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2 Addition of Segregated Addends – The Inherent Regioselectivity

2 Addition of Segregated Addends – The Inherent Regioselectivity

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290



10 Regiochemistry of Multiple Additions



Figure 10.1 Relative positional relationships of [6,6]-bonds in C60 adducts.

A denotes the location of the first addend.



benzene chemistry) within addend carrying [6,6]-bonds in C60 derivatives are labeled

as cis-n (n = 1–3), e′, e′′, trans-n (n = 1–4) (Figure 10.1).

This nomenclature is generally used in fullerene literature. However, it can only

be applied to [6,6]-bonds within C60-derivatives. But, since, for example, the regiochemistry of multiple [5,6]-adducts of C60 and multiple adducts of higher fullerenes

started to develop, a general site labeling algorithm has been introduced, which

can be applied to [5,6]- and [6,6]-positions of all fullerenes and their derivatives [4].

The basis for the algorithm is the contiguous numbering scheme of the C atoms of

a given fullerene in a spiral fashion, which is used for the logic structure assignment

of any fullerene adduct [5–7]. The corresponding labeling scheme for C60 derivatives

is represented in Figure 10.2. The priority of sites decreases in C60 derivatives in

the order I, II, III … eI, eII, … III*, II* I* and A, B, C … eA, H, H*… C*, B*, A*.

To describe fullerene derivatives with an inherent chiral addition pattern, two

mirror images of each labeling diagram with clockwise and anti-clockwise numbering of the C atoms have to be considered. This guarantees the absolute compatibility

of the general configurational description of fullerene derivatives with a chiral

addition pattern introduced by Diederich using the descriptors fC and fA. These

descriptors indicate the mode of numbering as fullerene clockwise or fullerene

anti-clockwise [8]. Figure 10.3 gives examples for the site labeling of fullerene

derivatives with an inherent chiral addition pattern.



Figure 10.2 Labeling of [6,6]-bonds (left) and [5,6]-bonds (right) in C60 derivatives.



10.2 Addition of Segregated Addends – The Inherent Regioselectivity



Figure 10.3 Scheme for determining the absolute configuration (fC vs fA) of

chiral C60 adducts and assignment of the absolute configuration of adducts.



10.2.1

Subsequent Cycloadditions to [6,6]-double Bonds



Cycloadditions to [6,6]-double bonds of C60 are among the most important reactions

in fullerene chemistry. For a second attack to a [6,6]-bond of a C60 monoadduct

nine different sites are available (Figure 10.1). For bisadducts with different but

symmetrical addends nine regioisomeric bisadducts are, in principle, possible. If

only one type of symmetrical addends is allowed, eight different regioisomers can

be considered, since attack to both e′- and e′′-positions leads to the same product.

Two successive cycloadditions mostly represent the fundamental case and form

the basis for the regioselectivity of multiple additions. In a comprehensive study of

bisadduct formations with two identical as well as with two different addends,

nucleophilic cyclopropanations, Bamford–Stevens reactions with dimethoxybenzophenone–tosylhydrazone and nitrene additions have been analyzed in detail

(Scheme 10.1) [3, 9, 10].

The results can be summarized as: (1) Product distributions (Figure 10.4) are

not statistical (in principle: one possibility for a trans-1 attack, two possibilities for

e′- or e′′-attacks and each of four possibilities for attack to the other trans- or cispositions); (2) in most cases e-isomers followed by the trans-3-isomers are the



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10 Regiochemistry of Multiple Additions



Scheme 10.1 Synthesis of regioisomeric bisadducts of C60.

(i) BrCH(COOEt)2, NaH, toluene, room temp.;

(ii) Ar2=NNHTs, BuLi, toluene, reflux; (iii) EtOOCN3, TCE, reflux.



preferred reaction products; (3) cis-1-isomers are formed only if the steric requirement of the addends allows their suitable arrangement in such a close proximity

(e.g. at least one imino addend is required, which unlike methano bridges contains

only one flexible side chain); (4) together with the e-isomers the cis-1-adducts are

the major products if their formation is possible at all; (5) an attack to an e′′-position

is slightly preferred over an attack to an e′-position; and (6) the regioselectivity of

bisadduct formation is less pronounced if more drastic reaction conditions are

used [e.g. less regioselectivity for nitrene additions in refluxing 1,1,2,2-tetrachloroethane (TCE) compared with reactions with diethylbromomalonate at room

temperature]. Similar product distributions were observed, for example, for twofold

additions of diamines [11], for bisosmylations [12], for twofold addition of azomethine ylides [13], for the formation of the tetrahydro[60]fullerenes [14] and for

the twofold addition of benzyne [15].



10.2 Addition of Segregated Addends – The Inherent Regioselectivity



Figure 10.4 Relative yields of isolated regioisomeric bisadducts of

C62(COOEt)4 (left) and C61(COOEt)2(NCOOEt) (right).



Interpretation of these experimental results requires distinction between the

properties of the reaction products and their precursor monoadducts [1]. Comparison of AM1-calculated energies of typical series of bisadducts (Table 10.1) shows

that (1) in bisadducts with two sterically requiring addends such as dialkoxycarbonylmethylene or diarylmethylene groups cis-1-adducts are energetically forbidden;

(2) the opposite case is observed in bisadducts with at least one imino addend where

the cis-1-isomers are the most stable; and (3) all other isomers exhibit very similar

stabilities, with the e-isomers being slightly stabilized and the cis-isomer slightly

destabilized. In all cases the trans-isomers have about the same calculated heat of

formation. As can be seen from space filling models, the instability of a cis-1-isomer

such as cis-1-C62(anisyl)4 (Scheme 10.1) is due to the pronounced steric repulsion

of the addends leading to considerable deformations of typical bond angles [10].

Conversely, a strain-free situation is provided if at least one imino addend is present,

since in low energy invertomers unfavorable interactions between the addends are

avoided. Analogous behavior occurs for the various regioisomers of C60H4 [14]

where, except for eclipsing H-interactions, no additional strain due to the addends

is present. The corresponding cis-1-adduct is the major product formed by a twofold

hydroboration followed by hydrolysis. AM1-calculations predict the cis-isomers to

be somewhat less stable than the trans- and e-isomers. According to ab initio

calculations (HF/3-21G) the cis-1- followed by the e-isomer is the most stable. These

considerations show that the thermodynamic properties of the bisadducts alone

cannot explain the observed regiochemistry [1].

What is the influence of the structural and electronic properties of the precursor

adducts on the product distribution? Analysis of various experimental and calculated

[6,6] monoadduct structures shows that the bond length alternation between [5,6]and [6,6]-bonds is preserved [1]. Significantly, independently of the nature of the



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10 Regiochemistry of Multiple Additions

Table 10.1 Relative stabilities (AM1HOF) in kcal mol−1 of the possible regioisomers

of bis-adducts with two identical and two different addends.



Positional

relationship



C62(COOMe)4



C62(phenyl)2



C60(NCOOMe)2



C61(COOMe)2(NCOOMe)



trans-1

trans-2

trans-3

trans-4



0.2

0.2

0.1

0.0



0.2

0.3

0.2

0.1



4.4

4.5

4.3

4.4



1.3

1.2

1.1

1.1



e′

e′′

cis-3

cis-2

cis-1

a)

b)

c)



0.0 a)

0.0 a)



0.0 a)

0.0 a)



1.3

1.8

17.7



2.3

3.3

24.9



4.2 a)

4.2 a)

5.9

6.7

0.0



0.8 b)

0.8 c)

3.7

3.8

0.0



e′- and e′′-isomers are identical.

e′-isomer referred to C61(COOEt)2 as precursor molecule.

e′′-isomer referred to C61(COOEt)2 as precursor molecule.



addend, the cis-1 bonds are considerably shorter than those of parent C60. A less

pronounced contraction is observed for the e′′-bonds. Another trend to emerge is

that the cis-2- and cis-3-bonds are somewhat elongated and that the opposite

hemisphere is less disturbed [1]. Similarly to the geometrical distortions the

polarizations (AM1-Mulliken-charges) of the C-framework in these monoadducts

is somewhat enhanced in the neighborhood of the first addend but essentially zero

in the opposite hemisphere. Computational analysis of frontier orbitals in monoadducts such as C61(COOEt)2 (1) revealed the following characteristics [1]: (1) In a

first approximation the distribution of the MO coefficients to specific sites within

monoadducts is totally independent of the nature of the addend. (2) The distribution

of MO coefficients to specific sites within monoadducts is related to that within

free C60, as can be seen from the diagrams correlating the HOMOs and LUMOs of

C60 with those of a monoadduct (Figure 10.5).

The lowest lying HOMO-4 of the monoadduct correlates with the hu orbital of

C60 having the highest coefficients in two opposing [6,6]-bonds (HOMO). Relative

to this HOMO three of the others have high coeffiecients in the equatorial sites and

not in the opposing I and I* (trans-1) sites (trans-1 effect). In the HOMO of the

monoadduct, highest coefficients are located in the cis-1 and e′′ site and in the

HOMO-1 in the e′ followed by the trans-3 and cis-2 sites. The LUMO of C60 with

high coefficients in opposing [6,6]-bonds correlates with the LUMO+2 of the

monoadduct. Due to the perpendicular orientation of the LUMOs of C60 and their

correlation with the LUMOs of the monoadduct there are no coefficients in trans-1

but preferably in the e-positions of the LUMO and LUMO+1 (trans-1 effect). In the

LUMO, pronounced coefficients are also found in trans-3 and cis-2. Only in LUMO+2

do the trans-1-, trans-4- and cis-3-bonds also exhibit enhanced coefficients. The

differences in [6,6]-bond lengths are influenced by the HOMO coefficients since



10.2 Addition of Segregated Addends – The Inherent Regioselectivity



Figure 10.5 (a) Correlation of the HOMOs of C60 with the HOMOs of C61H2.



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10 Regiochemistry of Multiple Additions



Figure 10.5 (b) Correlation of the LUMOs of C60 with the LUMOs of C61H2.



the shortest [6,6]-bonds are those with the highest coefficients in the HOMO.

Pronounced bonding interactions at a binding site in general cause a contraction

of the bond length. The fact the cis-1 bond is shorter than the e′′ bond, although the

coefficients in the HOMO are comparable, could be due to the removal of cyclic

conjugation within the six-membered ring involving the cis-1 bonds. For electrophilic attacks the highest lying HOMOs (e.g. HOMO and HOMO-1) and for

nucleophilic attacks the lowest lying LUMOs (e.g. LUMO and LUMO-1) of the

precursor adducts are the most important, and the distribution of their orbital

coefficients affects the regioselectivty of a bisaddition. In conclusion, the typical



10.2 Addition of Segregated Addends – The Inherent Regioselectivity



product distributions of twofold additions to [6,6]-bonds, especially the preferred

attacks of e-sites for sterically demanding and of e- and cis-1 sites for sterically less

demanding addends, correlate with enhanced frontier orbital coefficients in the

monoadduct precursors. At the same time the lengths of the corresponding

monoadduct double bonds are the shortest, implying pronounced reactivity. The

preference of cis-1 attacks leading, for example, to cis-1- C61(COOEt)2(NCOOEt)

(Scheme 10.1) indicates that not only the nitrene additions but also the cyclopropanations should be mainly HOMO-C60 controlled. Here, the cyclopropanation

via malonates would be due to addition or carbenes. If it is due to an initial

nucleophilic attack, then the preferred formation of e-adducts would also be reflected

by the distribution of the coefficients of the LUMO and LUMO+1 in the precursor

adducts. To explain the pronounced formation of cis-1 adducts, however, additional

factors have then to be considered, which could be thermodynamic arguments,

since for sterically non-demanding addends the cis-1-adducts followed by the eadducts are the most stable. Another driving force could simply be that cis-1 bonds

are the shortest, having the most double bond character. The extent of pyramidalization (curvature) at a given site, which is a predominant factor governing

the preferred bond of an attack, for example, to C70 ([6,6]-bonds at the poles,

Chapter 13), does not play a role for successive additions to C60. This can be clearly

seen from the fact that the reactive cis-1 sites are the least pyramidalized in a

monoadduct.

For higher adducts of C60 the number of possible regioisomers increases

dramatically with an increasing number of addends. At the same time it becomes

even more difficult to experimentally identify a specific addition pattern. There

are few systematic investigations to date of three successive additions of C2v- or C2

symmetrical addends to [6,6]-double bonds of C60 [3, 16–20]. For trisadducts, in

principle, 46 different regioisomers are possible. The number of regioisomers that

can theoretically be formed starting from a given bisadduct depends on its existing

addition pattern. For example, 14 different trisadducts can be produced from a

precursor with the addends bound in e-positions. However, for many addends,

such as malonates, the number of preferably formed regioisomers is considerably

smaller, since, for example, additions in cis-positions can be neglected. For

bis-malonates or related adducts with either e- or trans-positional relationships,

only attacks into e- or trans-positions to addends already bound have to be considered

[20]. The number of allowed addition patterns is then reduced to ten (Table 10.2

see page 300).

Seven of these were found after cyclopropanation of e- and trans-n-C62(COOEt)4

(n = 2–4) (4–7) with diethylmalonate (Figure 10.6) [20]. In a few cases, such as the

C3-symmetric adduct 8 (e,e,e-addition pattern), the structure can be assigned based

on NMR spectroscopy alone. NMR spectroscopy allows for the determination of

the point group of the adduct. The e,e,e-addition pattern is the only one that has

C3-symmetry and as a consequence the assignment is unambiguous. The same is

true for the D3-symmetrical adduct 9. Conversely, C2-, Cs- or C1-symmetry of

trisadducts can arise from different addition patterns. The structural assignment

of such adducts requires the additional analysis of their possible formation pathways.



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10 Regiochemistry of Multiple Additions



Figure 10.6 Trisadducts obtained from cyclopropanation of bisadducts 4–7.



10.2 Addition of Segregated Addends – The Inherent Regioselectivity



Figure 10.6 (continued)



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10 Regiochemistry of Multiple Additions

Table 10.2 Relative and absolute positional relationships, symmetry and number of possible

formation pathways of trisadducts that can be formed out of e and trans-n (n = 1–4) bisadducts

(first row) neglecting cis additions.



trans-1

I, I*



trans-2

I, II*



trans-3

I, III*



trans-4

I, IV*



e

I, eI



e, e, t-1 (I) a)

I, eI, I*

Cs

2



e, t-4, t-2

I, eI, IV*

C1

2



e, t-4, t-3

I, eI, III*

C1

2



e, t-4, t-2

I, eI, IV*

C1

2



e, e, e

I, eI, eII

C3

2



e, e, t-1 (II)

I, eII, I*

Cs

2



e, t-3, t-2

I, eII, III*

C1

2



e, t-3, t-2

I, eII, III*

C1

2



e, t-4, t-3

I, eI, III*

C1

2



e, t-4, t-2

I, eI, IV*

C1

2



t-4, t-4, t-2

I, IV*1, IV*3

C2

1



t-4, t-3, t-3

I, IV*, III*

Cs

1



t-4, t-4, t-2

I, IV*1, IV*3

C2

2



e, e, t-1 (I)

I, eI, I*

Cs

1



t-3, t-3, t-3

I, III*, III*

D3

1



t-4, t-4,t-4

I, IV*1, IV*4

C3v

1



e, t-4, t-3

I, eI, III*

C1

2



t-4, t-3, t-3

I, IV*, III*

Cs

1



e, t-3, t-2

I, eII, III*

C1

2

e, e, t-1 (II)

I, eII, I*

Cs

1



a)



t = trans.



For example, adduct 12 was formed from e-4, trans-3-6 and trans-2-7. Consequently,

it must involve these three positional relationships. Therefore, its structural

assignment is unambiguous. Similarly, various trisadducts carrying C2-symmetrical

bis(oxazoline) addends could be isolated and structurally assigned [19, 20]. The

regioselectivities of these cyclopropanations strongly depend on the precursor

bisadduct. Whereas, for example, all possible trisadducts 9, 10, 12, 14 that can be

obtained from trans-3-6 were formed in about equal amounts the cyclopropanation

of trans-4-5 is more selective. Among the four isolated isomers 10 was the most

abundant. The fifth isomer, with a C3v–symmetrical trans-4, trans-4, trans-4- addition

pattern, was not found.

Only three of the six possible trisadducts were obtained upon cyclopropanation

of e-4. Especially, the adducts with the addition patterns e,e,-trans-1 (I) and e,e,trans-1 (II) did not form [20]. Access to corresponding adducts requires trans-1

precursors and/or tether strategies (Section 10.3.3). Although the relative yield of

e,e,e-8 constitutes about 35–40% of the trisadducts formed and the yield of

e,trans-3,trans-2-12 is higher, the preferred mode of addition is e relative to the



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