Cyclopropanes shifts

Figure 7.9. Calculated and experimental h and C NMR spectra chemical shifts relative to TMS, H, and C, respectively. The calculations were with the default NMR method (GIAO) implemented in Gaussian 94W [54], The experimental values are from [89] (a value of —3.5 has been given for the cyclopropane shift [90]).

BUTENE. As shown in Figure 38, a group attached to C-1 can migrate from position 1 to 3 (1,3 shift) to produce an isomer. If it is a methyl group, we recover a 1-butene. If it is a hydrogen atom, 2-butene is obtained. A third possible product is the cyclopropane derivative. The photochemical rearrangement of 1-butene was studied extensively both experimentally [88]... 

Small shift values for CH or CHr protons may indicate cyclopropane units. Proton shifts distinguish between alkyne CH (generally Sh = 2.5 - 3.2), alkene CH (generally 4, = 4.5-6) and aro-matic/heteroaromatic CH (Sh = 6 - 9.5), and also between rr-electron-rich (pyrrole, fiiran, thiophene, 4/ = d - 7) and Tt-electron-deficient heteroaromatic compounds (pyridine, Sh= 7.5 - 9.5). 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

In the C NMR spectrum two signals with unusually small shift values [(07/2)2 5c = 7.7 CH 5c = 10.6] and remarkably large CH coupling constants Jch= 161.9 and 160.1 Hz) indicate a mono-substituted cyclopropane ring A. The protons which belong to this structural unit at = 0.41 (AA ), 0.82 BB ) and 1.60 (M) with typical values for cis couplings 8.1 Hz) and trans couplings 4.9 Hz) of the cyclopropane protons can be identified from the CH COSY plot. 

Along with the minimal barrier for H shift, the 2-butyl to t-butyl rearrangement gives the energy surface shown in Fig. 5.9. This diagram indicates that the mechanism for C-3/C-4 scrambling in the 2-butyl cation involves the edge-protonated cyclopropane intermediate. 

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

The formation of alkyl shifted products H and 14 can be explained in terms of the formation of endo-intermediate 21 formed by endo attack of bromine to 2 (Scheme 4). The determined endo-configuration of the bromine atom at the bridge carbon is also in agreement with endo-attack. Endo-Intermediate 21 is probably also responsible for the formation of cyclopropane products 12 and 15. The existence of cyclopropane ring in 12 and 15 has been determined by and 13c NMR chemical shifts and especially by analysis of cyclopropane J cH coupling constants (168 and 181 Hz). On the basis of the symmetry in the molecule 12 we have distinguished easily between isomers 12 and 15. Aryl and alkyl shift products IQ, H, and 14 contain benzylic and allylic bromine atoms which can be hydrolized easily on column material. 

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in superacid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl or hydride shifts unless primary carbocations (which are highly unlikely) are intermediates. However, the reaction can be explained " by postulating that (in the forward reaction) it is the 1,2 bond of the intermediate or transition state 15 that opens up rather than the 2,3 bond, which is the one that would open if the reaction were a normal 1,2 shift of a methyl group. In this case, opening of the 1,2 bond produces a tertiary cation, while opening of the 2,3 bond would give a secondary cation. (In the reaction 16 14, it is of course the 1,3 bond that opens). 

Hypothetically, 32 could have arisen from a 1,3 shift (direct or through a protonated cyclopropane) or from two successive 1,2 shifts ... 

However, the same reaction applied to 2-methyl-2-butanol gave no 32, which demonstrated that 35 was not formed from 34. The conclusion was thus made that 35 was formed directly from 33. This experiment does not answer the question as to whether 35 was formed by a direct shift or through a protonated cyclopropane, but from other evidence" it appears that 1,3 hydride shifts that do not result from successive 1,2 migrations usually take place through protonated cyclopropane intermediates (which, as we saw on p. 1382, account for only a small percentage of the product in any case). However, there is evidence that direct 1,3 hydride shifts by way of A may take place in superacid solutions." ... 

Two competing reactions are the homodienyl [1,5] shift (if a suitable H is available, see 18-29), and simple cleavage of the cyclopropane ring, leading in this case to a diene (see 18-3). 

Esters of a-diazoalkylphosphonic acids (95) show considerable thermal stability but react with acids, dienophiles, and triphenylphosphine to give the expected products. With olefinic compounds in the presence of copper they give cyclopropane derivatives (96), but with no such compounds present vinylphosphonic esters are formed by 1,2-hydrogen shift, or, when this route is not available, products such as (97) or (98) are formed, resulting from insertion of a carbenoid intermediate into C—C or C—H bonds. The related phosphonyl (and phosphoryl) azides (99) add to electron-rich alkynes to give 1,2,3-triazoles, from which the phosphoryl group is readily removed by hydrolysis. 

When CH2F is a substituent on most alicyclic rings, such as a cyclohexane ring, the 19F chemical shift of this group is not significantly altered from that of an acyclic system (Scheme 3.2). On the other hand, when it is attached to a cyclopropane ring, a unique deshielding influence is observed. 

Both the 13C chemical shifts and the F—C coupling constants for CF2 carbons are quite characteristic in value, as can be seen from the examples in Scheme 4.9. A review article on 13C NMR spectra of fluorinated cyclopropanes has recently appeared.3... 

A trifluoromethyl group attached to a cyclohexane ring is unremarkable with respect to its chemical shift, absorbing at -75 ppm, with a 3/fh = 8Hz (Scheme 5.2). There are no data available for trifluorometh-ylcyclopentane or cyclobutane. The chemical shift for trifluoromethyl-cyclopropane reflects additional shielding, such CF3 groups appearing the farthest upheld of any CF3-substituted hydrocarbon. 

Scheme 6.26 provides chemical shifts for all of the fluorines in a representative group of perfluorocarbons. The various environments exhibited should allow one to estimate the chemical shift for almost any fluorine in a perfluorocarbon system. The fluorine chemical shifts of four-, five- and six-membered ring perfluoroalicyclics are quite consistently in the range of -133 to -134 ppm, but as usual, fluorines on a cyclopropane ring appear at a much higher field than those of other fluorinated alicyclics, perfluorocyclopropane having a chemical shift of -159 ppm. 

In order to form the biradical (133), the cyclopropane molecule becomes vibrationally excited by collision with another molecule the C—C bond may then break provided the extra energy is not lost too rapidly by further collision. There is driving force here for a 1,2-shift of hydrogen—unlike in mono-radicals (p. 335)—because of the opportunity of electron-pairing to form a n bond (with evolution of energy) in (134). There is evidence that this H-migration is commonly the rate-limiting step of the reaction. 

In the end, chemical data are matters of fact, but mechanisms are matters of opinion on balance, we believe an adsorbed cyclopropane-type intermediate to be unlikely for bond-shift processes on platinum catalysts. 

Fig. 1. a) The highest occupied and lowest unoccupied Walsh orbitals of cyclopropane, b) Ground configuration (left) and excited configuration (right) of cyclopropane, and level shifts upon increase of a. 

Copper-catalyzed cyclopropanation of benzene and its derivatives by a diazoacetic ester yields a norcaradiene 230 which undergoes spontaneous ring opening to cyclo-heptariene 231. At the temperatures needed for successful cyclopropanation, sigma-tropic H-shifts leading to conjugated isomers of cycloheptatriene carboxylates cannot be avoided. The situation is complicated by the formation of regioisomers upon cyclopropanation of substituted benzenes, and separation of the cycloheptatriene isomers may became tedious if not impossible. 

As opinions regarding the metathesis reaction pathway have shifted in recent years to favor a nonpairwise carbene-to-metallocyclobutane transformation, increasing attention has been given to the mechanistic significance of cyclopropane olefin interconversions. This interconversion process seems to occur primarily when certain relatively inefficient catalysts are employed, which in itself raises questions. Under ideal conditions, "good metathesis catalysts are remarkably efficient promoters of transalkylidenation, and consequently are well suited for olefin and polymer syntheses. Thus, most early studies focused primarily on applications. When side reactions did occur, they were usually ignored or presumed to be trivial cationic processes. 

Additional evidence for a second intermediate in supposed carbene reactions comes from numerous studies.17-29 In the earliest experimental approach, the carbene precursor, frequently a diazirine, was photolyzed in the presence of increasing quantities of an alkene, which trapped the carbene with the formation of a cyclopropane (5 in Scheme 1). If carbene 2 were the sole product-forming intermediate, as depicted in Scheme 1, then the ratio of its alkene addition product (5) to its 1,2-H shift rearrangement product (4) would vary linearly with alkene concentration Eq. 9. 

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