Thioborates

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

The cis thioboration of terminal alkynes with 9-(arylthio)-9-BBN is catalyzed by Pd(Pli3P)4 in the presence of styrene. The product 136 is converted into the vinyl sulfides 137 and 138 by the treatment with MeOH or by Pd-catalyzed cross-coupling with aryl or alkenyl halides using K3PO4 in DMF[68]. No thioboration takes place with internal alkynes. 

The products resulting from such reactions should, therefore, have analogous names. If KBO2 is a borate, KBS2 is a thioborate and KBF is a fluoroborate. Similarly, the replacement of an oxygen atom by a sulfur atom or two fluorine atoms is understandable. However, the relationship of K2BN2 is less obvious, until one considers the dehydration and deammoniation schemes ... 

A possible intermediate for Pd(PPh3)4-catalyzed thioboration of 1 -alkyne... 

Catalyzed Thioboration Reaction of Alkynes. Differences between Pd(0) and Pt(o) Catalysts and between Thioboration and Diboration , Organometallics, 17, 1383. 

Monomeric iminoboranes containing only sulfur substituents have resulted from the thioboration of organic thiocyanates 32). 

A DFT study reveals that the thioboration reaction proceeds via a metathesis-like pathway between Pd-G and S-B bonds forming Pd-S and G-B bonds, rather than an oxidative addition of the B-S bond to the Pd(0) complex (Scheme 74).284... 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

Figure 10. The reaction mechanism of acetylene thioboration reactions catalyzed by Pd(0) complexes calculated by Morokuma and co-workers.

Can Pt(0) complexes serve as active catalysts for the alkyne thioboration reactions Morokuma and co-workers also carried out calculations on the mechanism of the Pt(0)-catalyzed acetylene thioboration reaction with a smaller model HS-B(OH)2. They found that the reductive elimination in the last step (from TH5 to TH6) needs to overcome a very high barrier of 27.9 kcal/mol because the Pt(II) analog of TH5 is calculated to be very stable. It is predicted that the Pt(0) complex is not a good catalyst for thioboration reactions. 

For the thioboration reactions of alkynes preferentially catalyzed by Pd(0) instead of Pt(0), the reaction mechanism involves a metathesis-like process. The reason for not having an oxidative addition step can be related to the electron richness of the alkylthio group, which prevents the oxidative addition of thioborane to the metal center. Because of the preference for having a metathesis-like process, Pd becomes a better candidate due to its relative less electron richness in comparison to Pt. 

Cui, Q., Musaev, D. G., Morokuma, K., 1998b, Density Functional Study on the Mechanism of Palladium(O)-Catalyzed Thioboration Reaction of Alkynes. Differences between Pd(0) and Pt(o) Catalysts and between Thioboration and Diboration , Organometallics, 17, 1383. 

Unactivated halopyridines including 3-halopyridines undergo substitution at room temperature using thioborates prepared in situ from K-Selectride and phenylethanethiol in 1,2-DME in moderate to good yield. 3-Chloropyridine is converted to the thiol in 63% yield using these conditions <1999TL5565> (Equation 99). 

Many compounds with B-S-R groups have been synthesized, including thioborates or trialkyl- and tiaryl-thioborates, B(SR)3, the sulfur analogs of trialkoxy- and triaryloxybo-ranes, and trialkyl- and triarylborathiins, (SBR)3, the sulfur analogs of boroxins. Simple thioborates are usually prepared by reactions of boron halides with thiols (equation 13). 

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