b. Furthermore, 1b has one imaginary frequency and is not a local energy minimum. Sander also optimized the structures of 2a and 2b¸ finding little effect due to the fluorine substitution.

1a

1b

Figure 1. UBLYP/cc-pVTZ optimized structures of 1a (2A1) and 1b (2B1).

The computed IR frequencies are listed in Table 1. The computed frequencies (and their relative intensities) of 2a match up strikingly well with those of the experiment. Sander concludes that 2a has in fact been prepared and characterized.

References

(1) Venkataramani, S.; Winkler, M.; Sander, W., "Trifluoro-1,3,5-tridehydrobenzene," Angew. Chem. Int. Ed. 2007, 46, 4888-4893, DOI: 10.1002/anie.200700536

(2) Cristian, A. M. C.; Shao, Y.; Krylov, A. I., "Bonding Patterns in Benzene Triradicals from Structural, Spectroscopic, and Thermochemical Perspectives," J. Phys. Chem. A 2004, 108, 6581-6588, DOI: 10.1021/jp049007j.

InChI:

1: InChI=1/C6H3/c1-2-4-6-5-3-1/h1,4-5H
2: InChI=1/C6F3/c7-4-1-5(8)3-6(9)2-4

benzynes &DFT Steven Bachrach 20 Aug 2007 No Comments