Retroaddition

It remains unanswered whether observed cyclopropanations and carbene retroadditions [Eq. (3)] always share a common transition state with olefin metathesis. 

In a significant series of reports describing the formation of metathesislike products from cyclopropanes via a carbene retroaddition reaction, Gassman (68) also presented results that were interpreted to mean that carbenes which normally participate in conventional metatheses are negatively polarized (nucleophilic). 

Metallocyclobutanes from cyclopropanes have been frequently invoked in transition metal-catalyzed rearrangements of strained ring hydrocarbons, and this body of chemistry is quite rich and diverse, as evidenced in the excellent review by Bishop (72). Because of this diversity, the significance of isolated observations should not be overstated nevertheless, certain reactions outlined by Bishop are closely related to the carbene retroadditions reported by Gassman and co-workers using metathesis catalysts. 

An electrocyclic reaction is defined as a concerted closure of a ring starting from a conjugated open system in which a k orbital is converted into a o orbital. It is therefore an example of intramolecular cycloaddition or retroaddition. 

The 5-benzoyl triazoline (Scheme 76) upon heating gives aziridine although it enters into competition with the retroaddition reaction, forming the starting azide and olefin.292... 

Product 19 in equation 39 is formed by trapping extruded Me2Sit, but 20, 21 and 22 are rearrangement products of silacyclopent-3-ene 23. Their formation is in accord with stepwise retroaddition with silylene extrusion from a vinylsilirane that is an intermediate in a reversible addition mechanism. As expected from such a mechanism, 20, 21 and 22 are coproducts with 23 in the addition of Me2Si to the piperylenes20. 

In oxadiazoles, 1,3-retroaddition (Section 4.21.7.2.1) of the nitrile oxide and nitrile (split a + b in 92 Scheme 15) does not compete well with the break at c, the weakest bond, followed by the break at d, except when X is NH2 <76AC(R)57,77AC(R)37i, 77AC(R)62i, 80OMS573). 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

Bicyclo[2.2.0]hex-l(4)-ene (30) dimerizes cleanly at 0 C in very dilute solution the products eventually isolated plausibly stem from a [2 + 2] cycloaddition to form (31), followed by [2 + 2] retroadditions in two different ways to give (32) and (33). ... 

Cyclopropanes are less reactive than alkenes retroaddition also occurs less readily, and cyclopropane adducts have a high acid tolerance. The nucleophiles that react include OH, OR, OCOR, O H, O R and RCN. 

The studies reported last year on the stereoisomeric control of the photo-induced Diels Alder reaction of maleic anhydride with homo-chiral anthracene derivatives such as (46) have been extended using 320-400 nm radiation, and this gives the head-to-tail anthracene dimer as well as the previously reported adduct (47) with excellent diastereoselectivity. The thermal and photochemical retroaddition process has been examined and the results suggest that this facile process may promote the anthracene as a new chiral auxiliary. 

A new synthetic route for the preparation of polyisobutylene (PIB) based block copolymers was developed by combining living carbocationic and anionic polymerizations. Living PIB chains were quantitatively end-capped with 1,1-diphenylethylene (DPE) leading to 1,1-diphenyl-l-methoxy (DPOMe) and/or 2,2-diphenyl vinyl (DPV) termini. This end-capping process is very sensitive to temperature, and retroaddition of DPE occurs in an equilibrium reaction above about -70 °C. Both the DPOMe and DPV terminated PIBs, and the mixtures of the two endgroups were quantitatively metalated with K/Na alloy, Cs metal and Li dispersion in THF at room temperature. 

In order to further investigate this phenomenon, DPOMe-ended PIB was prepared, purified, and then dissolved in CH2Cl2/n-hexane solvent mixture (40/60 v/v), cooled to -78 °C, and polymerization conditions were created by adding N,N-dimethylacetamide and TiCl4. This system was also allowed to warm up, and samples were withdrawn at different temperatures. Results identical to the experiment shown in Figure 1 were observed, indicating that retroaddition of DPE takes place from the DPOMe-ended PIB. 

Although C4 addition occurs with phenol complexes even for cases where C4 is substituted, in many cases, ortho addition is thermodynamically favored. In this scenario, the regiochemistry can be effectively controlled by adjusting reaction variables such as temperature, time, and catalyst [44]. Under basic conditions, the active form of the phenol complex is the phenoxide species, which can undergo reversible Michael reactions at C4 and C2, provided that the resulting enolate is not protonated. For instance, the addition of MVK to the osmium complexes of para-cresol (31) or estradiol (27, Fig. 8) occurs at C4 to give the 4H-phenol product (28,32) at -40 °C with an amine base. However, if the reaction is carried out at 20 °C or is run in the presence of a Zn " co-catalyst, the initially formed enolate may undergo retroaddition, and ultimately, the reaction yields the orthoalkylated product (30,33 see Fig. 8). Electrophilic addition at the ortho... 

Before analyzing Julia s results, it seems worthwhile to point out two observations of some importance in reversible cyclizations. The first one is relative to the retroaddition process, i.e., the opening of the (Cy ) radicals can be considered a )ff-scission process and to be governed by the same principles. 

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