Cyclization secondary cycles

In this case, the crosslinking density is the sum of the effective crosslinking density and the secondary cyclization, since both crosslinkages are elastically effective. The calculation conditions are the same as Figure 1 except that the number of secondary cycles formed per effective crosslinkage T) is 20. 

Scheme 15.11 Proposed catalytic cycle for catalyst 5, capable of cyclizing secondary aminoalkene substrates.

Several ways to suppress the 2-oxonium-[3,3]-rearrangements might be envisioned. Apart from the introduction of a bulky substituent R at the aldehyde (Scheme 23) a similar steric repulsion between R and R might also be observed upon introduction of a bulky auxiliary at R. A proof-of-principle for this concept was observed upon by using of a trimethylsilyl group as substituent R in the alkyne moiety (Scheme 25, R = TMS). This improvement provided an efficient access to polysubstituted dihydropyrans via a silyl alkyne-Prins cyclization. Ab initio theoretical calculations support the proposed mechanism. Moreover, the use of enantiomerically enriched secondary homopropargylic alcohols yielded the corresponding oxa-cycles with similar enantiomeric purity [38]. 

In a more recent study Co(dppe)I2 was used as a catalyst for reductive additions of primary, secondary, and tertiary alkyl bromides or iodides 249 to alkyl acrylates, acrylonitrile, methyl vinyl ketone, or vinylsulfone 248 in an acetonitrile/water mixture using zinc as a stoichiometric reducing agent [305]. The yields of the resulting esters 252 were mostly good. The authors tested radical probes, such as cyclopropylmethyl bromide or 6-bromo-1-hexene (cf. Part 1, Fig. 8). However, the latter did not cyclize, but isomerized during addition, while the former afforded complicated mixtures. On this basis the authors proposed a traditional two-electron mechanism to be operative the results do not, however, exclude a radical-based Co(I) catalytic cycle convincingly (Fig. 61). 

The major product from the oxidation of n-heptane [83, 84] is the conjugate 0-heterocycle 2-methyl-5-ethyltetrahydrofuran. The predominant chain cycle therefore involves initial attack at a secondary C—H, followed by addition of oxygen, 1 6-hydrogen transfer from another secondary C—H and decomposition of the 7-hydroperoxyalkyl radical by simple cyclization and loss of OH, e.g. 

The peroxide is a source of benzoyloxy radicals (PhCOJ) and these capture hydrogen atoms to t the most stable radical. The best radical is the tertiary one stabilized by both CN and CC. Cyclization on to the alkene gives a secondary radical on a six-membered ring and this abstri-hydrogen atom from the starting material to complete the cycle. 

Gagosz extensively investigated on the gold-catalyzed cyclizations of ether-alkyne systems 145 and 147 bearing both terminal and internal alkynes in 2010. A wide range of structurally important spiro or fused dihydrofurans 146 and dihy-dropyrans 148 were dexterously constructed via a 1,5-hydride shift/cyclization sequence using alkyne as hydride acceptors (Scheme 12.64) [68]. This hydroalkylation process, which could be applied to the terminal as well as ester-substituted alkynes, allows the efficient conversion of secondary or tertiary C(sp )—H bonds into new C-C bonds under practical conditions. The stereoselectivity of the cycloisomerization process toward the formation of a new five- or six-membered cycle appears to be dependent on steric factors and alkyne substitution. 

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