High-performance MoS transistors are developed using atomic hexagonal boron nitride as a tunneling layer to reduce the Schottky barrier and achieve low contact resistance between metal and MoS . Benefiting from the ultrathin tunneling layer within 0.6 nm, the Schottky barrier is significantly reduced from 158 to 31 meV with small tunneling resistance.

It is reported that 2,4,6-tri-tert-butylpyrimidine (TTBP), a highly sterically hindered base available through an efficient, cost-effective one pot sequence, is a replacement for 2,6-di-tert-butylpyridine and its 4-substituted analogs in glycosylation reactions and in the formation of vinyl triflates.2,6-Di-tert-butylpyridine (1) was introduced by Brown and Kanner 1 as a mild base capable of distinguishing between Brønsted and Lewis acids. 1 Subsequently 2,4,6-tritert-butylpyridine (2) and 2,6-di-tert-butyl-4-methylpyridine (3) have also been described, and all three bases and polymer-bound versions thereof, 2 have been extensively employed as non-nucleophilic bases in a broad variety of contexts 3 including glycosylation reactions, 4,5 and the formation of vinyl triflates. 6-9 Kahne and coworkers exploited the inability of these bases to form amine-borane complexes, as noted originally by Brown, 1 by conducting glycosylations in the presence of 3 and BF 3 , so avoiding the formation of orthoesters seen in the absence of the Lewis acid. 10 In this laboratory we have made extensive use of 3 in our various b-mannosylation sequences, 11-14 but, in applying this chemistry to the synthesis of higher oligosaccharides on larger scales have been confronted by the irksome but real issue of cost. In effect 1, 2 and 3 as purchased from the usual chemical supply houses are expensive (>$12/g in the year 2000) even when purchased in 25g lots. Syntheses of these bases making use of multiple, one-pot or sequential additions of tert-butylmetal reagents to pyridine or substituted pyridines, with rearomatization, necessitate the use of large excesses of the nucleophile so rendering them less than optimal for scale up as well as inefficient. 1,15-18 The various syntheses involving the formation of the corresponding tert-butylated pyrylium ions followed by treatment with ammonia are attractive but nevertheless require multiple steps or give only moderate yields. 19,20 The most direct of these approaches is Stang's 21 but even this only allows for the production of 65 g batches of 3 and only reduces the cost to an estimated $9.4/g. Bates' synthesis of 3, 22 the condensation of dilithio isobutylene with two equivalents of pivalonitrile, is low yielding and does not appear to be scalable. We now report that 2,4,6-tri-tert-butylpyrimidine (TTBP, 4) serves as an admirable replacement for 1-3 in a range of glycosylations, and in the formation of enol triflates, and, moreover, that it can be produced in 90 g batches in a one-pot process that brings the cost down to approximately $4.3/g. TTBP (4) was first prepared in modest yield by van der Plas and Koudijs, by Minisci-type homolytic alkylation of pyrimidine. 23 A much improved synthesis was later described by García Martínez, Hanack and coworkers involving the condensation of pinacolone with two equivalents of pivalonitrile, mediated by triflic anhydride. 24 We have found that this sequence may be readily scaled to provide 90g batches in a one-pot, chromatography-free sequence. The poor nu...

Insulating hexagonal boron nitride suppresses the electrochemical corrosion and provides a more effective long-term corrosion barrier than graphene.