Vertical and Nonvertical Participation by Sulfur, Selenium, and Tellurium

Abstract

Bulk electron transport in a high mobility n‐type polymer is studied by time‐of‐flight photocurrent measurements and electron‐only devices. Bulk electron mobilities of ∼ 5×10−3 cm2/Vs are obtained. The analysis of the electron currents suggests the presence of an injection barrier for all conventionally used low workfunction cathodes.

Mechanistic Understanding of the Divergent Cyclizations of o‐Alkynylbenzaldehyde Acetals and Thioacetals Catalyzed by Metal Halides

“…This latter intermediate readily cyclizes via B1‐TS3 a to form Pt–carbenoid B1‐IN3 a , which is the most stable intermediate in the present mechanism. The geometric structure of B1‐IN3 a shows the C 3 ⋅⋅⋅S distance is 2.590 Å, indicating the stabilization interaction between the two atoms 47…”

Modes of Neighbouring Group Participation by the Methyl Selenyl Substituent in β-Methylselenylmethyl-substituted 1-Phenylethyl Carbenium Ions

“…This mode of participation is analogous to that provided by the trimethylsilyl substituent in the carbenium ion 5 (Figure 1) which is the basis of the silicon β-effect [7,8,9,10]. Application of the variable oxygen probe to ether and ester derivatives of the antiperiplanar β-phenylselenyl alcohol 6 (Figure 1) provided crystallographic evidence that the C-Se bond is a strong σ-donor and can therefore effectively stabilise a neighbouring carbenium ion by hyperconjugation alone [1]. More recently NMR, crystallographic and computational studies on phenylselenylmethyl-substituted pyridinium ions 7 and 8 (Figure 2) revealed that a number of orbital interactions involving the selenium substituent were responsible for stabilisation of the charge on the adjacent carbon (Figure 3) [11].…”

“…Unimolecular solvolyses of the conformationally biased β-phenylselenyl trifluoroacetate 1 (Figure 1) occurs at a rate which is 10 7 times faster than the corresponding unsubstituted derivative 2 (Figure 1) suggesting that the selenium substituent provides strong assistance in the departure of the trifluoroacetate leaving group [1]. The mechanism of participation by the selenium substituent might reasonably be described by conventional neighbouring group participation [2].…”

The effects of σC—Se–π hyperconjugation in 1-methoxy- and 1-nitro-4-[(phenylselanyl)methyl]benzene

“…These two modes of participation can be differentiated as being examples of nonvertical and vertical participation (Hanstein et al, 1970;Lambert, 1990). Application of the variable oxygen probe to -phenylselenyl-substituted esters with varying electron demand provides strong evidence for the importance of vertical participation in the ground-state structures of these esters (White et al, 2002), while calculations on and structural studies of systems in which the C-Se bond interacts with orbitals having much higher electron demand suggests that nonvertical participation becomes of increasing importance (Lim et al, 2011;Harris & White, 2013) as the electron demand increases.…”

“…Phenylselenium substituents have been observed to accelerate the rates of unimolecular solvolyses in systems having a leaving group at the -position with respect to the selenium substituent. For example, the conformationally biased phenylselenylcyclohexyl trifluoroacetate (3) (Scheme 1) solvolyses at a rate 10 8 times faster than the corresponding selenium-free system (4) (White et al, 2002). The origin of this rate enhancement is believed to be from a combination of 'conventional' neighbouring-group participation in which a selenium nonbonded pair of electrons provides the nucleophile to displace the trifluoroacetate leaving group, with the formation of the seleniranium ion (5) and stabilization of the intermediate carbenium ion (6) by C-Se -hyperconjugation.…”